Mosely Pro 57b vs. Force 12 5BA

[John.Evans@comsat.com]

     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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