Build a Wi-Fi antenna using household materials

 

Who'd have thought that a toilet-brush holder, of all things, would turn out to be an excellent Wi-Fi antenna? The lesson is that you can achieve great results for little expense - and half an hour's work.The range of a WiFi router can be considerably extended simply by connecting a directional antenna. Standard omni-directional stub antennas are at the lower end of the performance scale, and they quickly come up against their limits when you need to give your own home better coverage, provide your neighbour with DSL, or pick up as many radio networks as possible while war driving.If the access point is three rooms further on, or even in the house on the other side of the road, you need a directional antenna. If you have to make a connection to your nearest DSL-equipped acquaintance at the other end of the village street, or to bridge even longer radio links to reach the free radio[1] node in the next block but one, you may even require two directional antennas.

Neighbourhood WiFi routers with omni-directional aerials are in any case the worst sources of interference in a city. A single block of flats can easily contain over ten wireless networks, all chattering away simultaneously. Mutual interference is inevitable, with the result that range and connection stability are drastically reduced.Replacing just one of two antennas at the base station can be a way of improving WiFi coverage, for example, down to the bottom of the garden. The near zone is served by the remaining stub. All current WiFi modules automatically use the most suitable antenna for each client, a process called antenna diversity. Even with models having only one external antenna, it's worth having a look inside the casing. Usually, a tiny socket for the second antenna is fitted on the WiFi module. Depending on the manufacturer, this type of plug is called U.FL or Ipex. The connection can easily be led out through a ventilation slot with a short adaptor cable ("pigtail"). On some WiFi notebook cards and USB sticks, there is also an antenna plug, and a look at the data sheet will tell you its type – normally SMA or RPSMA.

The simply made tin-can antenna, with the dimensions given here, is suitable for base stations and for clients who transmit on 2.4 GHz in accordance with the IEEE 802.11b and 802.11g standards. 802.11a uses the 5-GHz band, requiring different antenna dimensions. The necessary background for a recalculation is given in an article on building tin-can radio antenna (Building a Wi-Fi Antenna Out of a Tin Can) [2]

Very recent base stations that comply with the draft standard 802.11n also use the 2.4 GHz band. But they automatically use a number of methods to combine their antennas for optimal range and speed. However, this only works if the antennas have the characteristics expected by the WiFi chipset.

 

http://www.h-online.com/features/Build-a-Wi-Fi-radio-relay-using-household-materials-747382.html?view=print

Hex Beam antenna

 

I am currently looking at building a Hexx wire beam antenna. Due to space restrictions and various things w.r.t antennas in New Zealand I do think that this antenna can work for me.
I had a browse around and found a couple of websites referring to hexx beams and how to make it.It seems fairly easy and with a 5 band version all on one feed line it is well worth investigating.
Here is a picture of a typical Hexx beam:

So, how should I start,I managed to get a aluminium base plate at a scrap metal shop for a good price, Picked up some old PVC 20mm tube in 2 m lenghts and have wire I collected over the years.I made the PVC into 2m lengths to accommodate a 3 band antenna which will host 10m / 12m / 15m (28mhz / 24mhz / 21mhz).Now I made the aluminium base plate into a HEXX shape. With my thinking cap on I managed to make a Hexx shape. What a story if you do not have a compass.

The Wire needs to be added and this is what it should look like from above once completed.

 

I will post pictures as I progress so be sure to check back to follow my progress.

http://zs6lw.blogspot.com/2009/11/hexx-beam-antenna.html

MOSFET-BASED PREAMPLIFIER FOR FM RADIO Project

 

FM transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. FM DXing refers to hearing distant stations (1500 km or more) on the FM band (88-108 MHz). The term ‘DX’ is borrowed from amateur radio operators. It means ‘distance unknown’; ‘D’ stands for ‘distance’ and ‘X’ stands for ‘unknown.’ For an FM receiver lacking gain, or having a poor signal-to-noise ratio, using an external preamplifier improves the signal level.

The dual-gate MOSFET preamplifier circuit shown in Fig. 1 gives an excellent gain of about 18 dB. It costs less and is simple to design. Field-effect transistors (FETs) are superior to bipolar transistors in many applications as these have a much higher gain—approaching that of a vacuum tube. These are classified into junction FETs and MOSFETs. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate.In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain. In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate. The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The MOSFET is more flexible because it can be controlled by a positive or negative voltage at gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer. Thus the MOSFET has an extremely high input impedance. Dual-gate MOSFETs (DG MOSFETs) are very popular among radio amateurs. These are being used in IF amplifiers, mixers, and preamplifiers in HF-VHF transceivers.

The isolation between the gates (G1 and G2) is relatively high in mixer applications. This reduces oscillator pulling and radiation. The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the (large) incoming signal. A DG MOSFET can also be used for automatic gain control in RF amplifiers. DG MOSFET BF966S is an n-channel depletion-type MOSFET that is used for general-purpose FM and VHF applications.

