Sunday, July 6, 2014

Make a magnetic loop antenna for 7-21 mhz


  • Magnetic Loop Diagram

    Magnetic Loop Diagram

  • Magnetic Loop antenna

    Magnetic Loop antenna

This antenna has several advantages, not least being only 1 metre diameter! This loop relies on being horizontally polarised and receives only the magnetic wave, thus as most noise in the domestic environment is vertically polarised and electrical wave, it delivers low noise to your transeiver/receiver, which makes for nice clean listening. In addition any signal arriving in the direction of the loop end on will be nulled out, this can be useful to get rid of an interfering signal by simply rotating the loop leaving the desired signal in the clear. It can be used indoors with ease and works well at ground level which is not the case for long wire/dipole antennas at shortwave wavelengths.
So what are its disadvantages? Well its tuning is critical, such that for a small change in frequency the antenna will need to be retuned at the loop end. This is even more important for transmitting where a high reflected wave (swr) due to not being tuned correctly will damage the output stage of your transmitter! In addition due to the very high "Q" of the loop, very high voltages can build up on the loop tuning capacitor even with low amounts of power from your transmitter. It is for this reason I recommend this loop is used with a transmitter of no more than 8 watts, any more and the ordinary broadcast tuning cap will arc over with spectacular results. Of course should you wish, a higher spec/bigger air spaced tuning cap would allow higher power output transmitters to be used. Also I consider the use of remote tuning using a fairly high geared motor and insulated coupling on the tuning cap essential. For shortwave listeners manual tuning would suffice.
In setting up the tuning of the loop, connect to a receiver and tune to 14 mhz. Now tune the loop which as it nears peak tuning will cause a whooshing sound. Stop the tuning you should now hear good strength signals in your receiver. For tuning for a transmitter, 1st use receive method then apply low power and fine tune loop tuning and tweak gamma match for lowest swr.

Magnetic loop dimension details

  • Diameter of loop 1000mm
  • Diameter of tube 15mm
  • Width of base 780mm
  • Diameter of support pipe 42mm
  • Loop end spacing (for tuner) 50mm
  • Height of support 1590mm
  • Nylon board 210x240mm
  • Nylon board 240x70mm
  • Gamma match width 310mm
  • Gamma/loop spacing 110mm

Construction Tips

  • Use a bicycle wheel with no tyre on to help form the curves of the soft annealed copper tube
  • Clean the tube with wire wool before any soldering
  • Use a 100 watt soldering gun for the joints, but use a small blow torch first to get the copper at temperature to take a joint
  • Force some timber with the corners planed off down the plastic plumbing pipe this will stiffen the pipe as the loop is quite weighty
  • Use inverted shelf brackets to support the mounting pipe and make a wooden frame wide enough to hold up the loop



The small single turn magnetic loop (SSTML) antenna consists of a single winding inductor, about 3 feet (1 meter) in diameter, and a tuning capacitor. A second loop, which is one fifth of the diameter of the large loop, is connected to the feedline and this small loop is positioned in the large loop on the opposite side of the tuning capacitor.

Magnetic Loop Antenna

The SSTML has some very interesting properties:

a) It has a small footprint. The loop I describe here looks like a circle in the vertical plane and is just a little over 3 feet (1 meter) in diameter.

b)It is a rather quiet antenna. It doesn’t pick up as much man-made noise from nearby sources as a wire antenna would in the same situation.

c) This antenna is somewhat directional, which can benefit you in two ways. You can either aim (rotate) the antenna for maximum signal strength, or for minimum noise pickup. I prefer to do the latter, and here’s why. This antenna has what is called a deep null on each side of the antenna, the broad sides, meaning that signals coming from that direction will be attenuated quite a bit (30 dB is an often-quoted figure). However, this is mostly true for signals we receive directly, like noise sources, and not so much for signals from broadcast stations coming to us through skywave propagation. I aim the antenna for minimum noise pickup, which results in the best signal to noise ratio. In some situations it is quite possible to fully tune out a noise source such as a TV or computer monitor.

d) Since this antenna is really a tuned circuit, it also acts as a preselector. It only receives well in a narrow bandwidth of a few hundred kilohertz (kHz). The antenna requires retuning if you change the frequency on the radio by a hundred to two hundred kHz. This may sound like a disadvantage, but if you have ever tried a long wire antenna on a rather sensitive receiver, you probably have noticed that your receiver may get overloaded, resulting in hearing multiple stations at once or hearing broadcast stations on frequencies where there really aren’t any. This may make it impossible for you to pull in that DX station you’re really interested in or even make listening to a strong broadcast station rather unpleasant. This antenna will help prevent overloading your receiver.

Thursday, July 3, 2014

Inverted L Antenna for 80m and 40m (and some other HF Bands from 80m to 10m)


Inverted L antenna
The basic layout of the Inverted L Antenna (Practical Wireless)

The first antenna that I installed was for HF. I decided on an Inverted L that incorporates a 7MHz trap so that it can be used on both 7MHz (40 metres) and 3.5 MHz (80 metres).
The design of this Inverted L is well known and a good design has been published previously in Practical Wireless by Len Paget GM0ONX. It is based on one half of the famous W3DZZ trapped dipole antenna.
It can be made entirely from scratch as a DIY project, or the 7MHz trap could be purchased commercially as a ready made item, or whole antenna can bought as a complete kit from Tony Nailer, G4CFY, at Spectrum Communications. I opted to buy the 7MHz trap from Spectrum Communications, as I already had most of the other materials required - rope, egg insulator, plastic box, and some good aerial wire. The Spectrum Communications trap is solid and well made and 'potted' to protect against the elements.
This antenna is tuned for 40 metres and 80 metres, but the VSWR is acceptable on several other bands being in the region of 2:1 to 5:1. The designer anticipated that this antenna would be usable on five of the H.F. bands between 80m and 10m.
I have found that with the use of the Antenna Tuning Unit it can be used on all of the H.F. bands. However the polar radiation pattern may very well be less predictable on bands other than the intended 40 and 80 metres, and it may well be less effective than might be desirable - but it does work!
The antenna is in the back garden, while the shack (radio room) is in a bedroom at the front of the house. It is fed by a 30 metre length of RG213 coaxial cable (it is not possible to use twin feeder for this type of antenna as the Inverted L is an UN-Balanced aerial, whereas twin feeder is balanced). With this length of cable I estimate the loss in the feeder alone to be about 1dB at 7MHz. The feed point of the aerial is located at the base of a 16 foot high wooden pole near the bottom of the garden. The horizontal top wire returns to a fibreglass pole installed at the apex of the roof.

Sunday, June 29, 2014

Wideband RF Field Strength Meter


Field strength meter is extremely useful when working with RF devices. It can be used to quickly diagnose whether a transmitter circuit is working, and can be used to detect RF signals in the environment. The simplest field strength meter could be built with a tuned LC circuit and a germanium diode, just like the way of a building a crystal radio except replacing the ear piece with a high sensitivity current meter. While this approach fits the needs of most simple applications, it has a pretty narrow frequency range (~100 MHz) and requires tuning the LC circuit to the correct frequency before measurements can be made and the design can become complicated if wider frequency range tuning is desired.

Wideband RF Field Strength Meter

Another option is to use an RF detection chip. Most of such chips (from Linear Technologies, Maxim and Analog Devices) offer a very broad testing range and have much higher sensitivity and accuracy than a simple diode signal detector can offer. Here I will use Linear Technologies’ LT5534 RF detector chip as the field strength meter’s front end. Similar circuits can be build with other RF detection chips as well, depending on the types of the specific application.

