I currently have dual Nav/Comms that share one antenna for the Nav side.  This includes VOR/LOC/GS functions.  The antenna coax goes to the input of a diplexer and the two outputs each go to one of the Nav receivers.  The nav/comms are Apollo SL30 which have built in recievers for VOR/LOC/GS.  The problem I've seen since I acquired the airplane has been poor performance with the LOC & G/S from either nav indicator.  VOR from both is good however.

 

Has anyone noticed degraded performance using a single antenna for VOR/LOC/GS functions?  Are there any special diplexers that could be used to improve degraded performance? or possibly a need to install a seperate antenna for each receiver?  I'm not sure what kind of Nav antenna the builder used, but it is installed in the horizontal stab.

Steve,

 

The glide slope operates a quite a different frequency than VOR/LOC (LOC 109.5 MHz has the paired GS operating at 329.6 MHz) - thus, the same antenna (VOR/LOC) is not the best match for it.  On the other hand, one is usually within 15 NM of the GS transmitter and a less efficient antenna may be ok.  Usually the nav radio would have a separate antenna connection for VOR/LOC and GS.

 

Maybe the problem is that each receiver is looking for 2 inputs and you probably have three splitters (one to feed each radio and then one at each radio to split for the VOR/LOC and GS).  There is a signal loss for each connector and each splitter (even more as the connections age without cleaning occasionally).  Try this experiment (if you have the coax length to do it), take the antenna lead and connect thru only one of the splitters for one radio and see if the performance improves.  Also, you could have a bad connector or bad splitter in the mix too.

 

I have only one nav radio (inside the Garmin 430) that is fed directly by the VOR/LOC antenna in the horizontal stabilizer and the GS is fed by the 12 inch center tapped foil antenna located in the most outboard leading-edge part of the right stub-wing D-section. 

 

The VOR is only a seldom used backup for the GPS.  Nothing is better than an ILS (LOC-GS) approach ---- Uh, well until my 430 becomes a 430W...........