In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor. Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies. Capacitor C4 (100 kpF) at the gain control electrode (gate 2) decouples any variation in G2 voltage at radio frequencies to maintain constant gain. Set preset VR (47k) to adjust the gain or connect a fixed resistor for fixed gain. The output of the circuit is obtained via capacitor C5 and fed to the FM receiver amplifier.

For indoor use, connect a ¼- wavelength whip antenna, ½-wavelength 1.5m wire antenna, or any other indoor antenna set-up with this circuit. You may use a 9V battery without the transformer and diode 1N4007, or any 6V-12V power supply to power the circuit (refer Fig. 1). The RF output can be taken directly through capacitor C5. For an improved input and output impedance, change C1 from 1 kpF to 22 pF and C5 from 1 kpF to 100 kpF. For outdoor use at top mast, like a TV booster, connect the C5 output to the power supply unit (PSU) line. Use RG58U/ RG11 or RG174 cable for feeding the power supply to the receiver amplifier. The PSU for the circuit is the same as that of a TV booster. For TV boosters, two types of mountings are employed: The fixed tuned booster is mounted on the mast of the antenna. The tunable booster consisting of the PSU is placed near the TV set for gain control of various TV channels. (For details, refer ‘High-Gain 4-Stage TV Booster’ on page 72 of Electronics Projects Vol. 8.) Mount the DG MOSFET BF966S at the solder side of the PCB to keep parasitic capacitance as small as possible. Use an epoxy PCB. After soldering, clean the PCB with isopropyl alcohol. Use a suitable

enclosure for the circuit. All component leads must be small. Avoid shambled wiring to prevent poor gain or self oscillations. Connecting a single-element cubical quad antenna to the circuit results in ‘Open Sesam’ for DXing.You can use a folded dipole or any other antenna. However, an excellent performance is obtained with a cubical quad antenna (refer Fig. 2) and Sangean ATS- 803 world-band receiver. In an amplifier, FET is immune to strong signal overloading. It produces less cross-modulation than a conventional transistor having negative temperature coefficient, doesn’t succumb to thermal runaway at high frequencies, and decreases noise. In VHF and UHF, the MOSFET produces less noise and is comparable with JFETs. DG FETs reduce the feedback capacitance as well as the noise power coupled to the gate from the channel, giving stable unneutralised power gain for wide-band applications. This circuit can be used for other frequency bands by changing the inputand the output LC networks. The table here gives details of the network components for DXing of stations at various frequency bands.

HF Magnetic Loop Antenna

Ten Steps to QRP Success

1. Use efficient antennas
A half wave dipole or better is preferred.

2. Know your capabilities - do not expect DX every time
It would be nice to work Europe with one watt to a mobile whip on forty metres, but do not expect such contacts to come easily (if at all). Instead, you should cast your sights a little lower and enjoy the closer-in contacts that are more achievable.

3. Have frequency-agile equipment
Many articles describe simple crystal-controlled QRP transmitters that can be put together in an evening. These are fun to build but frustrating to operate; 99 percent of such rigs sit on shelves, unused, gathering dust. Instead, use a VFO or 3.58 MHz variable ceramic resonator on eighty metres, or a VXO with at least a 10-15 kHz tuning range on the higher bands.

4. Use 'tail-ending' to advantage
When your signal is weaker than average (such as when operating QRP), 'tail-ending' is the most effective way of obtaining contacts. Simply tune across the band, noting the contacts that are ending. When all stations sign clear, call one of the stations. They will most likely reply to your call, even if only to give a signal report.

5. Have a quality signal
A transmitter that clicks and chirps is harder to copy at the other end than a signal from a clean and stable rig. This is particularly the case when the receiving station is using narrow CW filters.

6. On CW, know the relationship between your transmit and receive frequencies
It is possible for a station to miss your call if you are transmitting on the wrong frequency. Set direct conversion QRP rigs so that they transmit about 800 Hz below their receive frequency. Conversely, if calling CQ, tune around your normal receive frequency (with the RIT control) just in case a station is calling you on the wrong frequency.

7. Have an efficient transmit/receive switching system
A homebrew station that requires the operator to flick two or three switches to switch from receive to transmit is inefficient and may result in missed contacts (particularly during contests). Use just one T/R switch or experiment with the many break in and timing circuits available.

8. Use the best receiver you can afford
Most of the complexity in a QRP station is in the receiver. While simple receivers are fine for casual SWLing, active operating requires a somewhat better class of receiver. Aim for good frequency stability, adequate bandspread, reasonable selectivity, good strong-signal handling and an absence of microphonics. A well-built direct-conversion receiver should satisfy on all five counts for all but the most hostile band conditions.

9. Enter contests to boost your operating skills
Many people think that high power is necessary to participate in contests. This is untrue, particularly for the local VK contests. Contest rules are given in Amateur Radio magazine and on the WIA website.

10. Don't be afraid to call CQ
On bands such as ten metres, the band can be wide open, but no one would know as every body is listening. Call CQ, particularly when you have grounds for supposing the band is open, for example reception of beacons or 27 MHz CB activity. Automatic CQ callers using tape recorders, computers or digital voice recorders are particularly handy here.

