This is a summary of the delta loops that the club put together for M0PCR's QTH in May 2014. Thanks go to G7RQD and 2E0VKZ for the assistance.
I wanted to put up some resonant antennas as I had for a long time been using a W3EDP (85' end fed with a 17' counterpoise) tuned with a T match and I wondered if loops would provide an improvement. The W3EDP had served me well for over a year, however on 15m and 17m the antenna was very twitchy and also gave rise to strong RF emissions in the house! I was keen to investigate loops having been recommended these by several members of the club since they can be easily hidden in trees and are low noise.
My shack is located 40' from where I wanted to hang the loops, which started me thinking about the best way to feed the loops. The traditional feeding arrangement being by dedicated coax with a 75ohm impedance transformer cut for the frequency of interest. G7RQD had seen 2:1 baluns used to feed loops from coax, which lead to the idea of using this to feed more than one loop reasonably efficiently.
Using the formula 1005/freq to give a loop length in feet as a starting point on 14.2MHz, a loop was soldered onto the balun terminals and checked out with an SWR bridge. Initial cut resulted in 2:1 SWR. The delta was fed at the centre bottom with the apex at the top. No particular care was taken to separate the two loops at the feed point, it is worth noting that others have noted increased interaction between the loops if spreaders are not used and this may be investigated on this antenna in future.
Before trimming the 20m loop a 17m loop was added (on 18.1MHz) and the SWR checked once again. After shortening of both loops by around 6 inches an SWR of 1.3:1 was attained on both bands. As further loops were envisaged (10 and 15) no additional trimming was undertaken.
Noise is nice and low on the delta loops - typically S1. So far only contacts made have been relatively local (southern Europe) however, I have heard a couple of VKs but couldn't make it through the pile up.
Thanks for reading.
M0PCR.