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An antenna in free-space may have the
nice toroidal, donut-shaped pattern, but when mounted on some vehicle, its
pattern may be distorted, leaving holes or nulls in various directions. These
nulls may be caused by blockage, but more often due to nearby metallic objects
which re-radiate the signal at some phase angle which may be destructive.
Consider, for example, elevator and aileron push-pull tubes and wiring. When
these are of a length that is an apprecible fraction of the average 105"
wavelength, and are within a wavelength, and somewhat parrallel, interesting
things happen to the toroidal pattern; it's as if your cat was taking bites out
of the donut.
If you have fibreglass wings, the LE of
the wing root is a good mounting place for the horizontally-polarized nav
signal. A signal-splitter has an inherent loss of dividing the signal power to
each receiver, so if you have two receivers, each will receive slightly less
than 1/2 power, a 3dB loss. That shouldn't be too bad for a close-up localizer.
A tip-off that destructive interference is taking place is that the reception
changes with frequency, some channels being better than others. As far as
GS is concerned, that's a much higher frequency, so it should not have
coincident loss of signal. That would seem to be more in line with some
blockage. Are the nav and GS antennas coincident in mounting? You might also
check each coax for poor connections that allow leakage at the connector which
also can be destructive, as well as possibly a shorted coax. Get hold of a
SWR tester and check these lines.
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