L Match Tuner for End-Fed wire antenna

 

In this tuner. a variable inductor made bv mutual coupling between two coils of near equal inductance is used as the L match inductor. The object is to have a variable inductor with no taps or rollers. These coupled coils, LI and L2 are connected in series (see Figure) to get total inductance.

A reverse switch connects the two coils either for additive or subtractive M to accomplish a wide range of total L. With LI = L2 = 10 uH and K = 0.8 the range is about 4 to 36 uH. One coil is wound on the plastic case of

a 12 ga empry shotgun shell and the other on a 20 ga empty shotgun shell . The winding is #23 magnet wire so that the outside diameter of the 20 ga coil slides nicely into the 12 ga shell to allow variable coupling by sliding the smaller coil in and out of the larger coil.

1. Use a low base 20 ga empty shotgun shell and drill out the primer end to clear a 114 inch screw.

2. Take a 1/4-20 nylon hex nut and glue it over the hole just drilled. This gives the shell a threaded nut that will run on a lead screw to move this coil in and out of the 12 ga shell.

3. In preparation for winding the coil cut off and discard about 318 inch of the crimped end of the 20 ga plastic so that a well formed coil form remains.

4. Locate and drill a 1132-inch dia hole in the plastic about 114 inch from the end just prepared. This is for the start of the $23 magnet wire winding.

5. Locate and drill another 1132" dia. hole in the plastic about 7116 inch from the fust hole toward the base. This allows 15 turns between the holes from start to finish, Drill a 1W-inch hole close to the base that will serve to bring both ends of the winding out for connections clear of the sliding fit. .

6. Wind 15 turns of #23 magnet wire between the two 1132 dia holes. Start with one end into a 1132 dia hole, into the shell and out the 118-inch hole. When 15 turns are on, put the end into the nearby 1132- inch hole, down the center of the shell and out the 118-inch hole pulling the wire tightly to maintain tight turns.

7. Take a 12 ga shell, cut the crimped end off to make a uniform coil form and wind on 13 turns of #23 magnet wire. No holes are used for the start and finish of the winding so tape must be used hold the winding and dress the leads. Clear fingernail polish may be useful as well.

8. A 1/4-20 x 2 inch nylon machine screw along with an extension of 114 inch wooden doweling is used to make the lead crew shown in the diagram in Figure

20-Meter Vertical dipole antenna

 

20-Meter Vertical dipole antenna

1.8mhz Inverted-L Antenna

 

The antenna shown in Fig is called an inverted-L antenna. It is simple and easy to construct and is a good antenna for the beginner or the experienced 1.8-MHz DXer. Because the overall electrical length is made somewhat greater than ë/4, the feed-point resistance is on the order of 50 Ù, with an inductive reactance. That reactance is canceled by a series capacitor as indicated in the figure. For a vertical section length of 60 feet and a horizontal section length of 125 feet, the input impedance is ≈ 40 + j 450 Ù. Longer vertical or horizontal sections would increase the input impedance. The azimuthal radiation pattern is slightly asymmetrical with ≈1 to 2 dB increase in the direction opposite to the horizontal wire. This antenna requires a good buried ground system or elevated radials. This antenna is a form of top-loaded vertical, where the top loading is asymmetrical. This results in both vertical and horizontal polarization because the currents in the top wire do not cancel like they would in a symmetrical-T vertical. This is not necessarily a bad thing because it eliminates the zenith null present in a true vertical. This allows for good communication at short ranges as well as for DX

Bent Dipole antenna

 

The simplest way to shorten a dipole is shown in Fig . If you do not have sufficient length between the supports, simply hang as much of the center of the antenna as possible between the supports and let the ends hang down. The ends can be straight down or may be at an angle as indicated but in either case should be secured so that they do not move in the wind. As long as the center portion between the supports is at least ë/4, the radiation pattern will be very nearly the same as a full-length dipole.

The resonant length of the wire will be somewhat shorter than a full-length dipole and can best be determined by experimentally adjusting the length of ends, which may be conveniently near ground. Keep in mind that there can be very high potentials at the ends of the wires and for safety the ends should be kept out of reach.

7-MHz Loop Antenna

 

The loop can be fed in the center of one of the vertical sides if vertical polarization is desired. For horizontal polarization, it is necessary to feed either of the horizontal sides at the center. Optimum directivity occurs at right angles to the plane of the loop, or in more simple terms, broadside from the loop. One should try to hang the system from available supports which will enable the antenna to radiate the maximum amount in some favored direction.