LT5534 can detect RF signal from 50MHz all the way up to 3GHz, which covers most of the spectrum one typically uses. If your frequency spectrum is significantly different, you may check out the other RF detection chips the above mentioned companies offer.

The core detector circuit is almost identical to the reference design. The LM324 op-amp forms a differential amplifier with a gain of 2. The main purpose of this differential amplifier is to provide the ability to “zero” the meter reading or adjust the sensitivity of the detector. Since the differential op-amp’s output is proportional to the voltage difference between the output of LT5534 and the wiper voltage of the potential meter, we can adjust the potential meter to set the reference point (i.e. zero reading) for the environment. Also, by raising the wiper’s potential, it would take a higher output from the RF detector for the differential op-amp to register an input voltage and thus effectively lowered the sensitivity of the detector.
The output bandwidth of LT5534 is tens of MHz, since we do not care about the signal details in this particular application, the relatively low bandwidth LM324 has no impact on performance. The above circuit uses a 5V regulated power supply.

Saturday, June 28, 2014

Homebrew Dummy Load for QRPers


Amateur Radio Antenna Projects and Information

Flower Pot Antenna -  - and interesting link to an interesting antenna design that was very kindly sent to me by Phil M6MRP
G4ILO - Stealth Antennas:
M0WYM - QRP Fan Dipole:
See Multi Band Dipoles Compared:
See Practical Dipole Antennas Compared:
The ALL Band HF Doublet on Ham Universe:
Multi-band Inverted V $4 Special by Joe Tyburczy, W1GFH:
The Norcal Doublet Antenna:
N4JTE - 6 Band Ribbon Dipole by N4JTE
W2BLC - Amateur Radio Antenna Ideas:

End Fed Half Wave Antenna


The End Fed Half Wave Antenna (EFHWA) is fed at a voltage node via a parallel resonant circuit against a ‘short counterpoise’, it is a favourite of backpackers and outdoor types.  It can be considered as a half wave dipole that’s end-fed at a voltage node rather than the current node, as is more usual. This is a very handy arrangement for portable QRP work.

End Fed Half Wave Antenna by AA5TB
End Fed Half Wave Antenna by AA5TB

I suspect that nothing new or radical has happened in the field of radio aerials in a VERY long time, like at least many tens of decades.  Most of the new wonder aerials are really a con.  Choke off the feed-line and then see how good they really are.  Prime among the baddies is the CFA.  It doesn’t really work, at least if you place a choke in the feed-line.  With any real aerial, there should be minimal radiation from the feed system… so a choke should really make no difference at all, but for the CFA it does!  The CFA is not alone, there are others.  The popular G5RV is another design with a radiating feed, deliberately so, but of course G5RV planned it that way.  He wasn’t cheating… merely being a bit devious, to make it multi-band

Lots of stuff to pass on to my fellow radio club members, most of whom are of the  ‘if it’s not expensive, it can’t be any good’ school of thought when it comes to aerials. Nothing of course could be further from the truth!  Aerials are one area where it makes a lot of sense to build our own." Website of GM1SXX -

Thanks for your email Allan. It's a good idea to point out that an antenna could be pressed into use on odd multiples of its resonant frequency, hence a 3.6MHz antenna for 80m could be useful near the 30 metre, 10.1MHz, band - near to the third harmonic of 3.5 MHz although, as you observe, the radiation pattern may be quite distorted from the traditionally expected dipole pattern and be more petal shaped. The same goes for a 7.1 MHz antenna for 40m being usable on its third harmonic of 21.3 MHz for the 15m band - a 40m dipole being three half waves an the 21 MHz band.

I have not experimented with a full size 80m dipole, but I would guess that it might be useful at 5 times 3.6Mhz in the 18 MHz / 17m band?
The point made about feeding a familiar dipole at the current node rather than the voltage node is obviously very important and, I imagine, sometimes overlooked.
PLANS: Download the pdf plans produced by G0KYA here > More from G0KYA here:

Friday, June 27, 2014

Inverted V Aerials

Another option maybe to support the dipole at its centre on a tall pole, or roof apex, with each end sloping downwards to lower fixing points. This will give the aerial an upside down V shape. As with a sloper, the Inverted V arrangement will give the aerial some directivity - a different radiation pattern compared to a straight horizontal dipole.

Spectrum Communications G4CFY Trapped Dipole in Inverted V configuration

Spectrum Communications G4CFY Trapped Dipole in "Inverted V" configuration

Using an Inverted V can help fit a dipole into a slightly restricted space. The Inverted V arrangement can be used for single band resonant dipoles, trapped dipoles and fan dipoles.

Inverted V (ref

At A, details for an inverted V fed with open-wire line for multi-band HF operation. A Transmatch is shown at B, suitable for matching the antenna to the transmitter over a wide frequency range. The included angle between the two legs should be greater than 90° for best performance. [ref:]

W3DZZ antenna by the Maidstone Amateur Radio Society


W3DZZ antenna by the Maidstone Amateur Radio Society that adds a dedicated 10 meter (28MHz) resonant element as a 'fan'.

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society

Moonraker supply a whole range of wire trap dipoles covering from 2 to 5 HF bands (MTD1; MTD2; MTD3; MTD4; MTD5; MTD6). Diamond also produce trapped wire antennas, the W-721, W-728 and W735. Comet and Diamond each produce similar interesting 5 band wire dipoles that utilize both traps and a fan arrangement - the Diamond W8010 and the Comet CWA-1000. If space really is limited then look out for KZJ Communications (dongo1950 on ebay) - he produces 'Limited Space Inductive Dipoles'. These are inductively loaded and shortened dipoles so they will have reduced efficiency, of course, but are very nicely made, so might be very useful in a tight spot.

To obtain good efficiency and achieve a low angle of radiation, desirable for longer distance DX, a horizontal dipole needs to be installed at a good height - over 20 feet would be desirable and it is quite common to install horizontal dipoles at around 30 to 40 feet above ground level. This might be a problem at some QTH's, it certainly is at mine!

Allan Copland, GM1SXX comments: "The dipole will operate well on the band it has been sized for , if placed at a suitable height, but will also operate as a’ three-half-wave’ aerial at three times the frequency and so on, so it’s not strictly a single band aerial.  An 80M dipole (132 feet typical) will work nicely on 30 metres  (three half waves) but not on 40m (two half waves)… because on 40M the feed-point  is at a voltage node and not at a current node, for easy feeding.  Most aerials are current fed.

The radiation pattern changes when a dipole is not used on its design frequency. The pattern will break up into multiple ‘petals’. This can be either a disadvantage or an advantage depending on what you expect from it.  Since most of us use co-ax, an UN-BAL  should really be used to connect the unbalanced feeder to the balanced aerial, but how many people actually bother? Not many I suspect.  It’s possible of course to use a balanced feed-line  system instead with a dipole and just have a delta match (no centre insulator… none needed).  There are many choices and permutations, but in general, dipoles are centre fed at a point of current maximum (and minimum voltage).

A normal dipole is current fed but of course can be voltage fed instead. This is what’s done in the EFHWA or Fuchs aerial where a resonant half wave wire is fed at one end (max voltage / min current) from an L/C tank, against a very short counterpoise wire.

