In my opinion, Front-to-Back ratio is an overrated characteristic.
Like SWR, it IS something that can be easily measured, or at least
observed, by the user. High F/B is usually attained by sacrificing
a little gain (usually on the order of 1 dB).
Before an ice storm destroyed my 2 element Mosley Quad, I used
it as a multiplier hunting antenna in contests. I parked it due East
and could hear everything from Europe to South America off the front.
With only a moderate F/B ratio, I also heard and WORKED stations
(multipliers) in the Pacific off the back.
When I replaced that Quad with a TH7 which exhibited a much higher
F/B ratio, I could barely hear the pacific stations off the back and had
to switch to another antenna to work them.
I recall hearing a story about W0ZV 's experiences on 20M using
a KLM monobander one year and the KT34XA the next. Of course
the monobander had a much better(?) F/B than the KT34XA. As I
heard it, Bill worked about 10 more multipliers the year he used
the KT34XA with a "poor" F/B ratio.
On the WARC bands, I have around 300 countries per band, all worked
with DIPOLES or long wires which exhibit ZERO F/B.
For contest antennas, I like about 10 dB of F/B. That is enough to
tell there is directivity but still works about as well as a dipole off
Some contesters on the coasts like to have high F/B to attenuate
other USA stations when running over the water.
A little less F/B might also help to keep your run frequency clear.
It comes down to personal preference.
de Tom N4KG
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