comm antennas
 

For my 73 P337 I am putting in an audio panel and 430 nav-comm-gps and given that my old radios didn't sound so good I wanted new coax and possibly antennas. What we found was that Cessna had installed a mickey mouse antenna switcher and only one antenna way back in the port fin.

Well, I am going to put in 2 new antennas and discard the antenna switcher and am trying to decide where to place them up front. I prefer not to put them on the belly as I may want to install a cargo pod and may go for the roof. But that brings up line of sight transmission worries to ground stations.

Can any of you comment on the location of your antennas, and my goal?

Dave Dillehay
N84E

GPS antennas have to go on top. Comm antennas normally are on the roof too away from the GPS antenna. I have seen a couple Skymasters with the Comm antenna on the wing just in front of the boom. The mounting out on the booms would stay away from the pressurized hull. I really thought of doing it to my plane when I had it all torn down last winter but I just had way too many other things happening.

Attached photos are comm antennas on the wings and I have two GPS antennas on the roof, in line (front to back)(the clecos in the picture are patches from my old comm antennas when they moved them for the GPS antennas. I had holes there that were never really covered correctly) and my 2 comm antennas are in front of the rear engine air intake scoop. The comm antennas are not exactly staged out parallel but that's life.

Hmmm... Picture no. 3 Looks like Oshkosh
 

The nice thing about the comm antenna being mounted like yours is that you can use them as a "gunsight" for backing your airplane in the hangar. It makes up for not having a center vertical stab.

Antennas
 

The antenna in the port fin is usually a VOR receiver antenna. Most companies have been sharing one antenna with two VOR's for years. They simply put a splitter in the coax line someplace before reaching the two VORs. You can get by with this because both radios are only receivers. The signal is weaker for each radio than two seperate antennas would provide, but still strong enough drive both receivers at a reasonable distance from the VORs.

Mine has the comm Antennas located on the top of the cabin (Where Herb shows Clecos shaping bases for two antennas). It is pretty rare to see two comm radios sharing one antenna. The transmitter fro one radio has a tendency to burn up the other radio's reciever in that configuration. It can be done, but requires special equipment to isolate one radio from the other. Two antennas are definately cheaper!

Dave, you might want to check and be sure you don't have a comm antenna in each fin. I found out ot my surprise that my plane did, so I had a spare antenna to use for my handheld. Also be sure and check with whoever is installing anything that punctures the pressure vessel. Some FSDO's require a DER sign-off with appropriate drawings, load analysis, etc before they will approve any new holes in a pressure vessel, and this will cost......a good reason to mount the antennas on the wings and route the coax thru existing wiring passages. Hopefully your folks are more reasonable, but the airplane wouldn't be airworthy in my area without the proper documentation of the basis for the new holes....