DIY Magnet Loop Antenna - Post 1

This and the following two articles are taken from a series I published in QRP Quarterly. I've adapted them very quickly, so I hope there aren't too many gaps. Go may want to go to Post 3 first to see my "final" version. Feel free to be in touch if you have questions or suggestions. - Howard K4LXY

I have a variety of portable antennas for QRP use, but they either require supports such as poles or trees, or they need counterpoises. None of them are very appropriate or effective for indoor use. For example, I’ve tried the short MFJ base-loaded whips, combined with counterpoises, extended from windows, but they aren’t very effective or easy to use in some situations.

Fortunately, my friend Russ, K3NLT, introduced me to the small magnetic vertical loop. After doing some reading and experimenting and then constructing one, I’m a believer. They can be made to roll up compactly, are quiet receiving antennas, are relatively unaffected by height, and best of all, are surprisingly good radiators. In fact, they provide both low and high angle paths by radiating in almost all directions, vertically and horizontally, except for a sharp null perpendicular to the loop. This null can be very useful for tuning out interference.

In the short time I’ve had this set up in my upstairs guest bedroom I’ve had many solid QSOs and even made contacts in far-off places such as Slovenia.

I am not going to explain loop theory here – there are lots of good articles on the internet.[i] For practical ideas, the articles I drew upon most were the G4ILO Wonder Loop,[ii] and an excellent piece by Leigh Turner VK5KLT.[iii]

At Russ’ suggestion, I made the transmitting loop from LMR-400 coax because it has a fairly large diameter, a heavy double shield, and is rigid enough to keep its shape. Yet it can be coiled into a rather compact loop for packing. RG-213 could also be used but the shield isn’t as heavy and it does not hold its shape as well.

A variety of loop sizes will work; I made one with a 9’ circumference for compactness and portability, and another 12’ one to use when there is adequate space. Both tune and transmit successfully on 20, 30 and 40M though the larger one is of course somewhat more effective, especially on the lower bands. Combinations of loop sizes and capacitance and the resulting efficiency can be found with the convenient calculator provided by 66pacific.com.[iv] Unlike the Wonder Loop, which only uses the shield as radiator, I connected the center conductor and the shield together in the PL-259 as well as on the SO-239s in the tuner. According to some sources this closes the tube, reducing the tendency of RF to flow inside the shield and thus increase resistance.

On each end of the coax I used gold plated PL-259 connectors as suggested by G4ILO’s design and easily available on eBay. This makes for easy dismantling and setup as well as a good electrical connection, which is important given the currents developed by such antennas.

To couple it to the transmitter, I used a simple loop – this should be 1/5 the size of the larger transmitting loop. This “Faraday loop” is hung at the top of the large loop, and fed by 50ohm coax directly to the tuner/transmitter. (See photo) This inner primary loop can be made from RG-58 or similar small coax, using only the shield. However, for improved noise reduction, I am now using a shielded Faraday loop made from small coax, as described in the VK5KLT article. Velcro straps keep it close to the larger coil and allow for some coupling adjustment if SWR is a problem.

The tuner is primarily a tuning capacitor with sufficient spacing that it doesn’t arc (some sources say that if you can slide a card between the meshed plates, it will be fine for QRP). The capacitor I used had a maximum of 250pf. One end of the loop is connected, through a SO-239 connector and copper braid, to the rotor of the capacitor and the other thorough a SO-239 to the capacitor’s frame. A plastic project box makes a good container for the tuner, though it could be of wood or other insulating material.

The major drawback of a magnetic loop antenna is its very narrow bandwidth. This means that one often needs to retune when changing frequency, and that tuning is very sharp. I solved this by putting two planetary drives in series, giving me a 36:1 tuning ratio, and adding a spinner knob. (See photo) Also, to keep hand capacitance from affecting tuning, it is well to have some distance between capacitor and tuning knob, so the separation created by the two ganged drives and shaft is an advantage.

I fashioned a support from 3/4” PVC, with a PVC tee at the top to hold the coax. The tee was opened to take the coax by cutting out the top with a Dremel tool. Both loops work with a single support but for the 12’ loop, I added two horizontal supports from PVC to help keep its circular shape – the more open the loop, the better the antenna. (See photo)

The basic plan looks like the diagram at the top of this post except that in mine the loop is at the top and the capacitor is at the bottom.[v] This makes for easier tuning and after strapping the feed coax to the PVC support so it doesn’t move, I haven’t had a problem with fluctuating SWR because of the coax lead.

Using my Elecraft KX-1, I first peak the signal while listening to the band. I then use the built-in autotuner. If I can’t achieve a low SWR, I tweak the tuning capacitor slightly and try again. After a bit of practice, I can now usually land on the right spot simply by listening. I have not checked the SWR on the antenna without the tuner since this arrangement is working so well for me and the coax lead is so short. Since QRP power is low, I keep the antenna close enough that I can tune and rotate it from my operating position.

For convenience, I mounted a camera quick release on the bottom of the tuner that matches the quick release on a tripod. The tripod not only provides a secure and convenient mount but also allows me to pivot the antenna as needed. However, do not use a steel tripod as ferrous materials can affect the operation of the loop. Mine is carbon fiber, but aluminum or wooden tripods should work. It could also set on a table without the tripod, though you may want to put a weight on the tuner to keep the antenna from tipping backward.

In the next post, I describe my further experiments.

[i] For example, see “Small Transmitting Loop Antennas” by Steve Yatkes, AA5TB: http://www.aa5tb.com/loop.html [ii] http://www.g4ilo.com/wonder-loop.html [iii] http://www.ahars.com.au/wp-content/uploads/2015/10/The-Underestimated-Magnetic-Loop-HF-Antenna-V1.-2.pdf [iv] http://www.66pacific.com/calculators/small_tx_loop_calc.aspx [v] Diagram taken from here: http://www.aa5tb.com/loop05.gif