The overall length of the wire used in a loop is determined in feet from the formula 1005/f (MHz). Hence, for operation at 7.125 MHz the overall wire length will be 141 feet. The matching transformer, an electrical 1/4 ë of 75-Ù coax cable, can be computed by dividing 246 by the operating frequency in MHz, then multiplying that number by the velocity factor of the cable being used. Thus, foroperation at 7.125 MHz, 246/7.125 MHz = 34.53 feet. If coax with solid polyethylene insulation is used, a velocity factor of 0.66 must be employed. Foam-polyethylene coax has a velocity factor of 0.80. Assuming RG-59 is used, the length of the matching transformer becomes 34.53 (feet) . 0.66 = 22.79 feet, or 22 feet, 91/2 inches. This same loop antenna may be used on the 14 and 21-MHz bands

Loop Skywire antenna

 

Are you looking for a multiband HF antenna that is easy to construct, costs nearly nothing and yet works well?

You might want to try this one. The Loop Skywire antenna is a full-sized horizontal loop. Early proponents suggested that the antenna could be fed with coaxial cable with little concern for losses, but later analysis proved that this was a bit of wishful thinkingthe relatively low values for SWR across multiple bands indicate that cable losses were part and parcel performance. The best way to feed this versatile antenna is with open-wire ladder line, with an antenna tuner in the shack to present the transmitter with a low value of SWR.

The antenna has one wavelength of wire in its perimeter at the design or fundamental frequency.

If you choose to calculate Ltotal in feet, the following equation should be used:

Total L = 1005/F

Where F equals the frequency in MHz.

Given any length of wire, the maximum possible area the antenna can enclose is with the wire in the shape of a circle. Since it takes an infinite number of supports to hang a circular loop, the square loop (four supports) is the most practical. Further reducing the area enclosed by the wire loop (fewer supports) brings the antenna closer to the properties of the folded dipole, and both harmonic-impedance and feedline voltage problems can result. Loop geometries other than a square are thus possible, but remember the two fundamental requirements for the Loop Skywire—its horizontal position and maximum enclosed area.

There is another great advantage to this antenna system. It can be operated as a vertical antenna with top-hat loading on other bands as well. This is accomplished by simply keeping the feed line run from the antenna to the shack as vertical as possible and clear of objects. Both feed-line conductors are then tied together, and the antenna is fed against a good ground.

Antenna construction is simple. Although the loop can be made for any band or frequency of operation, the following two Loop Skywires are good performers. The 10- MHz band can also be operated on both.

3.5-MHz Loop Skywire

(3.5-28 MHz loop and 1.8-MHz vertical)

Total loop perimeter: 272 feet

Square side length: 68 feet

7-MHz Loop Skywire

(7-28 MHz loop and 3.5-MHz vertical)

Total loop perimeter: 142 feet

Square side length: 35.5 feet

The actual total length can vary from the above by a few feet, as the length is not at all critical. Do not worry about tuning and pruning the loop to resonance.

70 CM UHF 6 Element Yagi Antenna

Source - http://users.belgacom.net/hamradio/schemas/www.qsl.net/on6mu

3 Element VHF Yagi for 2 meters

J-POLE vertical antenna

Click the picture to get clear view

VHF/UHF Dualband vertical antenna

 

20m mini loaded dipole

 

It is very simple to build and you can tune it in your shack room (simulating a holyday installation in a Hotel...) using an antenna analyzer (I used my Autek VA1) but I think that you can do it also with a SWR meter and a general coverage rtx using low power.

Start with the ends some cm longer (ex. 400 mm instead of 330 mm) and then cut both sides until you have the resonance in 20m band.

Remember that the frequency is not critical because then you will "tune" the dipole with the Antenna Tuner Unit.

Thanks to author Guido, ik2bcp / iu2r / ab9dg

A simple long-wire antenna for 80 through 10 meters.

A simple long-wire antenna for 80 through 10 meters.

Typical coaxial cable transmission lines used in Amateur Radio

A typical Novice antenna installation for 40 meters.

 

How Increase BSNL Mobile Broadband EV-DO Signal Strength

 

The material used is a thick copper wire. Take copper wire about 20cm and bend at 5cm. make 5 turns of the wire on a screw or any thing that make the winding looks good. Ok then stretch the turns about 3mm wide. now make few turns on the other end.Place it on your EVDO antenna as shown in the figure. see the difference in Signal strength.

You can use this trick to get good signal in indoor from any mobile broadband networks like BSNL, MTS MBlaze etc.

The W8NX trap dipole antenna

This antenna was designed for amateurs with limited space who also wanted to operate the low bands.It was first described in July 1992 QST by A. C. Buxton, W8NX, and features innovative coaxial-cable traps.

Fig  shows the antenna layout; it is resonant at 1.865, 3.825, and 7.225 MHz. The antenna is made of #14 stranded wire and two pairs of coaxial traps. Construction is conventional in most respects, except for the high inductance-to-capacitance (L/C) ratio that results from the unique trap construction.

The traps use two series-connected coil layers, wound in the same direction using RG-58 coaxial cable’s center conductor, together with the insulation over the center conductor. The black outer jacket from the cable is stripped and discarded. The shield braid is also removed from the cable (pushing is easier than pulling the shield off). No doubt you will want to save the braid for use in other projects. RG-58 with a stranded center conductor is best for this project.

