Make a magnetic loop antenna for 7-21 mhz

 

 

• 

  Magnetic Loop Diagram

• 

  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

SMALL SINGLE TURN MAGNETIC 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.

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.

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

 

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.

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.

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.