Moebius Magnetic Loop Antenna

Description

I was running one of my IoT workshops at the library on SDR receivers; however, receiving signals below 50Mhz in an urban area is very difficult, particularly in a library. Conventional HF antennas are often many meters in size. VHF and UHF were relatively easy, but HF is almost impossible due to limited space and radio noise. One solution to this problem was using a Moebius magnetic loop antenna.

Details

SWL on 80M - 40M - 20M and 6M

One solution for receiving HF in urban areas is a magnetic loop antenna, also called a small loop, as it has a diameter smaller than half the operating wavelength (typically no more than ⁠ 1 /3⁠ to 1/4⁠ wavelength). Loop antennas are used mainly as receiving antennas but are sometimes used for transmission. The radiation pattern of a loop antenna is maximum at directions within the plane of the loop, so perpendicular to the maxima of large loops.

A loop antenna can be a single-wire loop, rode, or strip matched to the feed line with a tuned circuit and balun. Another loop antenna design was developed during the Cold War to prevent EMP (Electro-Magnetic Pulse) damage. These antennas use the feed line coaxial cable as the loop element. The output is balanced and only requires a 1:1 Balun to be unbalanced. Further, coaxial loops are less susceptible to electrical and electrostatic noise as electrically it is a DC short circuit. On searching the internet, I found that many Ham Radio enthusiasts make most loop antennas. 2 types in particular:

"Lt. Baum’s 1964 study compared conventional split-shielded loops, which have a long history of success in radio direction finding, against a relatively unknown type called a Moebius Strip Loop. What Baum’s analysis revealed was that a Moebius loop antenna has twice the sensitivity to the magnetic field of an electromagnetic (radio) wave as a conventional shielded loop while simultaneously exhibiting lower sensitivity to the electric component of the wave." - Sourced from Bob Thomas, W3NE.

I had an unused roll of N-terminated Andrew LDF4-50A HELIAX 1/2" Corrugated Coaxial Cable, which I've used for microwave experiments. It is quite ridged, and I thought this would make an excellent stiff loop. It is also very low-loss. So, I cut it into two lengths to make a loop approximately 1.2 metres in diameter, removing the ends for termination.

I found a plastic enclosure, fitted cable glands and soldered the two ends for the Moebius configuration.

I used a Diecast aluminium box and fitted 3 N Connectors at the other end.

Then, I set about winding a 1:1 Balun from instructions I found on Peter Miles, VK6YSF website,

This was roughly potted with epoxy to support the Balun and wires from vibration.

Putting it together was larger than expected and I realised I needed to put it on some stand.

As this antenna would be used for a workshop at the library, I used a speaker tripod and 40mm PVC Pipe, which allowed me to pack it all down for transport in my car and assemble it again quickly.