After finally getting my ham radio license it was time to build a magnetic loop antenna for short wave listening and QRP operation because the apartment I am living in does not allow me to put up large antennas.
A magnetic loop antenna is, as the name suggests, a simple resonant antenna that receives mainly the magnetic component of the electromagnetic field. The resonant main loop is capacitively fed by a smaller feed loop. This is good because most noise generated by power supplies has a stronger electric field component, i.e. this antenna should pick up less noise even in indoor environments.
Because the antenna needs to be tuned to resonance at the frequency of interest, an air variable capacitor out of an old shortwave radio receiver is used. The antenna is very narrow bandwidth which means that it has to be tuned even when changing to a frequency within the same amateur radio band. On the upside the narrow bandwidth again means less noise pickup, as total noise power is proportional to bandwidth.
After some search on the web I found a design by DL8NDG which seemed simple enough to replicate. The article calls for RG214 coaxial cable for the loop but I only had RG58C/U on hand which is significantly more flexible (not a good thing).
Using their spreadsheet I calculated the lengths for my application and began building the antenna.
My final measurements are about 80 cm main loop diameter (251.3 cm length), 16 cm feed loop diameter (51 cm length). The variable capacitor contains two stators which are wired in series, which means that the voltage rating of the cap doubles (power rating quadruples). In my case that configuration gives a capacitance between about 9pF to 280pF.
The variable capacitor was placed inside a small outdoor cable distribution box and the coax fed through the cable holes in the box. This helps the mechanical stability of the coax connection to the cap. Note that the outer and inner conductors of the main loop are shorted together on both ends before soldering to the cap.
The feed loop is exactly 1/5 of the diameter of the main loop and is coaxially soldered to the feed connector on one end. The other end is only soldered to the shield of the connector with the inner conductor, the outer conductor should be insulated on this end.
To achieve this I have simply bent back the outer braid and put a bit of heat shrink tubing over it to keep it from shorting to the inner conductor. I have attached the feed loop to the main loop directly opposite to the capacitor using some packing tape (cable ties may work better).
During a first test I have received some CQ calls on 20 meters which came in nice and strong. I could also receive some CW on 40 meters, all using a modified rtl-sdr stick and the antenna on a wooden desk.
The tuning is very sensitive, you really have to turn the rotors of the cap slowly in order to maximize the sensitivity at a certain frequency. I have also noticed that it seems to pick up a very strong noise source. This may be a strong FM broadcast station nearby.
Using RG58 is not ideal and I will probably have to rebuild the antenna with the actual RG214 which is not as flexible. As it is, I can not put up the loop vertically because it deviates from the circular shape way to much.