For the second night of the ARRL contest, I set up
three receiving loops with a switch to compare them in real
time. A Pixel circular loop (10 foot perimeter) using the
30 dB Clifton Labs preamp that comes with it was the first
antenna. The second one was a square 20 foot perimeter
shielded loop as I described in the Sept./Oct. 2009 National
Contest Journal:
http://www.n6rk.com/loopantennas/NCJ_loop_antenna_N6RK.pdf)
The third antenna was a square 40 foot version of the loop
in the NCJ article. No external preamp was used with the 20
and 40 foot loops. The receiver was a Yaesu FT-1000 with its
internal preamp turned on. All three loops were oriented
to null at 30 degrees azimuth on ground wave, which
minimizes the noise at this QTH.
After comparisons on dozens of stations in the contest,
the clear winner was the 40 foot loop. In general,
the noise levels of the Pixel and the 40 foot loop
were similar. However, the latter had 6 to 10 dB
more signal, resulting in a markedly improved S/N ratio.
There were numerous stations where a signal that was Q5
on the 40 foot loop was inaudible on the Pixel.
I ended up just leaving the switch on the 40 foot loop
for the balance of the contest. If I had to have only
one loop to use, it would be an easy decision to go with
the 40 foot one.
The 20 foot loop had signal levels comparable
to the Pixel, but the noise level was considerably lower.
It came in 2nd place. The signal level of the 20 foot
loop has always been marginal for the FT-1000 without
external preamp. It might help to have some preamplification
on it. I would rather use a larger loop than fool with
a preamp. We have strong BCB signals, including one at
1,700 kHz, so preamps are always problematical.
The 40 foot loop will work (on 160 meters only) with an
unmodified antenna circuit board, however, I ran into
BCB QRM of the tuning diodes. I removed one of the two
tuning diodes, which allowed the tuning voltage to rise.
This helped on the BCB QRM, but there was still some lite
hash heard. Finally, I went to four diodes in a series
parallel configuration as described in the article. This
fixed the BCB QRM. The final configuration tunes from
about 1.25 to 2.5 MHz, covering WWV.
I also compared to the transmit vertical. In most cases,
stations could be copied on the vertical, but maybe not
at 100%, and even when they could be copied, there was
the usual unpleasant impulsive noise, as compared to the
gentle rushing noise of the loops. From a listening
fatigue aspect, it is not good to have to listen on the
vertical. I have no technical theory to explain why
this is, I just observe it consistently.
The Pixel loop covers all the way to 30 MHz. It still
may be a useful receive antenna on the higher bands.
When I purchased it for evaluation, I suspected that
160 meters was not its strong suit. Below 160 meters
on BCB, the signals are much stronger and the Pixel should
have no problem. Thus the take away is only that
the Pixel isn't very useful on 160 meters.
The mechanical construction of the 40 foot version is
much more difficult that the 20 foot version. My initial
attempt was successful enough for these tests, but was
basically a discovery of what NOT to do. It did become
clear that it is much easier to guy the vertical support
rather than making it self supporting. I am interested
in hearing about successful large loop construction
techniques.
Rick N6RK
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