The 7-MHz traps have 33 µH of inductance and 15 pF of capacitance, and the 3.8-MHz traps have 74 µH of inductance and 24 pF of capacitance. The trap Qs are over 170 at their design frequencies. These traps are suitable for high-power operation. Do not use RG-8X or any other foam-dielectric cable for making the traps. Breakdown voltage is less for foam dielectric, and the center conductor tends to migrate through the foam when there is a short turn radius. Loading caused by the traps causes a reduced bandwidth for any trap dipole compared to a half-wave dipole. This antenna covers 65 kHz of 160 m, 75 kHz of 80 m, and the entire 40-m band with less 2:1 SWR.

Quarter-wave vertical antenna for VHF

Quarter-wave vertical antennas are useful for local communications when size, cost and ease of  construction are important.

The antennas shown are built on a coaxial connector. Use UHF or N connectors for the fixed station antennas. BNC connectors are good for mobile and portable antennas. BNC
and N connectors are better than PL-259 connectors for VHF/UHF outdoor use because: (1) they provide a constant impedance over the frequencies of interest, and

(2) they are weatherproof when the appro priate connector or cap is attached. The ground-plane antennas require a panel-mount connector (it hasmounting holes to hold the radials).
If the antenna is sheltered from weather, copper wire is sufficiently rigid for the element and radials.

Antennas exposed to the weather should be made from 1/16- to 1/8-inch brass or stainless-steel rod.Radials may be made from 3/16-inch aluminum rod or tubing and mounted on an aluminum sheet. Do not use aluminum for the antenna elementbecause it cannot be easily soldered to the coaxial-connector center pin.Where the figures call for #4-40 hardware, stainless steel or brass is best. Use cadmium plated hardware if stainless steel or brass is not available.

Off-Center-Fed (OCF) dipole for 3.5, 7 and 14 MHz

Fig  shows the off-center-fed or OCF dipole. It is not necessary to feed a dipole antenna at its center, although doing so will allow it to be operated with a relatively low feed-point impedance on its fundamental and odd harmonics. (For example, a 7-MHz center-fed half-wave dipole can also be used for 21-MHz operation.) By contrast, the OCF dipole of Fig, fed  1/3 of its length from one end, may be used on its fundamental and even harmonics. Its free-space antenna-terminal impedance at 3.5, 7 and 14 MHz is on the order of 150 to 200 Ω. A 1:4 step-up transformer at the feed point should offer a reasonably good match to 50 or 75-Ω line, although some commercially made OCF dipoles use a 1:6 transformer.   At the 6th harmonic, 21 MHz, the antenna is three wavelengths long and fed at a voltage loop (maximum), instead of a current loop. The feed-point impedance at this frequency is high, a few thousand ohms, so the antenna is unsuitable for use on this band.

10M Rectangular Loop Antenna.

With the large number of operators and wide availability of inexpensive, single-band radios, the 10-m band could well become the hangout for local ragchewers that it was before the advent of 2-m FM, even at a low point in the solar cycle.

This simple antenna provides gain over a dipole or inverted V. It is a resonant loop with a particular shape. It provides 2.1 dB gain over a dipole at low radiation angles when mounted
well above ground. The antenna is simple to feed— no matching network is necessary. When fed with 50-Ω coax, the SWR is close to 1:1 at the design frequency, and is less than 2:1 from 28.0-28.8 MHz for an antenna resonant at 28.4 MHz.

The antenna is made from #12 AWG wire and is fed at the center of the bottom wire. Coil the coax into a few turns near the feedpoint to provide a simple balun . A coil diameter of about a foot will work fine. You can support the antenna on a mast with spreaders made of bamboo, fiber glass, wood, PVC or other non conducting material. You can also use aluminum tubing both for support and conductors, but you’ll have to readjust the antenna dimensions for resonance.

This rectangular loop has two advantages over a resonant square loop. First, a square loop has just 1.1 dB gain over a dipole. This is a power increase of only 29%. Second, the input
impedance of a square loop is about 125 W. You must use a matching network to feed a square loop with 50-Ω coax. The rectangular loop achieves gain by compressing its radiation pattern in the elevation plane. The azimuth plane pattern is slightly wider than that of a dipole (it’s about the same as that of an inverted V). A broad pattern is an advantage for a general-purpose, fixed antenna. The rectangular loop provides a bidirectional gain over a broad azimuth region.

Mount the loop as high as possible. To provide 1.7 dB gain at low angles over an inverted V, the top wire must be at least 30 ft high. The loop will work at lower heights, but its gain advantage disappears. For example, at 20 ft the loop provides the same gain at low angles as an inverted V.

40M and 15M Dipole antenna

Two popular ham bands, especially for Novice and Technician class operators, are those at 7 and 21 MHz. As mentioned earlier, dipoles have harmonic resonances at odd multiples of their fundamental resonances. Because 21 MHz is the third harmonic of 7 MHz, 7-MHz dipoles are harmonically resonant in the popular ham band at 21 MHz. This is attractive because it allows you to install a 40-m dipole, feed it with coax, and use it without an antenna tuner on both 40 and 15 m.

