This will probably be the last of my RX antenna experiments for quite a
while. It's been warm and the grass is growing again. Mowing season is here
and temporary antennas have to go. I have been meaning to try a K9AY loop
for some time and finally managed to squeeze it in before the mowing
started. The results of this experimental installation are both impressive
and disappointing.
The loop was constructed and installed as per the original QST article.
Only a single loop was used to keep things simple and facilitate data
collection. The termination resistor was 390 ohms as suggested for optimum
160 use and was not changed during testing. A relay was used to switch
direction. The loop was installed on the front lawn in an area that has
previously shown to be relatively free from interaction with the other
antennas on the property.
The first thing that struck me was the ease of installation. With a single
26 foot wood pole with a pulley on top, the loop was put up in a matter of
minutes. The two loop model would be a little trickier, but not much I
suspect. Of course, "There ain't no free lunch". With the area required for
the guys and stakes, this thing is a hazard to foot traffic and vehicles
and impossible to mow around. However, at my previous QTH, in the open
desert, it would have been a godsend!
The second big plus is the relatively high output level. It is very much
more than a Flag and perhaps even enough to eliminate the need for a preamp
in some installations.
Certainly not least of all the all the advantages is the relay direction
switching. Having two directions from a single loop (four from the two loop
model) instantly available is fantastic. It sure beats having to rotate my
"Rotatable Flag". In the two loop model, this would be a real plus for
contesting.
A/B switching is the only sure way to compare on-the-air performance. This
time I had the luxury of A/B/C/D/E switching with the flag, a small loop,
and two shorty beverages to compare to. (TX antenna comparisons with RX
antennas are worthless and won't be done.) And now we are back to the "no
free lunch" thing.
F/B was not as good as the flag or the beverages. It was measured at about
14 dB for local BC stations, power line noise, and skywave signals. Being
used to the 30 dB null off the back of the flag, I found this to be
disappointing. It was pointed almost directly away from my noisy power
lines, to the SE. Used for the PW0T and others down that way, it never gave
me the S/N that I had with either the flag or the beverages. It was always
better than the small loop. I feel that if the F/B was better, it would
have been equal to the flag, but with that power line behind it, not so.
K9AY suggested that I could improve the F/B by adjusting the termination
resistance, but I ran out of time and was not able to try that experiment.
F/B was even worse on 80, but that was expected since I used the resistor
specified for 160. Perhaps relay selection of terminating resistances for
each band would be something to try.
This antenna has a lot going for it, and while the poor F/B as I installed
it will keep me from using it at this QTH, I think it has great potential.
As with all RX antennas, you don't know for sure until YOU try it, your
mileage may vary!
One final note: In general, I believe that compact RX antennas should be
optimized for ground wave rejection off the back, not sky wave. Small
antennas are most attractive in urban small lots where local noise
dominates the propagated noise by 20 to 40 dB. It makes no sense to me to
reject stuff that you can't hear anyway because of local noise. Just my 2
cents worth.
I would be interested in hearing about others experiences with the RX
antennas I've reported on. With all the possible variations in construction
and installation, a single users data is not always significant. Taken
together, many different inputs can be very valuable.
73,
Larry - W7IUV
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