For what it's worth, I had a 2-element Force 12 40M yagi on a 100-foot
tower. That antenna had identical elements, linear-loaded and a hairpin
match, and was tough to optimize.
What I finally wound up doing was putting a receive antenna about 200
feet from the tower and maye 30 feet up from the tower base. Then W2GD
went up the tower and adjusted the jumpers on the reflector. It started
out actually backwards, but in a couple of tries he managed to get the
F/B optimized in the CW band, and at that point we called it a day, or a
year, or an era (hi). Not theoretically perfect, for sure, but in
combination with Phillystran top guys that antenna turned out pretty well.
73, Pete N4ZR
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On 5/23/2021 4:43 PM, Lux, Jim wrote:
On 5/23/21 1:31 PM, Mike Zak wrote:
Is there a paper, article, YouTube video, Powerpoint deck, chapter in
a book, website, message-in-a-bottle, or anything else, that clearly
and succinctly describes the trade-offs and interactions between yagi
gain, pattern, and match?
A 15 minute session with EZNEC, and a candidate yagi design, is
plenty adequate for showing that there’s some serious systems
engineering involved in coming up with the right set of trade-offs,
starting with “which problem are we trying to solve?” But few of us
are ever going to capture the ideal design and then fabricate it. So
maybe if there was some insight around these tradeoffs we could all
do a better job with our selections of commercially available products.
Did W4RNL ever do anything along these lines? Or W2PV? How about the
Telrex archives? Anyone have any suggestions for something that is
readable by mere mortals?
Mike, W1MU
I don't think so. I think it's what you say, using a modeling code and
iterating to see what happens.
Kraus talks about how Yagi-Uda's work, but not much about optimization.
Orfanidis's book has examples, and matlab code to do a quick modeling
that does the interelement coupling, then computing the pattern -
that's where I started, because I'm fairly facile in Matlab.
Lately, I've been running models using Python and NEC 4.2 to try and
come up with a "grass roots" understanding of trapped multiband
antennas (using the ARRL antenna book example, with 32 feet inside
trap, 22 ft outside trap (on one side) and 8.2 uH and 60 pF for the
trap).
Do you want the trap resonance "in band" or "out of band". In band
gives better pattern performance (and potentially better efficiency),
but is less tolerant of changes in frequency or trap component tolerance.
Similar questions about "tuning components in middle of element" - an
inexpensive RCS-8V would potentially give you 32 capacitor values in
the middle of an element.
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