Adding 160m / Top Band to an Inverted L


The 160 metre Top Band can be added to this aerial by connecting a 3.5 MHz trap at the end of the 80 metre wire (where to monofilament joins the 6.55m section of wire below) with another length of wire on the other side, increasing the overall length of the antenna.
Find out how to do it here:

Adding Top Band to and Inverted L Antenna

Adding Top Band to an Inverted L by Len Paget GM0ONX (Practical Wireless magazine)

Inverted L - 80 metres to 10 metres


A typical Inverted L antenna will be trapped for 40m/80m using a 7.1 MHz trap. It is essentially one half of a W3DZZ dipole so can be accommodated very much more easily into a small plot or garden - especially as part of the antenna is running vertically up a wooden or fibreglass (non conductive) pole. This should allow it to be fitted into quite a small garden such as mine.

The Inverted L is also a very effective aerial because it has the benefit of both vertical and horizontal radiation. While Inverted L's might make good TX aerials, like ground mounted vertical aerials they can be quite noisy on RX.

The Inverted L is extremely easy to 'home brew'. Spectrum Communications can also supply the complete aerial as shown below. It should give excellent performance on 80m and 40 metres, with 20 metres also being good but allowing use on 15m and 10m and possibly one or two of the WARC bands:

Spectrum Communications 40m / 80m Trapped Inverted L

Small Loop for 20 metres to 10 metres:

A loop for 20 metres or 17 meters is relatively compact and could easily be installed in small 'postage stamp' sized gardens. A loop antenna could be triangular, square (Quad) or circular, but a square loop (and indeed a circular loop) would need more supporting points than a delta (triangular) loop, so a Delta loop is likely to be the easier option.

The loop is really a single band antenna cut for one wavelength on the band of interest, however it can also work quite well as a cheap and easy to install multi-band H.F. aerial. A loop consisting of a 17 metre length of thin antenna wire, for example, will work well on 17 metres but may also give 15m, 12m and 10m with an ATU. My own loop is made from an 16 metre length of wire, tuned for the 17m band, but can work on higher bands. A 40 metre loop will be considerably larger, but it might still possible to accommodate in many fairly compact gardens. Performance will depend on height and orientation.

Feeding the loop at the top or bottom will give horizontal polarisation, while placing the feed point on the side will give vertical polarisation. The apex can be at the top or the bottom, but performance should be better with the apex at the bottom with the flat wire across the top  - however for ease it may be more convenient to support a Delta Loop on a single pole, meaning that the apex would be at the top.

Ideally a loop should be fed with balanced line back to the shack, connected to a balanced line ATU or other ATU via a 4:1 balun. Alternatively use a 4:1 balun at the antenna end and run 50 ohm coax back to the ATU / txvr - though losses will be greater doing it by this method if the coaxial cable is quite long.

If one can install a separate antenna for the lower frequency bands of say 160m, 80m and 40m, then a Loop Antenna could be a good partner to allow operation on the higher bands of 20 metres to 10 meters or even 6 metres.

A loop should be really very easy to install using a single support pole and very cheap too! All that's needed is the supporting pole, some cheap wire, a 4:1 balun which can be 'home brewed' and some thin cord and insulators which should not be an eyesore either.

Multi Band Delta Loop using 4:1 balun at feedpoint

Diagram from the excellent article by W5SDC

Wednesday, June 25, 2014

The Moxon-Beam


The Moxon-Beam was introduced by L. Moxon (G6XN) in his book "HF Antennas for all Locations" (RSGB- Publications, Great Britain 1993). This beam is a 2-Element-Yagi with radiator and reflector and reduced size to about 75% of a normal beam. The 2-Element-Yagi with reflector has normally a 0,2-lambda-boom and an impedance of 50 W. The Moxon-beam has a 0,18-lambda-boom and still 50 Ohm. This is a good impedance for wire- beams.The ends of the two elements are bended backward (radiator) or forward (reflector) and act as a capacitive load. That is much better than inductive loading with coils. So we have greater bandwidth and lower losses.Through the reduced size we get a 0,5-0,7 dB lower gain than with a fullsize beam.

This type of a 2-Element-Yagi has an unbelievable F/B-ratio on the design frequency of >= 30 dB. That is higher than with any other 2-Element-Beam.

The gain is higher on the beginning of the band and lower at the end. The bandwidth for a SWR < 1,5 is great enough for the range of 28,0-28,7 and 21,0-21,45 MHz if the beam is built up with aluminium tubes. Wire-beams of the Moxon-type have a smaller bandwidth.

The design frequency should be for a frequency 1/3 from the beginning of the band, because the SWR raises more below the design frequency. For example look for the SWR of a tube-Moxon for the 15-m-band:

Designing a Moxon- Beam is very easy with a useful little program by D. Maguire, AC6LA with the name "Moxgen".

This freeware can be downloaded at:

Moxgen generates an output file for "EZNEC" for modifications (e.g tapering) and shows you the dimensions for  building a wire-Moxon.



This ATU was designed in about 1990 to allow QRP rigs to be used with portable antennas. The unit had to be small and light weight. Generally I am against using ATU's at all regarding them as a cop-out for poor antenna design. However, when portable operation is considered, it is not always possible to erect the ideal antenna. This ATU was designed with idea of achieving a better match to an antenna that was nearly right.

A later version was tried which had an additional coil switched in and also a balun to accomodate twin feeder. I was never happy with this version, as it didn't seem to work as well as the simple version. When it was deconstructed in 2007 to do some loss measurements, it was discovered that a constructional error during the mod may account for this.



Hi to all! Today I found a very interesting project that using a small whip for hear the long VLF waves. Adrian Knot says that with this simple and inexpensive system you have good results, but suggests the using outdoor far from electric sources.
References: (

Tuesday, June 24, 2014

Putting up a Long-Wire Antenna


Fig. 2 Longwire antenna

Fig. 2 Longwire antenna


I've actually used this kind of antenna. When I was a teenager, I used it with my old Realistic stereo receiver to improve its AM reception, and it did work. (My dad put it up for me.)

Note: RadioShack sells this antenna as a kit, catalog # 278-758, $9.99 in their 2000 Catalog. You might also find it from other electronics suppliers. They sell the antenna wire as catalog # 278-1329, $6.99 in the same catalog. The insulators can be ordered for shipment to your house through RadioShack RSU catalog # 12099248, $4.79 + shipping in the 2000 Catalog.

Find two points, such as two poles (NOT utility poles!), or as most people will do, the corner of your house and a tree. Any safe places above the ground from which you can string this wire will do. Remember that not just the mounting places, but the "air-space" through which the wire will be strung must be free of obstructions, such as trees, and especially power lines. Whatever you do, don't put it on the same corner of the house through which your electrical service enters! First, that will be dangerous, and second, you'll pick up power line noise.

This antenna picks up signals best at right angles. It picks up worst end-on. Therefore, remember to orient the wire to pick up from the direction in which you are most interested.

Using an eye-hook at each point, attach a length of nylon rope. Tie the other end of the nylon rope to each insulator. The bare copper wire is strung from between the insulators, as shown in the diagram.

Use an insulated wire to connect to the copper wire at the end closest to your radio. Strip a couple inches of it and wrap it around the bare copper wire. For best results, solder this connection. This is shown in Figure 2. 

If you string the wire from a stationary point (such as a house) to a tree, use this trick that my father taught me, and that Ham Radio operators use. You see, the tree will sway in the breeze, stretching the wire over and over, eventually breaking it. So on the tree, instead of using a plain eye-hook, put up a pully (such as a clothsline pully). Run the nylon rope through this pully, letting a few feet hang down, and tie a brick (or suitable heavy weight) to the end of the rope. This is what I have illustrated in the first diagram. As the tree sways in the breeze, the rope moves feely through the pully, and the weight of the brick keeps the whole thing taut.

