John:
What Gramin sez is perfectly true. The apparent cause seems to be the
antenna. There are, besides VSWR meters, a new development which amateur
experimenters will be acquiring (or have). The devices are called
Antenna Analyzers and are becoming more common. They not only give you
the VSWR at any frequency, but search out and display the Resonant
Frequency of your installation, along with several other qualities. This
would simplify the testing of your anteena immensely, and permit
accurate adjustment if necessary.
One other facet has not been mentionned. The polarity of your antenna.
As a standard, all aviation VHF antennae are vertically polarized. That
is, they stand up vertically (usually an electrical quarter-wavelength
for simplicity). If you have installed one horizontally, it will lose
about 20 decibels of signal strength by being cross-polarized - a
function of its design. This would mean a loss of signal from marginal
distance aircraft, but less noticeable from ground stations because of
their greater output power.
Hope this helps as well,
Happy Landings,
Ferg
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