To put this scheme to use, first measure, cut and adjust the dipole to resonance at the desired 40-m frequency. Then, cut two 2-ft-long pieces of stiff wire (such as #12 or #14 house wire) and solder the ends of each one together to form two loops. Twist the loops in the middle to form figure-8s, and strip and solder the wires where they cross. Install these capacitance hats on the dipole by stripping the antenna wire (if necessary) and soldering the hats to the dipole about a third of the way out from the feedpoint (placement isn’t critical) on each wire.

W5LAN's 2-meter mobile antenna

Brass tubing is available in some hardware and hobby stores. It comes in sizes from 1116 to 21/32 inch outside diameter(OD), in l/32 inch steps. Each size slip fits within the next larger size. It is usually sold in 12- or 36-inch lengths.The antenna is made from two 12-inch lengths of 5/32-inch tubing and two 12-inch lengths of 118-inch tubing. A V-shaped horizontal dipole is formed when the tubes are mounted through a short piece (6 inch or so) of 7/8-inch OD plastic pipe (see Fig). It is V shaped to reduce the overall size and provide a better match to 50 R coax.

Begin by drilling two 5/32-inch holes through the plastic pipe at right angles to each other. (Position one hole slightly below the other so that the dipole elements cross inside the plastic pipe without touching.) Enlarge the holes of two solder lug and force each over one end of the S/32-inch tubes and solder them in place.

Push the other end of those tubes through the holes in the plastic pipe until the solder lugs are flush against the pipe. Strip the end of a length of coax, then solder the braid to one solder lug and the center conductor to the other. Use sealant to weatherproof the coax end and feed point.

The antenna is adjusted to resonance by sliding the 118-inch tubing in and out of the larger tubing to achieve minimum SWR. If the fit is too loose, nick the end of the smaller tube slightly with diagonal cutters, and force it into the larger tubing. After performing the adjustment, cut the smaller tube to a length that leaves about an inch inside the larger tube and solder it to the larger tube. The element lengths on my antenna are about 20.5 inches each, and the SWR was near unity over most of the 2-meter band, with a slight rise at the high frequency end.

Demi-Quad Antenna

The demiquad is a single-element 1 quad antenna. The length of the antenna is,like the cubical quad beam antenna (see Chap. 12), one wavelength. Figure shows a type of demi-quad based on the tee-cross type of mast.

The impedance-matching section is a quarter-wavelength piece of 75-Ω coaxial cable (RG-58/U or RF-11/U). The length of the matching section is determined from:

where
L is the overall length, in feet 

FMHz is the frequency, in megahertz

V is the velocity factor of the coaxial cable (typically 0.66, 0.70, or 0.80)

Improved G5RV Antenna

 

The original G5RV antenna was developed by Louis Varney G5RV for 20 meters. Although his design was a good one, he used the 450 ohm ladder line as a feed-line-to-antenna impedance match, and without the use of a BALUN. We discovered that feeding the 450 ohm ladder-line directly with an antenna tuner, left us with a shack full of RF…HOT mics, hum, and in some cases, we had "squeals" from rectified RF getting into the microphone audio path, within the transceiver, a sure sign of RF-Feedback (base rectification).

To make the G5RV more "user-friendly" and with less RF exposure within the HAM-shack, we added an MM11 BALUN at the lower end of the 450 ohm ladder-line, and from the asymmetrical input of the MM11 BALUN (outside the HAM shack), we used 50 ohm (low impedance) coax to reach the antenna tuner inside the HAM shack. We’ve found that this improvement to the G5RV has put more of our transmitted RF into the elements of the antenna, and made the antenna virtually noise free and reduced re-radiation as much as 85 percent.

Without using an external antenna tuner, we’ve found that our transceiver will work into the 50 ohm coax and the MM11 BALUN with VSWR below 2:1 on the bands the G5RV is cut for.

By making the additional BALUN and coax improvement to the original 20 thru 10 meter G5RV, it is now possible to build the G5RV for more bands, and thus cover lower bands and frequency’s. We now have a means by which we can have an antenna that fits almost any real-estate configuration, from as little as 27 feet (8.2 m), (20 thru 10 meter bands) to 207 feet (64 m) (160 thru 10 meter bands).

80, 40, 20 and 10 M Multiband HF antenna

 

Multiband antenna using paralleled dipoles, all con-nected to a common 50 or 75-Ω coax line. The half-wave dimensions may be either for the centers of the various bands or selected for favorite frequencies in each band. The length of a half wave in feet is 468/frequency in MHz, but because of interaction mamong the various elements, some pruning for resonance may beneeded on each band.

The Multee Antenna

Two-band operation in limited space may be obtained with the multee antenna. The portion identified as H should remain as vertical as possible, as it does the radiating on the lower frequency band.

Two-band operation may be obtained on 1.8/3.5 MHz or on 3.5/7 MHz within the confines of
the average city lot by using the multee antenna shown in Fig. Dimensions are given for either pair of bands in the drawing.  If built for the lower frequencies, the top portion will do little radiating on 1.8 MHz; it acts merely as top loading for the 52-foot vertical section. On 3.5 MHz, the horizontal portion radiates and the vertical section acts as a matching stub to transform the high feed-point impedance to the coaxial cable impedance.