Grounding: From the ground terminal on your radio (next to the antenna terminal) run a wire to either a copper cold-water pipe (PVC pipe won't work) or a copper ground rod driven into the ground.

Some safety notes:

1. Keep your antenna far away from power lines! If you, your antenna or your ladder touches a power line, you could be KILLED.

2. Remember that you will be climbing ladders or trees to high place. Observe every safety precaution. Never extend the ladder beyond the manufacturer's reccomendation. Never stretch to reach a point-- instead, move the ladder. Have the ladder on a secure footing, and have someone there to hold the ladder.

3. Make sure your antenna is in a safe place, high enough so people or pets won't walk into it by accident. (Plus, the higher up you get it, the better it works).

4. Use lightening protection! Make provisions to disconnect and ground the antenna when not in use. C. Crane Company and RadioShack offer lightening protection devices. However, disconnecting and grounding the antenna is still the best option. RadioShack sells a simple knife switch you can use for this. Screw it to the wall or table next to the radio, hook one side to the antanna, the other to a wire leading to a copper grounding rod, or the copper cold-water pipe leading outside of your house. (Remember this doesn't work if you forget to throw the switch!)

Long Loopstick Antenna


Wound on a 3 foot length of PVC pipe, the long loopstick antenna was an experiment to try to improve AM radio reception without using a long wire or ground. It works fairly well and greatly improved reception of a weak station 130 miles away. A longer rod antenna will probably work better if space allows. The number of turns of wire needed for the loopstick can be worked out from the single layer, air core inductance formula:
Inductance = (radius^2 * turns^2) / ((9*radius)+(10*length))

where dimensions are in inches and inductance is in microhenrys. The inductance should be about 230 microhenrys to operate with a standard AM radio tuning capacitor (33-330 pF). The 3 foot PVC pipe is wound with approximately 500 evenly spaced turns of #24 copper wire which forms an inductor of about 170 microhenrys, but I ended up with a little more (213uH) because the winding spacing wasn't exactly even. A secondary coil of about 50 turns is wound along the length of the pipe on top of the primary and then connected to 4 turns of wire wound directly around the radio. The windings around the radio are orientated so that the radio's internal antenna rod passes through the external windings. A better method of coupling would be to wind a few turns directly around the internal rod antenna inside the radio itself, but you would have to open the radio to do that. In operation, the antenna should be horizontal to the ground and at right angles to the direction of the radio station of interest. Tune the radio to a weak station so you can hear a definite amount of noise, and then tune the antenna capacitor and rotate the antenna for the best response. The antenna should also be located away from lamp dimmers, computer monitors and other devices that cause electrical interference.

Magnetic loop antenna for 7-21 mhz

  • Magnetic Loop Diagram

    Magnetic Loop Diagram

  • Magnetic Loop antenna

    Magnetic Loop antenna

This antenna has several advantages, not least being only 1 metre diameter! This loop relies on being horizontally polarised and receives only the magnetic wave, thus as most noise in the domestic environment is vertically polarised and electrical wave, it delivers low noise to your transeiver/receiver, which makes for nice clean listening. In addition any signal arriving in the direction of the loop end on will be nulled out, this can be useful to get rid of an interfering signal by simply rotating the loop leaving the desired signal in the clear. It can be used indoors with ease and works well at ground level which is not the case for long wire/dipole antennas at shortwave wavelengths.

So what are its disadvantages? Well its tuning is critical, such that for a small change in frequency the antenna will need to be retuned at the loop end. This is even more important for transmitting where a high reflected wave (swr) due to not being tuned correctly will damage the output stage of your transmitter! In addition due to the very high "Q" of the loop, very high voltages can build up on the loop tuning capacitor even with low amounts of power from your transmitter. It is for this reason I recommend this loop is used with a transmitter of no more than 8 watts, any more and the ordinary broadcast tuning cap will arc over with spectacular results. Of course should you wish, a higher spec/bigger air spaced tuning cap would allow higher power output transmitters to be used. Also I consider the use of remote tuning using a fairly high geared motor and insulated coupling on the tuning cap essential. For shortwave listeners manual tuning would suffice.

In setting up the tuning of the loop, connect to a receiver and tune to 14 mhz. Now tune the loop which as it nears peak tuning will cause a whooshing sound. Stop the tuning you should now hear good strength signals in your receiver. For tuning for a transmitter, 1st use receive method then apply low power and fine tune loop tuning and tweak gamma match for lowest swr.

Magnetic loop dimension details

  • Diameter of loop 1000mm
  • Diameter of tube 15mm
  • Width of base 780mm
  • Diameter of support pipe 42mm
  • Loop end spacing (for tuner) 50mm
  • Height of support 1590mm
  • Nylon board 210x240mm
  • Nylon board 240x70mm
  • Gamma match width 310mm
  • Gamma/loop spacing 110mm

Construction Tips

  • Use a bicycle wheel with no tyre on to help form the curves of the soft annealed copper tube
  • Clean the tube with wire wool before any soldering
  • Use a 100 watt soldering gun for the joints, but use a small blow torch first to get the copper at temperature to take a joint
  • Force some timber with the corners planed off down the plastic plumbing pipe this will stiffen the pipe as the loop is quite weighty
  • Use inverted shelf brackets to support the mounting pipe and make a wooden frame wide enough to hold up the loop

Friday, June 20, 2014

QRP Fan Dipole


Fan Dipole Circuit The object of the exercise was to produce an aerial that would allow me to operate from 40 metres to 10 metres, specifically 40, 20, 17, 15 & 10 metres. The antenna was always going to be mounted in the attic as no external antennas are permitted at my QTH, the attic allows the antenna to 'beam' roughly northwest / southeast and the house is some 40 feet above sea level. Construction would be simplified by the fact that I intended to run a maximum of 10 watts which means that the antenna wires can be simply attached to the rafters. (Click on images for a larger version.)

Vertical antenna



This type of antenna exhibits an omni-directional pattern, with a low radiation angle. The length of the radiator is calculated by using the formula 234/Frequency in Mhz, for feet, or 71.5/Frequency in Mhz, for meters to make a 1/4 wavelength, at the desired frequency. The radiator can be made entirely from 1" aluminum tubing, but can also be made from several sections of tubing of different sizes (Below 20M it is not appropriate to use 1" tubing). These could be fastened together using pipe clamps, after splitting the lower section about 1", across the circumference, along the diameter to facilitate clamping of the upper section.

The radiator is mounted on a reasonable length square wooden post which is buried in the ground or fastened to the roof. Large diameter pipe clamps are used to fasten the radiator to the wooden pole. At about 1/2" from the base of the radiator a hole 1/8" should be drilled into the aluminum pipe. This hole is used for a bolt onto which the coax center conductor is fastened.

The radials should be slightly longer than the radiator. To facilitate multi-band operation at least four radials should be used for each band you wish to operate in, the radiator being cut for the lowest band. All the radials are then fastened, or soldered together, and a connection is made to the coax cable shield. The radials should be buried a few inches into the ground, and ideally spread out in a circle. An antenna matching unit is used for multi-band operation. Alternatively, in place of aluminum tubing, a PVC pipe could be used, with a 1/4 wavelength wire positioned inside as the radiator. Note: For this antenna the Antenna Matching Unit should be at the base of the antenna, for best performance, and can be remotely controlled.

