Figure 1: Hybrid 4:1 voltage balun / 1:1 current balun used in loop.
I have been playing around with loop antennas for a number of years, and have found that they are gratifying to use for receiving antennas, because they are so quiet. But, it seems that they are often lacking when it comes to using them for transmission purposes.
Recently, I decided to raise one of my loops from its original height of 25 feet, adding an additional 30 feet, putting it just short of the three quarters point on my North Carolina gum tree at 55 feet. When the loop antenna was at 25 feet, it was much flatter of course, and it was cut for twenty meters. I raised it and lengthened it to have a forty meter fundamental. Even with the apex raised, the bottom is still not high (maybe twelve feet on one end), so I did not necessarily expect a big improvement in the transmitted signal. A first week of reports seems to indicate results better than expected.
There was a substantial increase in the RST reports received after raising the loop. For instance, when the loop was in the low position, reports typically were in the range of 54/55/56 with an occasional 57/58/59. Since the loop has been raised, reports are trending towards 56/57/58/59. Another thing I noticed is that received reports are now more likely to be on par with those of the station I’m contacting. So, I guess the old saw is correct, when it advises to put it up as high as you can. A modest improvement in height of only 30 feet has been worth the little bit of time required to perform the task.
Antenna theory tells us to expect trouble with ground losses on certain types of antennas when they are put too close to the ground. I think that partially plays into the observed improvment, but I think there’s a gain increase as well. My antenna is patterned after the W1FB antenna mentioned in QST of October 1984, authored by W1FB and W1SE. Doug (W1FB) wrote that the theoretical launch angle for the antenna is ten degrees, and on harmonics of the fundamental, such as 20 and 15 meters, the gain appears to be 6 db in the favored direction.
The W1FB antenna was listed at 50 feet, so this loop should be on par with his. His antenna fundamental was cut for 80 meters. So, if you have a low loop and are unhappy with the results, see if you can add a modest height increase, and maybe you’ll be as happy with the results as I. Note that the box in the graphic is a hybrid 4:1 voltage balun / 1:1 current balun from BalunDesigns (IIRC model 116t) – 73’s WB8LZR
Added 12/24:
With the original loop, I had a piece of ladder line between the balun and the wire of the loop. It was changed to various lengths, and finally I settled on 42 feet, the latter of which worked well and avoided the quarter wavelength multiple problem. The SWR at 7.075 MHz was 1.0, and I had less than 1.5 to 1 SWR without using a tuner on all bands higher than 80 meters except 30, 17 and 12 meters, which would be expected. The SWR on 10.120 MHz was 2:1, on 18.075 MHz it was 3:1 and on 24.9 MHz it was also 3:1. I could tune all of those warc bands (1:1) with a tuner, and had QSOs on all those bands using the loop antenna. But, in the final analysis I removed the ladder line altogether as it seemed to make for more noise.
The ladder line was light and easier mechanically to attatch to the antenna, allowing the heavier balun to be attached to a tree. However, I ended up with the balun actually on the antenna, and like the results better. The SWR for all bands remained unchanged (or changed by less than .1) after the removal of the ladder line. I have the balun secured with its own lanyard.
WB8LZR 