The Oblong Loop SW Antenna Project

Hi all,

my goal was an antenna, which works on the shortwave bands for DX and also on the lower bands. I have an attic space of 5 m width and 2.5 m height, and used it to span an oblong (since it is not square) loop, similar as described in "Rothammels Antennenbuch". The antenna connector is slightly off-center on the lower horizontal part of the loop, making the polarization mostly horizontally.

I was inspired by an article in the "Funkamateur" journal, issue 2-2016 , "Lowband-Empfangsloop fuer Normalfunker", about the good reception properties of loop antennas on the low bands (probably because of their low impedance on the low bands, or perhaps because they rely more on the magnetic field on low frequencies). I ran an xnec2c simulation with the nec file for the antenna (below), for the 15m band, including the ground effect, and found that it has a broad vertical beam, good for nearby contacts, but also 2 very flat beams with about 6dB gain. This is good for DX, since they are so flat. The 2 horizontal portions of the loop push the beams down, like stocked dipoles, despite the ground which "wants" to push them up (which happens for a single dipole).

When I then tried the antenna, I was indeed surprised how good it was at DX. With the small Ground Plane and the dipole antennas which I had in the same space before I only had European contacts, with the loop I regularly have DX contacts to America and Asia, despite the antenna being under the roof. This can be seen in the picture below, from pskreporter.info, which shows automatic reception reports (I used the digimodes JT65 and JT9) for a day for the 20m and 17m bands. I found the antenna works well on the 20m, 17m, 15m, 12m, and 10m bands, and also on 40m and 80m. Surprisingly, or rather as expected, it also works well on the lower bands (all with an antenna tuner), 40m works well with my automatic antenna tuner, for 80m I need a matching circuit for transmission, reception is quite good, and 160m I still have to try.

So all together this seems to be a small and powerful antenna for almost all our shortwave bands.

The simulation file for Xnec2c is for the antenna 4 m above a cylindrical building with 5m radius. It has hence 2 grounds, one for the building and one for further away. This is simulated with the 2 ground cards GN and GD in the xnec2c file.

Vy 73, Gerald, DL5BBN

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