A Tree Friendly 2 Meter Halo Antenna


Having purchased an all-mode, all-band (160m - 70cm) transceiver, I became curious about what 2-meter weak signal operations have to offer. I have a 5/8th over 5/8th vertical collinear antenna hanging in a tree at some 30 odd feet high, but I never heard anything on it, except on FM. The reason for that, I learned, is most 2-meter weak signal operations take place using horizontal polarization. Cross polarization is good for about 20 dB attenuation, which easily translates into the difference between perfectly good copy and inaudible signals. So I decided I needed a horizontal polarized antenna.

As is usually the case with antennas, there are a bazillion designs to choose from and none of them really fulfills all your requirements. I do not have a mast or tower, and I love to use trees for supports, so I wanted something that I could hang from a tree branch. Since I have no means to rotate the antenna, I required that the new antenna have an omnidirectional radiation pattern. It didn't have to be the best performer, because I just wanted to get my feet wet in this new mode of operation. There are few designs that would fit that bill. I settled on the Halo antenna because of its small footprint. This is important because larger designs would require a longer branch, with sufficient clearance in all directions, to hang from. The Halo I describe here has a diameter of only about 12 inches and can be hung virtually anywhere in a tree.

The Halo Antenna

Halo stands for "HAlf wave LOop". The antenna is in fact nothing else but a half wavelength dipole with the legs bent in the shape of a circle. However, the ends do not meet, (especially near the end of the month) so technically it's not a loop. This loop can be fed with coaxial cable using a gamma match.

The Halo is certainly not a new design. Laurence M. Leeds and Marvel W. Scheldorf obtained a patent for this antenna in 1943. You can find their design at the U.S. Patent Office under Patent Number 2324462. Click on the "Images"-button to view the patent. You'll need a special browser plugin to access the patent. See the U.S. Patent Office website for more information on this.

Most Halo designs you find on the internet have moving parts. Often they require some sort of tuning capacitor and have a capacitor in the gamma match along with a slider construction that connects the gamma arm to the radiator. I prefer a design without moving parts so that the antenna doesn't get detuned easily when a bird decides the antenna makes a good resting place. I found the design that I describe here in a German antenna book "Antennen Buch" by Karl Rothammel, Y21BK.

Basic Design

The design of this antenna is very simple and straightforward. It basically consists of a half wavelength piece of copper tubing bent into a circle. Between the ends of the tube there needs to be a gap of at least 1 3/16". This is to minimize capacitive coupling between the ends. This antenna is fed by a coax feed line through a gamma match. The gamma match is constructed from 6 1/4" #4 or #6 copper wire. This wire is bent into an L shape. The short end of the wire is soldered on the inside of the loop at the point where the long end of the gamma arm aligns with the halfway point of the loop. See below:


You could feed the loop directly with 50 ohm coaxial cable as shown above. However, I added a 1:1 current balun (choke) to the original design. I did this to force all the RF current, on the inside of the braid of the coax feed line, to go into the antenna and not to come back down on the feed line on the outside of the coax braid. This will help keep the feed line from radiating, causing potential RFI problems and changing the radiation pattern of the antenna.

Building the Halo

Building this antenna is like making the pieces of the puzzle first, and then putting the puzzle together. First you build the antenna itself, then the support boom, the choke balun, the mount, and finally, you put these parts together.

The Antenna

Start out by cutting a string to a length of 41 inches. You'll use this to measure the correct tube length. Thick monofilament wire as used for garden trimmers works very well. Mark this wire at the halfway point. A piece of electrical tape will do fine. This is the point where you’ll later have to mark the copper tube and where the coax braid will get soldered to the tube.

You'll probably find the soft copper tubing material in a 10-foot length, coiled up in a bag. Fortunately, the coil diameter is about the same diameter as the final loop will be. So there will be very little bending involved to get the circle shape needed for the loop. Eyeball how much tube you'll need from the loop coil and cut it. Don't attempt to make an exact measurement yet. In fact, the measurement I give here for the main loop is deliberately somewhat too large anyhow. Put the part you cut off on a flat surface and now measure how much you really need using the string from the previous paragraph. Cut off the excess tubing. Use the string again to find the halfway point of the tubing and mark it. This will be the point at which you will later solder the coax braid. Make sure the tubing is shaped like a circle, and that the ends are at least 1 3/16" apart. To keep the ends apart, I cut a piece of hexagonal Bic pen tubing to length and put it between the ends. You can secure it with some shrink tubing, but don't shrink it until you're done with the final tuning later on.


Now build the gamma from a piece of #6 or #4 copper wire. Use the measurements from the detail diagram above and bend it as shown. Solder the short end of the gamma arm to the inside of the loop at the point where the long end of the gamma arm lines up with the halfway point on the main loop (as marked earlier).

The Support Boom

To support the antenna, I used a piece of 3/4" schedule 40 PVC pipe. Lay the antenna on the pipe and cut it so it's just a bit longer than the diameter of the loop. Now drill holes through the pipe to mount the antenna. On one end of the pipe you need two holes approximately 1 15/16" apart for the main loop and the gamma match to go through. Make sure you drill the hole for the tube very close to the end of the PVC pipe. This will make it easier to solder the coax braid onto the copper tube. Also be careful not to drill the hole for the gamma match all the way through the pipe. The gamma match only goes in halfway through the pipe and will not come out the other side.

On the other side of the PVC pipe, drill a hole in the same plane as the first two holes for the piece of plastic tubing (hexagonal Bic pen) that is being used to keep the ends of the main loop apart. Next, drill the holes necessary to mount the SO-239 (panel mount) connector.

The last hole you need to drill is for mounting purposes. This hole needs to be drilled in the middle of the PVC pipe, all the way through. Make absolutely sure that this hole is perpendicular to the plane of the antenna. This hole needs to be the size of a bamboo skewer you can buy at the grocery store (sold in a bag of 20 or so). This skewer is later used to mount the antenna. You can use something else if you like as long as it's thin, strong, sturdy and straight.


The Balun

Even though the original design does not call for a balun, I decided to add a 1:1 current balun in order to prevent RF currents from flowing back onto the outside of the coax braid, perhaps causing the feed line to radiate, create interference and change the radiation pattern of the antenna. The balun I built for this antenna consists of 12, 1/2" long, type 43 ferrite beads that slide over a short piece of RG-58 coax with the outer jacket removed. The hole in the beads I used was not big enough for the coax to slide through with the outer jacket intact.

The length of the piece of coax needed for the choke can be measured from the end of the boom where the gamma match is to the farthest edge of the SO-239 connector, plus about 3/4". The balun itself is about 6 inches long. If you use a different size of beads, just make sure you have enough beads to make a 6-inch long balun. You can secure the beads with some shrink tubing or electrical tape. This should be enough to make an effective balun for VHF that can handle up to 100 Watts of power.

One side of the coax should be prepared so you can solder it to the SO-239 connector. You can already solder the connector to the coax if you wish. The other side needs to be prepared so that the braid will reach the middle of the loop, and the center conductor meets the gamma match. Do not cut the center conductor to length. Instead, only remove the insulation from the center conductor so that it can be soldered later to the gamma match. Leave the excess wire intact. Put some shrink tubing or electrical tape around the braid to insulate it, except for the very end, of course.

