Re: F-B = 22 dB =/- 5 dB.
Reading spec sheets on multi-element designs can be a bit trying, and a
curve of the F-B would be nice from any antenna manufacturer.
In general, Yagi designs can optimize the F-B to have narrower limits than
+/- 5 dB, or they can optimize gain and cast F-B to the winds of
fortune--or they can compromise. On such designs, multi-element Yagis
will tend (but not absolutely always) to show a rise in gain across the
band, with a F-B peak somewhere in the middle of the operating bandwidth.
If we slide that F-B peak down in frequency a bit, we tend to get a bit
more gain with a lower F-B at the upper end.
There may be some modification of this generalization for multi-band
Yagis. A set of curves is therefore the best--both of gain and F-B. If
everyone asked, I'll bet more producers might supply them to potential
buyers. When you compare both gain and F-B curves for various antennas,
you can learn some important things about the design philosophies of the
various producers and better match your wishes to their products.
One producer makes available computer models of some of their products,
and you can get a sampling of their design ideas from these. You can also
model some antennas from spec sheets of those who have one. Model very
carefully and you can see what the design is driving at, even if your
model cannot match every last tenth of a dB of the producer's product.
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