In a message dated 11/23/2006 1:07:56 P.M. Central Standard Time,=20 elippse@sbcglobal.net writes:

<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>

An antenna in free-space may have the nice toroida= l,=20 donut-shaped pattern, but when mounted on some vehicle, its pattern may be= =20 distorted, leaving holes or nulls in various directions. These nulls may b= e=20 caused by blockage, but more often due to nearby metallic objects which=20 re-radiate the signal at some phase angle which may be destructive.=20 Consider, for example, elevator and aileron push-pull tubes and wiring. Wh= en=20 these are of a length that is an apprecible fraction of the average 105"=20 wavelength, and are within a wavelength, and somewhat parrallel, interesti= ng=20 things happen to the toroidal pattern; it's as if your cat was taking bite= s=20 out of the donut.

If you have fibreglass wings, t= he LE=20 of the wing root is a good mounting place for the=20 horizontally-polarized nav signal. A signal-splitter has an inherent=20= loss=20 of dividing the signal power to each receiver, so if you have two receiver= s,=20 each will receive slightly less than 1/2 power, a 3dB loss. That shouldn't= be=20 too bad for a close-up localizer. A tip-off that destructive interference=20= is=20 taking place is that the reception changes with frequency, some=20 channels being better than others. As far as GS is concerned, that's=20= a=20 much higher frequency, so it should not have coincident loss of signal. Th= at=20 would seem to be more in line with some blockage. Are the nav and GS anten= nas=20 coincident in mounting? You might also check each coax for poor connection= s=20 that allow leakage at the connector which also can be destructive, as well= as=20 possibly a shorted coax. Get hold of a SWR tester and check these=20 lines.

Thanks for a more technical explanation. Hmmmmm, the old swept-ba= ck=20 nav antenna was a center tap arrangement running the width of the small=20 horizontal - maybe about 4 feet or about half the wavelength - The push= =20 pull tubes are not at right angles since the antenna "wings" are swept back=20= just=20 like the external whiskers often observed on those funny "standard" tin=20 airplanes - but, on a good approach, the durn thing has to look thru th= e=20 people in the cockpit, the engine and the carbon in the main wing spar, abou= t 3=20 degrees down from the antenna placement. Oh well, works pretty=20 good for me......