If you do not have any ferrite beads available, you can construct an air core choke balun instead. There are several ways to make one. One method is to wind about 7 turns of the feed line on a coil form made from 3/4" PVC pipe. Place this choke near the antenna. If you prefer the choke to be part of the antenna, then you can wind 7 turns of the coax going from the SO-239 connector to the gamma match around the PVC boom.

The Mount

Since I've chosen to hang this halo from a tree branch, I needed to find a way to mount this antenna onto a rope coming down from a branch in such a way that the antenna itself will remain in the horizontal plane. I came up with a method that will use gravity to hold the antenna perfectly horizontal.

When you built the PVC boom, you drilled a hole in the middle for a skewer. Now imagine you put the skewer through the boom, put the support rope alongside of the skewer and then tie the rope to the skewer with some wire ties. If you'd hold just the rope above the skewer and boom, the boom would just dangle in all kinds of directions and stay far from being horizontal. However, if you'd make a small loop in the bottom end of the rope and hang a weight from it, you'll see that the boom stays perfectly horizontal. See the diagram below.


Putting It All Together

Take a piece of pen tubing, or whatever you chose to fill the gap in the main loop, and push it in the PVC boom.

Next you can join the antenna with the boom. Take one end of the antenna and guide it through the hole next to the hole for the gamma match in the PVC boom. Slowly move the tubing through the PVC support pipe. You'll notice some resistance because the loop is round and the holes through the pipe are in a straight line. This causes some friction. With a small amount of force you'll see that that the tube will go through the pipe quite easily. Stop when you reach the gamma match. Do not push the gamma match into its hole in the boom yet.

Bend the end of the braid of the coax that goes inside the boom at a 90-degree angle. Just over 1/4" from the end should be sufficient. This will make it easier later on when you solder the braid to the tube. Also bend the exposed center conductor at a 90-degree angle about 1/4" from the end.

Slide the coax inside the PVC boom through the hole for the SO-239 connector. While you do this make sure that the end of the center conductor kind of scrapes on the inside wall of the pipe, on the side where the hole for the gamma match is located. At some point you'll notice that the wire will get caught in this hole. Pull the wire through the hole with needle nose pliers while you continue to slide the coax inside the pipe. Stop when the insulation around the center conductor appears at or through the hole. If you bend the wire a bit at this point, it will stay in place.

Solder the center conductor to the gamma arm. You can cut off the excess wire, but I simply bent the remaining wire along the gamma match. Or, wind it around the gamma match, just in case you have to do it again. Now slowly push the gamma arm in the hole of the PVC boom. If it doesn't quite fit, you can cut a tiny wedge out of the hole where the center conductor passes through the hole. Stop when the gamma match reaches the middle of the pipe and the center marking on the copper tubing is in the middle of the PVC pipe.

When you look into the pipe you'll see the braid near the copper tube on the inside of the pipe. Pre-tin the copper tubing at the halfway mark. Make sure this mark is in the middle of the pipe. Now you can solder the braid to the tube.


If you haven't already soldered the SO-239 connector to the coax, then do so now. Push the remaining coax into the boom and fasten the SO-239 connector to the boom.

Slide a piece of shrink tubing over each end of the antenna. Mate each end of the copper tube with the piece of pen tubing protruding from the support boom and slide the shrink tubing back a bit so it covers the piece of plastic tubing also. Do not heat the shrink tubing yet. This will hold the ends in place. You can also use electrical tape to do this. Just make sure the ends of the copper tubing stay flush with the plastic tube.


Mounting the antenna

The antenna is now finished and we're ready to mount it for testing and tuning. Find a place where you can hang the support rope, like a tree branch. Make sure that there are no metal objects nearby, as they will detune the antenna. You can tune the antenna at a different place than the final destination if you wish.

Put a wire tie just below the middle of the skewer. This will prevent the antenna from sliding down the skewer. Secure the bottom half of the skewer onto the rope with two or three small wire ties. Make sure the skewer cannot slide along the rope. Drop the bottom end of the rope through the loop of the antenna and push the skewer all the way through the support boom from the bottom up. You may have to wiggle it a little bit when you're halfway into the pipe in order to get past the coax inside. Once the skewer is all the way through the support boom and the boom is resting on the wire tie on the middle of the skewer, you can fasten the top half of the skewer to the rope with two or three wire ties.


Your antenna can now dangle freely from the rope. You'll see that the antenna does not really stay horizontal yet.

Make a small loop at the bottom end of the support rope and hang a weight from it. I used a brick. Even if the antenna is going to be mounted very high up in a tree, try to keep the weight near ground level. This serves two purposes. First, if for some reason the weight would fall, it will not fall on top of someone and no one will run into it. Second, it will keep the whole system very stable in the wind. If the weight would be higher than just a few feet of the ground, the wind would catch it also and start swaying the weight. Not only is this dangerous, it will also take a long time before the system is stable again once the wind drops.

Of course you could opt not to use a weight and simply tie the bottom of the rope down. However, that makes the whole mechanical system very inflexible. The rope would move back and forth over the branch when it's swaying in the wind, and eventually the branch might be cut.

If you later find that the rope and weight are swaying too much, you can minimize this with a guide rope that is tied to, say, a post, or the trunk of the tree, and goes around the support rope. The guide rope will then allow the support rope to move mainly in the vertical direction.

Once finished, the antenna should look something like this:


Testing and Tuning

Before you test the antenna, double check you made all the right connections. When you use an ohmmeter to check the connections, you should be measuring a short (zero ohm resistance) between the center and outer connections of the SO-239 connector. This antenna is what is called "DC grounded", which may help reducing static buildup on the antenna.

Now you can attach a 50-ohm coaxial feed line to the antenna to test and tune it. Use a wire tie to attach the feed line to the skewer. This will make the feed line run along the support rope and help stabilize the system. If you plan on weatherproofing the antenna, then read the part on weatherproofing below first before you tune the antenna.

I borrowed an antenna analyzer to tune the antenna, but you can also do it with just an SWR meter. When you use an SWR meter and cannot find a near 1:1 SWR anywhere in the 2 meter band, you need to make note of three SWR measurements. One at 144 MHz, one at 146 MHz and one at 148 MHz. If you find that the SWR is lower at 144 than at 146 and 148 MHz, then you know the antenna is tuned below the 2-meter band. If you find that the SWR is lower at 148 than at 146 and 144 MHz, then you know the antenna is tuned above the 2-meter band.

You will probably find that the antenna is tuned somewhat below the 2 meter band. I deliberately listed the measurement of the main loop slightly too large which results in a lower than desired resonance frequency. Cut a tiny bit of each end of the copper loop until your antenna resonates near 144.200 MHz. That is the SSB calling frequency in the U.S.A. Of course you'll have to decide at which frequency you want the antenna to be resonant. The 1:1.5 SWR bandwidth of the antenna is about 1 MHz.

Since the gamma match is fixed, you will probably not be able to get an exact 1:1 SWR reading. More realistic is 1:1.1 to 1:1.3. Don’t let this scare you. SWR readings other than 1:1 are perfectly fine as long as they do not go much higher than 1:1.5. At that point many rigs will throttle back the power. If you really cannot sleep peacefully if the SWR isn't perfectly flat, then by all means desolder the gamma match from the loop and solder it at a different point until you've found that serene 1:1 SWR spot. I will warn you though that things can get messy real quick while it really isn't worth the effort.

