A Loop on the Balcony
After a year with the wire I built a small magnetic loop to see whether the claims about noise rejection would hold up in a concrete building. The loop is a metre across, made from a length of 20 mm copper water pipe bent round a plastic bin as a former, with a gap at the top where a small amplifier box sits, and a feed of coax down to the kitchen. It is mounted on a broom handle in a plant pot full of sand so that I can turn it.
A loop of this kind is a receive-only antenna that responds mostly to the magnetic part of the signal, and a lot of local electrical noise is predominantly electric field at close range, which is the theory behind it being quieter. It is also directional, with deep nulls off the sides of the loop, so a noise source or an unwanted station can be rotated away from.
The comparison with the wire was not the clean win I had hoped for. On the lower bands, 75 and 60 metres, the loop was clearly better: the noise floor dropped by two S-units and signals that were marginal on the wire became readable. On 31 and 25 metres the two were about equal, with the wire pulling in slightly stronger signals and the loop slightly less noise, so the readability was similar. On the higher bands, 19 and 16 metres, the wire was simply better; the loop is small relative to the wavelength there and the amplifier adds its own hiss.
The nulls turned out to be the most useful feature. The neighbours’ evening buzz comes from roughly east, and by turning the loop so that its side points that way I can reduce it sharply without losing the stations to the south and west that I most want. On one evening in November I used the null to separate two stations on 9,650 kHz that were on top of each other on the wire; rotating the loop forty degrees took the stronger one almost out and left the weaker, which turned out to be a Brazilian station with a football commentary.
So the loop stays, for the low bands and the evenings, and the wire stays for the daytime and the higher bands. I now have two antennas on one balcony, which the neighbours have not commented on yet.