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Antennas

whittfic

Well Known Member
Patron
I have searched through the threads on this, and while there is plenty of info and opinion, much of it varies and is sometimes contradictory to the point that my head is starting to spin!

So what I am looking for is some direction with regard to the best locations for a single bent whip (Commant) comms antenna, Commant transponder antenna and a Rami AV200 whip ELT antenna. Aircraft is RV6 slider with Lyc 0-360 and fixed pitch prop. There is a Whelen Strobe power unit mounted under the baggage floor.

Equipment being installed is Garman GTR 225 comm, Garmin 327 transponder with Trans-Cal SSD 120 NE encoder and Kamad 406 AF-Compact ELT.

My current thinking is:

- ELT antenna on top of rear fuselage about 2? forward of the vertical stabiliser
- Comm antenna mounted under pilot seat about 6? inboard from the edge of the fuselage (midway between seat ribs)
- Transponder antenna bottom centre of the fuselage about 12? aft of the rear baggage area bulkhead (not sure if this will be adversely affected by exhaust plume or relative proximity to the Whelen power unit)

I have conduits running both sides of the fuselage that will be hidden by upholstery panels. Stbd side conduit is used for electric trim wiring while the Port side is clear, so I?m hoping there will be no electrical interference problems running antenna and PTT wiring through this (Port side) conduit.

Last question: Best place to mount the ELT?? I?m tossing up between:

- On the floor behind the passenger seat? (in the way?)
- Behind the rear baggage bulkhead? (access to unit?)
- In the tail under the fibreglass stabiliser fairing? (access and w/b issues?)


So what do the electrical gurus think? Am I on the right track here?

Clive Whittfield
Auckland
New Zealand
RV6 still building but getting closer
 
My commant bent whip is in centerline right about the forward lower seat attach hinge. I get good transmit/receive to the tower and planes in the pattern when on the ground. In the air I get great performance of over 30 miles at pattern altitude and over 60 miles at 3000 ft agl. Of coarse MO is flat. I get a little exhaust and breather oil on the antenna but not much.

Transponder antenna located on centerline about a foot aft of firewall. I seem to get strong pings registering on my SL70 from STL class B on the ground at home airport 10 miles from STL as well as in the air 60 miles out (maybe more as I have never really checked).

My opinion is ELT antenna needs to be external and on aft fuselage to best survive the crash and get best transmit performance. ELT antenna for me is located on top aft fuselage a few inches aft of max aft position of slider. ELT itself needs to be back there as well to stay connected. Mine is on RH sidewall of baggage area. Really do not like it there as it is in the way when laying down to get head under the panel for maintanence. I usually only take RH seat out to get under panel since I usually have RH stick out.
 
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A transponder antenna does not need a large ground plane (radius commensurate with length of antenna). I saw other RVs had it mounted near a fuel tank vent. I put mine near the left vent. It keeps the feedline short and simple.
I have had no indications of operation difficulty with ATC.

I have two Com antennae. A bent whip under the passenger seat and a straight one on top about mid fuselage. The bottom antenna is on my more powerful transmitter.

Traditionally, I use the top antenna for ATIS and Ground and then switch to the other after runup for tower and departure and remainder of flight. Occasionally (not often) I have to go back to the top antenna because tower communication is weak. (Meaning they do not hear me and their response is low volume).

My current assumption is that I am getting some blanking or transmission pattern distortion by the wing or landing gear (A model) and the randomness comes from how I am pointing the craft relative to the tower antenna. Operation in the air has been no different than other craft I've flown (who can really understand ATC anyway :confused:)

My ELT is an in-cabin 406 (EBC) that does not require the remote control panel. Even though I did not do it, I agree with Phils commentary on ELT antenna placement.
 
There seems to be a difference between belly antennas on tail and nose draggers. Must be a lot more blanking of signal by the fuse and wing with conventional gear. On my 7 I could never get good ground reception. In the air it was great, but at certain airports on the ground I would lose comms altogether. Bent whip was left side, between ribs about 6" behind the spar.
With help from the forum I was able to get some improvement but never enough. In the end I took the easy way out (for a tip up ) and put a straight whip behind the canopy.

My thought would be, if you really have to use a belly mount, find someone with a 6, not a 6A, who has good reception and copy his antenna position exactly.
Good luck.
 
....Last question: Best place to mount the ELT?? I?m tossing up between:

- On the floor behind the passenger seat? (in the way?)
- Behind the rear baggage bulkhead? (access to unit?)
- In the tail under the fibreglass stabiliser fairing? (access and w/b issues?)

.....

How about option one, modified?

UNDER the floor behind the passenger seat? (now not in the way)

Just make an access panel in the baggage floor...
 
Good thought Gil. I already have an access panel in the floor of the baggage area so tried the ELT there but unfortunately it wouldn't quite fit. It's close but....:( There is a little more room further foward but this would require the floor to be taken up and even then I'm still not sure if there would be enough room.

I am leaning more toward the second option. I'm mounting a remote switch on the instrument panel so access to the unit wont be that much of an issue.

As for the transponder antenna, I have re-thought that one and will mount it lower fuselage opposite side to the comms antenna.

Thanks for the input guys, much appreciated.
 
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