If you find yourself in the position where you cut too much of the antenna ends in order to find that elusive 1:1 SWR, don't panic! There's an easy solution. Solder a half inch or so piece of solid copper wire (#14 will do) to the inside of each tube end as shown in second diagram on this page. The wire ends protruding from the copper tubing will fit inside the plastic separator tube. This will help maintain a clean look of the antenna while you get a second chance at tuning the antenna. Now simply cut small pieces, like 1/16" or so each time, off of each wire until the antenna resonates at the desired frequency.

When the antenna is tuned you can heat up the shrink tubing around the copper tube ends so that everything remains in place.

Weather Proofing

You can weather proof the antenna by filling all holes and gaps with RTV, or silicone sealant. Care should be taken when you want to seal the end of the boom where the ends of the loop meet. Putting RTV in that side of the pipe will detune the antenna. Make sure the sealant only goes to the inside of the PVC tube, and don't be tempted to put any sealant on the loop ends. The reason for this is that there is some stray capacitance between the loop ends. By adding sealant you change the dielectric between the tube ends, and therefore the value of the stray capacitance. This in turn changes the resonant frequency of the antenna. So it is better to seal this end of the support boom first before tuning the antenna if you plan on weather proofing the antenna.

If you feel that you need to seal the area around the small piece of spreader tubing, then use something like a very thin layer of nail polish. Also, if you want to make the loop very shiny, use some very fine steel wool to polish it.


I described how you can build a Halo antenna for two meters that does not require a mast, has a very low part count and can easily be built with a minimum of tools. This project description may seem more complex than similar ones you can find on the internet, but that is simply because most other plans leave out a great deal of detail, especially in the area of construction. I like to include the lessons I learned along the way when I built the antenna.

This article also described a unique way to mount these types of antennas on a rope. This makes the antenna an attractive alternative for use in the field where the usual support structures may not be available or for those folks who, like me, do not have a tower or mast. Of course you can mount the antenna on a mast with a U-bolt if you wish.

I have built a 2 and 6 meter version of this antenna, mounted them on the same support rope and feed them with separate feed lines.


The 2 meter Halo is mounted at 24 feet, and the 6 meter Halo is mounted at 20 feet high. As you can see, the Halo makes for a stealthy antenna even though that was never one of the design goals. If you really want the antenna to blend in with the background, paint it light gray or light green, and add some random black strokes here and there. When you break up any symmetrical lines and patterns, any object can be made invisible against the background.

These Halo antennas allow me to dabble a bit in VHF weak signal operations given the restrictions mentioned in the beginning of this article. While the performance of this type of antenna is limited compared to other types of antennas, I'm rather surprised with the DX contacts I’ve been able to make with a modest 50 Watts of power.

If you have any questions or suggestions, please do not hesitate to drop me an email.

--Alex, KR1ST




    The dipole is one of the most important and popular antennas, it forms the basis of many other different types of aerial which are directional and have gain in certain directions.

    The dipole in its basic form consists of two identical lengths of wire with the feeder connected in the middle as in the diagram above. It can be any number of electrical wavelengths long. The most popular is the half wave dipole, normally thought of as a single band aerial.  Although it has little gain, it can be easy and cheap to construct, and they often prove to be the ideal solution with amateur radio operators who are only interested in one particular band .



    The wire dipole is normally mounted horizontal, vertical or sloping between two supports, but it can be erected in many different shapes such as the inverted-V or a Z shape. On the lower HF bands the the lengths become rather long so it is sometimes necessary to bend the antenna to fit the average garden.


    The formula to calculate a half-wave dipole (in feet) on any HF frequency is 468/frequency in MHz. This is total wire length tip to tip, or 234/frequency in MHz for each element length. For example = If you want the antenna for 40 meters, choose the centre part of the band or centre part of the frequencies you want to work, 468/7.05 MHZ = 66.3 feet. It is better to make the wire slightly longer and then trim the wire to resonance. Make sure you trim the same length of each end.

    In free space the feed point impedance is 78W so it is a good match to 75W coax. However when erected in the garden or over the house the impedance changes and it is acceptable to use 50W coax cable.

    Although it is not essential, it is best to use a 1:1balun. This is connected between the two elements and the feeder cable. This ensures the correct operation of the antenna. It does not provide any impedance match, but it will balance the load by causing equal but opposite phase currents to flow in the conductors reducing radiation on the transmission line.



  • The end fed wire has to be one of the best cost effective and simplest multi band antennas. They are easy to erect and can cost almost nothing if you have enough wire handy. Almost any type of wire can be used, any medium gauge multi stranded pvc coated wire or single strand enameled copper wire. (I use 16 gauge pre- stretched enameled copper wire). This antenna will suit most houses and gardens as it can be horizontal, part horizontal, vertical or sloping to fit your available space.

    Any length of wire can be used, but the length will determine which bands you can use. For shortwave listeners the lengths are not critical but for transmitting the chart below will give a good starting point.



    40,30,17,15, and 12m

    80,40,20 and 12m

    All bands (tuning may be difficult on 10m)

    The wire lengths above may need some alterations due to the geometry of your QTH, height and nearby buildings.

    There are some drawbacks with this type of antenna, it is unbalanced and in some cases may rise to TVI problems in built up areas. It also requires the use of an ATU (antenna tuning unit) and you will also need a good earth connection. As you can see from the diagram the ground wire is connected to the the earth terminal of the ATU,  the other end is connected to a copper earth rod driven into the ground.  The longer the rod the better but the ground wire should be kept as short as possible. Use a thick heavy gauge wire such as the type used for car battery leads or the braid from a heavy gauge coaxial cable.

    If your shack is on the first floor or higher it may be difficult to get a good RF earth.  If the ground lead length is more than a small fraction of a wavelength you may need to use an artificial ground. This is a quarter wavelength of wire for each band in use, they must all connect to the earth terminal of the ATU.  The wires can be run outside, concealed around the skirting board or under the carpet of your shack, make sure the end of these wires are well insulated or use pvc coated wire as the free end of a counterpoise wire will carry very high RF voltages when transmitting.  The counterpoise wires will also reduce or remove any RF feedback or transceiver instability.  

    Another idea is to use a length of wire one half wavelength at the lowest operating frequency. This requires a less extensive earth system (An earth rod driven into the ground without any counterpoise wires.)




    I would recommend the end fed wire as a good antenna to start the hobby as it requires very little expense. I have worked stations all over the world on 20 meters,  all over Europe on 80 and into central Russia on 40 meters. Although I have not called for any real Dx on 40 meters, I have heard stations as far as central Japan.

Tuesday, June 17, 2014

Build a 2 Meter, 5/4 Wave Antenna

Many RASON members truly enjoyed last month's collinear antenna. This month I decided to build a 2 meter 5/4 wave antenna. This antenna is unique in that it is enclosed entirely in 3/4" PVC which makes the design a little more complicated. The primary problem is that PVC tubing has a significant velocity factor which causes RF to slow down. This means that an antenna encased in PVC will normally need to have it's physical length reduced by about 19%. To further complicate the design, a 5/4 wave antenna's impedance has a highly inductive component which must be tuned out to get a good match. Fortunately, the design in Figure 1 solves all of these problems.

Oz Report's Antenna

2m (144-148 mHz) Antenna with molded BNC connectors (the only kind worth anything in hang gliding harnesses).  

Five foot long total RG58C/U coaxial cable. 19.5" of unshielded center cable (for 144-148 meters) which you attach to your harness mains (using tape).

The ground wire is the exposed shielding wire, the mesh wire that normally surrounds the interior wire. This is how you hook it up in your harness:  

You can just tape down the braid also.

