> ...sometimes we can hear / be heard
> from a good distance away
> but on occasions, we can't hear Rochester until we
> are within about 5 miles of them (122.25Mhz).
If at all times off the nose, or same azimuth point elsewhere, it
could be a simple matter of which of their transmitters they're using.
They need not have the same xmit power, but if an installation is
performing poorly, above can happen. When you have squares and logs
in the math, seemingly small matters can then cause extreme results!
Like get this. At only 100AGL over really flat terrain, theory says
at 5 miles you should hear ATC if they were using a 100milliwatt
transmitter! Something's not working well at all.
> 1) the aerials gain isn't over the required range -
It's the loss, not the gain. Only a directional antenna can have gain
of practical use. There is a small, omnidirectional gain of 2.2db due
to pure physics. If there's greater than 2.2 in some directions,
other points around the circle _must_ have a loss. But if a loss in
one direction, there may not be a gain anywhere else, depending upon
what a nearby metal element is doing (like changing the signal's
polarization).
> this could easily be fixed by
> a more conventional aviation aerial
The radiating pattern of a whip antenna pointed down is somewhat
better than a dipole. However, more significant is where it's located
within a fuselage. External to the fuselage, and any problems go
away.
> 2) The squelch is either over enthusiastic or being
> triggered by noise...from the engine, GPS etc
GPS can't do anything to VHF, but engine or strobe etc. noise is
nothing but audio carrier modulation which makes no sense as spoken
words. Same as ATC with talent as a voice comic mimicing an
alternator whine into the mic. So squelch may break, and we hear
noise. It won't decrease ability to receive carrier, but rather as a
combined signal makes carrier stronger -- in a cheap circuit, squelch
can be broken. So Narco in its 810 uses a fancy circuit which
attempts to determine if words are being received. They call it
"squelch comparator and carrier override." IOW, ain't voice, so let's
ignore the stronger, noisy carrier and nothing is heard in the audio.
Works great!
Reg,
Fred F.
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