Transponder antenna location?

[gregpro50 64]

I want to ad a transponder.  For the people that have one, where is your antenna mounted and how is it mounted?

[KFfan 151]

Mine on a mod 2, bought it built, is on top of the cabin just forward of a cross tube above the seat back. It is mounted about 4" from the ELT antenna. Not good at all.

I've got to rearrange antennas and add a third for comms. Thinking of putting a plate under the fabric and on top of the fuselage for that one.

Lou

[marksires 484]

Mine is under the floor, center of the cabin, sticking down.  Had to get a 90degree adapter so I could connect the coax, it is right under the support that goes between the pilot and copilot.

[Geoffrey Thorpe 31]

I have a Merlin GT which is similar but different than the Avid - but I put a piece of aluminum on the inside of the bottom fabric behind the baggage area, attached it to a couple cross tubes with Adel clamps and stuck the antenna in the middle. The antenna is attached to the plate and sticks through the fabric. The only issue is that it needs a long cable to get from the transponder to the antenna. Seems to work just fine.

[nlappos 413]

Mine is a cheap ($22) dipole from Ebay, mounted on the belly between the pedals, a little toward the right side, and the coax drops right down from the panel above. That belly area under the cockpit is aluminum, so there is a fine groundplane. I put the altitude encoder on top of the unit. The unit is an Edo Aire RT-777 I bought on Ebay for about $100. Works like a charm, it tests strongly at least 40 miles from the radar site.

Edited 14 Dec 2015 by nlappos
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[dav8or 20]

I added a transponder a couple of years ago. I went with a Trig transponder which has a black box that I mounted under the passenger seat. From there the antenna cable run was not bad to a spot on the bottom of the fuselage about a foot and a half behind the seat. I have a small plywood deck there that the battery is mounted to. I cut a fair sized round hole for access through it then cut a 6" square piece of aluminum thick enough to hold its shape. Cheap Ebay dipole antenna is mounted to that with a small hole melted through fabric for the antenna to pass through then sandwiched with the nut from the outside. I used some 1/2" material as a spacer between the aluminum and the plywood deck which is mounted on top of the fuselage tubing. Hope that makes sense. The shop that tested the transponder said it has a very strong signal.

 

Dan

Avid Model C Speedwing

[gregpro50 64]

I need some help with the ground plane.

I made up a piece of 8" diameter .040" aluminum to use as the ground plane.  I was thinking I would install it in the belly right behind the header tank.

Can I just adhere it to the fabric with some RTV sealant or similar?

Does the ground plane actually need to be physically grounded to the metal airframe or can I just center it in the middle of the fabric?

[KFfan 151]

Typical antenna installation has the ground, braid, of the cable attached to frame for a ground. The "ground plane" must also be attached to the metal of the air frame to be effective.

 

Lou

[gregpro50 64]

I found this picture on the internet.

So this type of ground plane would not be effective because it is not grounded to the aircraft frame?

Isn't the coaxial cable itself grounded through the transponder?

Edited 22 Dec 2015 by gregpro50

[Knuckledragger 135]

I found this picture on the internet.

So this type of ground plane would not be effective because it is not grounded to the aircraft frame?

Isn't the coaxial cable itself grounded through the transponder?

Most likely.  The ground plane is bonded to the transponder via the cable shielding - that's the important part.  If the return for power feeding the transponder is bonded the same way (most likely is) then the ground plane is bonded to the air frame.

Edited 22 Dec 2015 by Knuckledragger

[wypaul 448]

The ground plane will work just fine without bonding to the airframe.  I used the same setup in my Q2 which is a composite aircraft for many years with flawless results.  Just attach the shielding of the coax to the "ground plane" and the center to the "antena".

[dav8or 20]

My ground plane is the 6" square piece of aluminum which is isolated from the fuselage. The antenna is grounded to the ground plane through the mounting nut that holds it in. There is some interesting reading out there on ground planes including size and shape and it does not necessarily have to be a sheet form of metal.

I am not sure I would attach it directly to the fabric without having additional support of some kind. Think about what would happen if the antenna catches debris or gets hit by something. Mine is held against the fabric but secured to a plywood platform that is mounted across the fuselage tubing for support. Just something to consider. I fly from rough runways occasionally with tall grass that can hide debris and I worry about catching anything with the antenna which should be on the bottom for best reception.

Edited 22 Dec 2015 by dav8or
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[gregpro50 64]

Can the antenna be inside the fuselage or does it need to protrude from the bottom? 

[dav8or 20]

Can the antenna be inside the fuselage or does it need to protrude from the bottom? 

If it is inside the signal could be reduced by the fuselage I would think. I have my ELT antenna mounted inside for now until I find a better location but I am risking my signal not getting out as far as it could in an emergency (or am I protecting the antenna from crash damage?).

[gregpro50 64]

Well I have my transponder installed and all wired up.  Now I just have to run the cable from the transponder to the antenna.  I had to order some RG400 cable.

[gregpro50 64]

I figured I might as well post up some pictures of the install if it helps anyone out in the future.

I got a great deal on a used but like new KT-76A that someone was selling because they did an upgrade.  It came with the tray, harness, and encoder.  I was able to get the wiring diagram for the transponder and encoder online so the wiring was pretty easy.

I made up a ground plane from .040" scrap aluminum I had laying around at the shop.

I mounted the encoder to the top of the radio with industrial Velcro.

The encoder was tied into the static port.