I have also had adequate success just cutting off the braid exposed ground line like this:

Click on above to view a higher resolution image. 

Connect the antenna, as shown in the diagram above, to your mains. I suggest in a manner to protect the junction at the end of the shielding from flexing, so run the shielding up the mains for a couple of inches, if you have the room. 

I have normally cut off the ground as it is sometimes a bit of a hassle to place horizontally along the back plate, but by leaving it on you have better matching of your antenna to your radio to get the highest signal power output. I've had good luck even with the ground gone.

The antenna is cut to 19.5 inches which corresponds to 144 mhz. You can cut the antenna a bit shorter for high frequencies if you like, although the differences are quite small. To know how long to cut the antenna (the single center strand), it is length (in inches) = 2808/freq in mHz. For example, for 147 mHz, length equals = 2808/147 = 19.1 inches.

Single 5/8 Flower Pot Antenna

The Single 5/8 version of the Flower Pot simply substitutes a 5/8 wave-length section for the top quarter wave of the basic half wave antenna design. The arrangement is shown in the sketch below. The 5/8λ radiator uses a 0.2λ (shorted) co-ax phasing stub to resonate the 5/8λ element. In a conventional 5/8λ mobile whip, an inductor is used to bring the 5/8λ element to resonance; however, in this Flower Pot style of antenna, using a co-ax phasing (or delay) stub suits the construction technique and has the advantage of being able to be precisely determined and cut at the construction stage.

The antenna configuration is similar to, but slightly shorter than, the “Gain Sleeve” antenna described in the RSGB Hand-book (6th Edition – figure 13.99, which itself is derived from the reactance – or shunt – fed 5/8λ monopole antenna at figure 13.84 of the handbook).
The Gain Sleeve antenna achieves an effective radiating element length of one wavelength and, since the aperture is twice that of a half wave dipole, a theoretical gain of 3dBd (gain over a dipole) could be achieved.

However, note that the Handbook indicates that in practice, the Gain Sleeve antenna would realise about 2.5dBd. The effective radiating element length of the Single 5/8 Flower Pot is 7/8λ suggesting it would have somewhat less than 2.5dBd gain.

2m Construction

2m Single 5/8 Flower Pot

Simple 1/4 Wave Ground Plane Antenna

If you are just starting out or have the desire to build an antenna here is a simple and fun project. This antenna is perfect for those hams living in the primary coverage area of the .075 repeater. The radials can be made of no. 12 copper wire. The vertical radial (A) should be soldered to the center connector of the SO239. The four base radials (B & C) and (D & E) can be soldered or bolted to the SO239 mounting holes using 4-40 hardware. The four base radials then should be bend downward to a 45 degree angle. The antenna can be mounted by clamping the PL259 to a mast or even passing the coax through a 3/4 ID PVC pipe and compression clamping the PL259. Either way let your creativity flow. If you plan on mounting it outside experience teaches to apply RTV or sealant around the center pin to keep water out of the coax.

Make each radial a 1/4 wave of your desired frequency. Sometimes it helps to add a little extra length to the radials. This will give you some snipping room when you adjust the SWR.
example calculation:
Freq (mhz)    A (inches)    B&C/D&E (inches)
146 mhz            19-5/16            20-3/16

VHF/UHF Ground Plane antennas for under $20

The entire antenna consists of a panel mount SO-239, some stiff wire (like aluminum welding wire, brazing rod, 12 GA bare copper wire, electric fence wire, etc) or thin brass tubing, some solder, and four 4-40 3/8" long screws, washers, and nuts

Cut the wires to length in accordance with the chart provided for the band you want. You will end up with 5 pieces of wire, 4 the same length (radials) and one slightly longer (driven element). Use a good high wattage soldering iron to tin one end of the driven element

HINT:Cut your radials extra long, mount them on the SO-239, mark the point to start the bend, remove the radials, make the 45deg bend, remount them, then measure and trim to length. When tuning for lowest SWR, remember that the angle of the radials affects the value, so feel free to adjust them anywhere between 30 and 45 deg for best reading. Also, if tubing is used, one end of each radial can be soldered to a ring terminal or hammered flat and drilled

There are several ways to attach the radials:

  • Bend the wire through the corner hole and solder in place — Requires a lot of heat, no longer removable for transport but better electrical connection
  • Use a pair of needle nose pliers to fashion one end into a tight loop just big enough for the 4-40 screws to get through — Takes a little work, can loosen over time [use split washers for extra tension], but no soldering
  • Solder (or crimp) ring terminals on one end — clean, professional looking, but may break over time with repeat bending
Notice that the length of the radials is measured from the 45 deg bend to the tip, so which ever way you choose, allow for this extra length in your calculations.

Thursday, January 16, 2014

Homebrewed Off-Center Fed Dipole


Building A Homebrewed Off-Center Fed Dipole Scanner Antenna.

Aluminum/copper tubing construction:

You will need to check the fit of the tubing with the T connector and the caps while you are at the store. One combination that fits nicely is 3/4" copper pipe with 3/4" CPVC fittings (not to be confused with 3/4" PVC fittings which will be too large). The tubing/connector is held in place with 2 stainless steel sheet metal screws for connecting the balun to each element.
Find a "U" bolt to fit your mast. Drill two holes in the support pipe to fit the U bolt.The support pipe is 18" from the "T" to the mast.

Remember, bandwidth increases as diameter of the elements increases. I think, if I remember correctly, at the hardware store, that a few CPVC fittings will fit copper tubing perfectly!

Some say that the 18" element on top mounted works best,Some like the 48" element on top.It does'nt matter,it works the same. If you use the copper tubing,be sure to paint it with some good,non-conductive paint.I used to paint mine light grey. -Have fun! (Teraycoda)

For an alternate/temporary mounting option, drill a hole in one of the end caps and put in an eye bolt with a nut on the underside of the cap to secure. Be sure to secure this end cap to the copper tubing somehow, perhaps with an additional small stainless sheet metal screw. Be sure that the eye bolt itself doesn't make electrical contact with the tubing. Also, drill a small weep hole in the bottom end cap to allow any moisture to escape that may accumulate inside. Use the eye bolt and some rope to pulley the antenna up high in a tree, or use a hook to hang it somewhere. Give careful consideration to safety and grounding depending on your particular usage scenario. (Qdude)

Variation for Off-Center Fed Dipole Using Simple Wire and 75 to 300 ohm TV Balun Transformer

Offset dipole scanner antenna.jpg
Electrically, this version is the same as the one using copper tubing (above) but can be assembled quickly and is quite portable. While not as broadbanded as an OCFD using copper tubing or other metal with a larger diameter, the OCFD made from simple wire turns in great receive performance in all the commonly scanned bands, as reported here on RR in multiple message threads.
The legs/ends of the dipole are simple bell wire and shown here coiled up. Uncoil them and hang them vertically; doesn't matter if the long or short leg is at the top... works the same either way. The wire terminal lugs shown at the end of the legs of the dipole antenna should NOT be connected electrically to the wires - just crimp them on over the wire insulation. They are used as convenient hangers for the antenna, and not meant for electrical connection. Obviously, the lugs at the TV transformer/balun ends of the wire should be stripped before crimping on the terminal lugs to ensure contact with the antenna wires when you attach the TV transformer. Ensure the 75 ohm coax feedline that you connect from the balun/transformer runs away from the antenna at as near a 90 degree angle as possible.