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. 

Paul,

 

Thanks for a more technical explanation.  Hmmmmm, the old swept-back nav antenna was a center tap arrangement running the width of the small horizontal - maybe about 4 feet or about half the wavelength - The push pull tubes are not at right angles since the antenna "wings" are swept back just like the external whiskers often observed on those funny "standard" tin airplanes - but, on a good approach, the durn thing has to look thru the people in the cockpit, the engine and the carbon in the main wing spar, about 3 degrees down from the antenna placement.  Oh well, works pretty good for me......

 

My biggest problem is the comm antenna in the vertical tail - the nulls are greatest radiating along the aircraft centerline. Bummer.