Since the antenna must work against ground on its lower frequency band, it is necessary to install a good ground system. Minimum requirements in this regard would include 20 radials, each 55 to 60 feet long for the 1.8/3.5-MHz version, or half that for the 3.5/7-MHz version. If not much area is available for the radial system, wires as short as 25 feet long (12 feet for 3.5/7-MHz) may be used if many are installed, but some reduction in efficiency will result.With suitable corrections in length to account for the velocity factor, 300-Ω TV twin-lead may be substituted for the open wire.  .

bobtail curtain thorne antenna

 

The bobtail curtain antenna is a fixed array consisting of three individual quarter- wavelength elements spaced a half-wavelength apart, and fed from the top by ashorting element or wire. The inverted bobtail curtain, or Thorne array, consists of an upside down bobtail curtain as shown in Fig. 12-11. The radiator elements are each a quarter-wavelength long. Their lengths are found from

The lengths of spacing between the elements are exactly twice above the value or

Feed method  worked out by the late J. H. Thorne (K4NFU/5), feeds the end elements from the shield of the coaxial cable, and the center element of the array is fed from the center conductor of the coaxial cable. A coaxial impedance-matching section is used between the cable transmitter and the antenna feedpoint.

Bisquare loop antenna

The bisquare antenna, shown in Fig, is similar to the other large loops, except that it is wavelength/2 on each side, making a total wire length of two wavelengths. This antenna is built like the diamond loop shown earlier (i.e., it is a large square loop fed at an apex that is set at the bottom of the assembly). In this case, the loop is fed either with an antenna tuning unit (to match a 1000-Ω impedance) or a quarter-wave length matching section made of 300-Ω or 450-Ω twin-lead transmission line. A 1:1 balun transformer connects the 75-Ω coaxial cable to the matching section.

The bisquare antenna offers as much as 4-dB gain broadside to the plane of the antenna (i.e., in and out of the book page), in a figure-8 pattern, on the design frequency. It is horizontally polarized. When the frequency drops to one-half of the design frequency, the gain drops to about 2 dB, and the antenna works like the diamond loop covered previously.

Delta loop antenna

The delta loop antenna, like the Greek uppercase letter “delta” (∆) from which it draws its name, is triangle-shaped (Fig. 14-8). The delta loop is a full wavelength, with elements approximately 2 percent longer than the natural wavelength (like the quad). The actual length will be a function of the proximity and nature of the underlying ground, so some experimentation is necessary. The approximate preadjustment lengths of the sides are found from:

 

The delta loop antenna is fed from 52-Ω coaxial cable through a 4:1 balun trans former. The delta loop can be built in a fixed location, and will offer a bidirectional pattern.

QRP Magnetic Loop Antenna for 40,30,20 Metres

Build an Indoor FM Antenna With These Plans

 

The easiest way to improve your FM reception is to build an indoor FM antenna, instead of using your FM stereo’s internal FM antenna. This indoor FM antenna is easy to build, and cheap. It works every bit as good as other FM antennas that you can buy for as much as $100.

In order to build this indoor FM antenna, all you need is two 3/8” dowel rods 48” long, 10 ft. of 20 ga. wire, and some 75 ohm RG-59 or RG-6 coax (for TV’s). All of this can be picked up at your local hardware store. However sometimes hardware stores don’t have dowel rods 48”. If you can’t find any that long, you can always take two 36” dowel rods and tie them together with cable ties to the correct length.

This FM antenna is what is called a Full Wave Loop antenna. The diagram below shows the design of this indoor FM antenna:

The red is the wire, which is to be 30” on each side. The brown represents the 3/8 inch dowel rods. Also notice that the coax is fed from the side. This is not necessary, as typically full wave loops are fed from the bottom. I was interested in receiving one particular station that transmits a vertically polarized signal. Almost all FM stations transmit circularly polarized, which is both vertical, and horizontal polarization. Also feeding the Fm antenna from the side seemed to be a stronger, more reliable means of connecting the coax to the FM antenna.As far as construction of the FM antenna, the first thing to do is cut 4 inches off each dowel rod. This will then make each dowel rod 44 inches long. Next cut a slit aprox. ½ inch on each end of the dowel rods. These slits will be how you mount the wire to the dowel rods of your FM antenna.

Here is a photo of what I am talking about:

This not only shows the slot cut in the dowel rod, but also the wire, as well as the use of a cable tie to secure the wire to the end.

On the last end, where we will attach the coax to the FM antenna, put both ends of the wire into the slot leaving about an inch extending past. Next strip off the insulation and attach one end of the loop to the center conductor of the coax, and the other end of the loop to the shield of the coax.

Here is a photo of the coax being attached to the FM antenna.

Next, secure the coax to the dowel rods with it coming off the bottom dowel rod. Lastly, take a couple of cable ties and put one on the top of the vertical dowel rod to create a loop to attach a string to hang the FM antenna.

