> Perhaps we could focus on the antenna. What design shortcoming would
> cause a weak receive in this specific frequency range. It is not
there at
> less than 120MHz, and it is not there at more than 124MHz. Indeed!
It's not design shortcomings, but installation issues. A dipole is an
actual reference antenna, as a baseline to publish specifications for
any antenna (the spec is "dBd"). Ergo, you don't need an RF test
chamber to market one. Build it, and it will work!
This then means that any issue is installation location in the
airframe, which may require experimentation in different locations.
An antenna analyzer looking at any antenna expected to receive a band
of freqs shows what's happening. Suspend the test item well out in
the open and introduce like a long metal ruler near to it in various
orientations. Some locations are harmless; others cause bad behavior
which is frequency dependent...or not. Try different lengths of
stuff, as it can be a tuned element like the antenna. The analyzer
cannot detect radiation pattern problems which don't affect its
"reactance." That's a separate test, but implies also we can compound
our troubles where reactance is affected too. A virtually complete
null in a swath of direction can be demonstrated.
The condition/quality of the receiver is then important, if it's
expected to deal with common conditions affecting input signal
strength. If they say "2.0 microvolt sensitivity," that means nothing
(besides being a bit on the high side for Icom A-200 sales literature,
where they won't say "min" or "hard"). It seems like a tiny number
for just fringe reception purposes, but the actual tested number of a
given box dominoes upward to how well automatic squelch will work on
weaker signals, and requires auto squelch be well-designed, unless
maybe it actually can pull in .5uV at "6 dB S/N."
I've had breakfast every Tuesday since 1997 with a group which
includes a retired, aircraft antenna engineer. If I were to ask Bill
why something doesn't work -- sketch it out on a napkin, here's a long
control tube, here's a strobe wire -- he will say, "Because I guess it
doesn't!"
Reg,
Fred F.
|