K9YC said:
"Further, on 6M, my primary interest is weak signal work, which is all
concentrated within a little over 200 kHz."
Jim, for solar cycle 22 I used YO to automatically optimize a 5-element
Yagi for forward gain, F/R, and SWR over 50.0 to 50.2 or 50.3 MHz (I
forget which). In free space it had 10.2 dBd forward gain and a
narrowband pattern with three equal lobes to the rear, each about 20 dB
down. At the time YO was calibrated to MININEC, which uncorrected has a
small frequency offset. On the air I noticed that the three backlobes
were unequal at 50.110. I used portable phone signals just below the
band to confirm that the equal-backlobe response was a couple hundred
kHz lower than expected. Shortening the elements and recalibrating YO to
NEC solved the problem.
One thing I noticed was that SWR would rise slightly when it rained. I
figured it was just water getting into the gamma match. Later I
understood that element tuning can change when wet. This turns out to be
a common issue with narrowband designs. Once Mike Staal, K6MYC,
realized this, he installed a sprinkler system to test his M-squared
Yagis for rain immunity!
Narrowband optimization can be exhilarating due to the relaxation of the
constraints that come into play at wider bandwidths. But you need to be
careful about environmental sensitivity. Otherwise, an automatic
optimizer (or an over-enthusiastic manual optimization) can lead you astray.
With the optimized Yagi and a single 3-500Z amp on a hilltop, I had a
lot of fun during cycle 22. One thing I will never forget is the massive
JA SSB pileup I generated one afternoon. There were so many signals that
it sounded like white noise. I had known theoretically that the sum of a
large number of independent signals approaches a Gaussian distribution,
but it was something else to experience it with human voices.
Brian
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