On 1/13 I posted the following queerie:-
"I am thinking of replacing a Mosely Pro 57b,which has been quite
trouble free since it was put up 5 years ago,with a Srike Force-5BA.
According to the available data this has a claimed gain improvement
of about 2 dB on most of the bands. The boom is only slightly longer
(33 ft. vs. 28 ft.) and the wind area is the same at 11 sq. ft. (This
BTW I find surprising given that the 5BA has 15 elements and the
Pro 57b only 7.) Any comments pro or con? Is it worthwhile making the
switch (the Pro 57b is at 80 ft.)? Anything to watch out for?
John N3HBX."
I received five replies. There seems general agreement that the low
wind resistance of the Force 12 antennas is due to the use of thin,
tapered elements. Because they must support the weight of the traps
these can't be employed with trap designs. The traps themselves also
contribute to the wind loading. There was less agreement over whether
making the swap is worthwhile. The Force 12 people argue that their
gain figures are clearly stated and supported by model calculations,
while other manufacturers often fail to state what assumptions they
have made (for example,on ground reflections) and/or are unsupported.
Others counter that the gain of a yagi is largely controlled by the
boom length, and while trap losses are undesirable,they don't amount
to 2 dB.
One is left wishing there were side-by-side comparisons of the older
trap designs versus the newer computer-optimized trapless ones.
Unfortunately, this is hard to do in a carefully controlled way -
especially at the elevations of interest. If I recall correctly,
the Bureau of Standards (now NIST) did some work on yagi designs using
scale models in anechoic chambers, but this was probebly limited to
measuring the free space gain vs.the number of elements, spacing etc.
I guess I am not much further along! John N3HBX.
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