By Kelvin Brayshaw
For the last few years I’ve been dabbling with lengths of wire that ended up becoming full-wave HF loop antennas. My ‘experiments’ have been based on the ‘cut-and-try’ method, and I’ve usually had a dipole in the backyard to compare results. If the loop exceeded, or filled gaps in the dipole’s performance, it had a good chance of surviving for a few months, or until the next bright idea came along.
Various magazine articles stated that loops could provide good reception at low heights, didn’t mind being close to other physical objects and tended to have a low noise level. As a move to a smaller property in a few years was a real possibility, this seemed to be the way to go, and subsequently I was very interested to read Cliff Couch’s DX Times article on his experiences as an SWL in a retirement village setting.
In the early 90’s I tried (without much success) a half-wave, open, delta loop during an 80 metre Ham contest. More recently I’ve had better results, as an SWL, with horizontal full-wave 40 metre and vertical 20 metre loops. The latter was made using the outer braid of 50 ohm coax for the whole antenna, and gave quite adequate results across a number of bands.
My latest construction owes more to Cliff Couch (for its shape) and Paul Ormandy (for its termination) than to any technical skills on my part. “It” started life as two separate 40 metre full-wave loops, mounted under the eaves of the garage (timber). PWO told me about the possibility of rectangular loops having some directional properties, so I had a feed on one corner, and a termination to ground at the opposite corner, through a variable resistor, on the 90º/270º diagonal, and the other on the 150º/330º axis.
While they were showing directional trends along their axes, they weren’t giving much improvement over anything the dipole (with 1:1 baluns at each end of the coax) could produce in signal reception or noise rejection. Sooner or later they had to face the inevitable.
What has resulted is a “double loop” fed at one corner (90º bearing) with coax, the inner going to loop ‘A’ and the outer to loop ‘B’. The opposite corner of the loops (270º) are parallel terminated (i.e. the loops are not “open”) to ground through individual 5k ohm variable resistors, which are adjusted to match the frequency.
There are enough permutations of this arrangement to keep even the most avid tinkerer happy for hours. I’m content for the moment to keep the pots set around 300 ohmss, and enjoy better signals for on 16 & 20 metres than my dipole is able to deliver.
The loops are spaced 20 cm apart and average 2.4 metres above ground-level. With the right coloured wire they’re virtually invisible to the neighbours!