The photo below shows the completed FM antenna:

http://www.mikestechblog.com/joomla/misc/54-fm-antenna/130-build-indoor-fm-antenna-plans.html?start=1

2 Meter Turnstile Antenna For Amateur Satellite Communication

 

Here are construction plans of a Turnstile antenna that I use for space communication on the 2 meter amateur radio band. Specifically for 145.80 mHz.

A Turnstile antenna with a reflector underneath it makes a good antenna for space communications because it produces a circularly polarized signal pattern and also has a broad, high angle pattern. Due to these characteristics, there is no need to rotate the antenna.

My design goals were that it had to be cheap (of course!) and made from easily available materials. In looking at other turnstile antenna designs, one thing that has always bothered me is that they use coax (un-balanced feedline) and directly feed the antenna (balanced load). According to the antenna books, this situation tends to cause the coax to radiate, and upset the overall radiation pattern of the antenna.

The Antenna
What I decided to do is to use "folded dipoles" instead of traditional ones. Then feed the turnstile antenna with a 1/2 wavelength 4:1 coaxial balun. This type of balun also takes care of the "balance-to-unbalance" problem usually encountered as well.
The drawing below shows how to make a turnstile antenna. Please note, this is not to scale.

Construction of a turnstile reflector antenna consists of two 1/2 wavelength horizontal dipoles that are oriented 90 degrees from each other (like a big X). Then feed one dipole 90 degrees out of phase of the second one. One problem with Turnstile Reflector antennas is that the structure to hold up the relector part can be cumbersome. Fortunately (some might disagree) I decided to build my turnstile antenna in my attic. This solves another problem in that I also don't have to concern myself with is weatherizing the antenna.
For the folded dipoles I used 300 ohm TV twinlead. What I had on hand was low loss "foam" type. This particular twinlead has a velocity factor of 0.78. You will also notice in the above drawing that the lengths ot the dipole aren't what you would expect for 2 meters. This is the length I ended up when I was finished adjusting for minimum SWR. Apparently the velocity factor of the twinlead figures into the resonance of the folded dipole. As they say, "Your mileage may vary" on this length. I would also like to point out that in the drawing above the feedpoint of the folded dipoles is actually in the center of the folded dipole. I made the drawing this way for clairity.

The Reflector
In order to get the radiation pattern in the upward direction for space communications the turnstile antenna needs a reflector underneath it. For a broad pattern the antenna books recommend 3/8 wavelength (30 inches) between the reflector and the turnstile. The material I chose for the reflector is ordinary window screen you can pick up at a hardware store. Make sure it is metal screen as there is a non-metal type of window screen they sell as well. I purchased enough to lay out an 8 foot square on the rafters of my attic. The hardware store couldn't give me one big piece for all of this, so I overlapped pieces of screen by about a foot on the seam. From the center of the reflector, I measured up 30 inches (3/8 wavelength). This is where the center, or the crossing point of the folded dipoles are located.

http://www.mikestechblog.com/joomla/ham-radio/ham-radio-antennas-category/65-2-meter-turnstile-antenna-for-amateur-satellite-communications.html

2M + 70cm Open Sleeve Vertical Dipole

 

Johnny Pedersen (LA3AKA)

I was playing around with the MMANA Antenna analysis software and wanted to design a 2m/70cm vertical antenna. I tried different antenna models, J-pole, half wave dipoles, Ground Planes …. I then remembered a chapter in the 18. edition of the ARRL Antenna Handbook covering Open Sleeve antennas to make Broad band antennas. I thought that this might be useful for making a dual band VHF/UHF antenna.

The Antenna is planned built using 6mm aluminium rods. According to MMANA this will with a distance of 3.2cm between driven element and sleeve elements give an Feedpoint Impdance of 75 ohms on 2 meter and 50 ohms on 70cm. One nice thing with this antenna is that you get a good gain on 70 cm (approximately 3dB over a g Gain:

http://www.eham.net/articles/8808

A Magnetic Loop Antenna for Shortwave Listening (SWL)

Now that we’re on the downward slope of sunspot cycle 23 (2004) you may have noticed that some of your favorite broadcast stations don’t come in as strong as they did a few years ago. This is especially apparent on weaker DX stations. The whip on your shortwave receiver used to be sufficient to pull in some good DX, but now you find yourself looking for something better.

Maybe you have been thinking, or even have already tried, putting up a wire antenna. This may be a great solution if you live in a reasonably quiet area, noise wise, and your shortwave receiver doesn’t easily overload in the presence of strong signals. Perhaps you live in an apartment or are situated where installing a wire antenna is simply not feasible. Or maybe you’re looking for something that offers a bit more mobility so you can take it into different rooms of your house. Consider the small single turn magnetic loop antenna if any of the above situations apply to you.

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.

More at…… http://www.kr1st.com/swlloop.htm

Simple 1/4 Wave Ground Plane

 

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 extralength 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

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.

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:

http://www.kr1st.com/2mhalo.htm