transponder antenna on bottom of wing?

I've had my transponder antenna in two positions on the bottom side of the fuselage but have moved it because of the addition of a belly pod and a Service Bulletin for a new fuel drain. So now I'm thinking of positioning it out on the bottom side of the wing near the jury strut where there is an inspection cover.

Anyone else put a transponder antenna out there? Any reason not to? My avionics shop said they'd done this before on a hull type amphib.

Trial and error. It can work but may 'shadow' the transponder signal if the fuselage is between the antenna and the tower. In the case of belly pods, I've had good luck with the antenna moved forward, on the bottom of the boot cowl.

Also, make sure to allow for a ground plane for that antenna if you mount it in a rag wing.

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We moved mine from the boot cowl to the tail metal clean-out for Mode S issues but works well, no complaints from anyone and I have a pod.

You can see where I had it mounted originally on the belly skin. To allow the pod to go on, I moved it to the boot cowl, where I had access through a cover that opened up behind the gascolator. This is the only place on the boot cowl with good access and room for mounting an antenna and getting the coax to it. But the SB for the new fuel drain has it going to exactly the same place as I had the antenna. I actually used the same hole for the drain as the antenna had been penetrating.

I thought about the tail area but I'm concerned that rough strips and snow when I'm on skis would cause problems with it back there.

The leading edge on a CC EX has aluminum skins that come back about 6" under the wing. This looked like both a solid mounting area for a new antenna, with a doubler and located close to a rib, and also a decent ground plane. With the tank lid above much of this area removed, I also have good access to lay a new coax to the antenna.

I realize there may be some shadowing effect. But there is with gear legs and exhaust when it was on the boot cowl. The bottom of the wing seemed like a workable solution. But I wanted input from other Cub minds :smile:

Placing it out underneath near the wing tip will minimize the fuselage shadow. As I understand it this will require a special cable to minimize signal loss.

skywagon8a said:

Placing it out underneath near the wing tip will minimize the fuselage shadow. As I understand it this will require a special cable to minimize signal loss.

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Look in the install manual. RG400 or RG142 is usually good until around 20 ft of cable length. Beyond a stated length you need to step up to special (i.e REALLY expensive) triple shield cable.

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My Dec has it on the lower boot cowl. Only complaint I ever get is I am showing 100' low, and even then that is only half the time. Gremlins.

Someone may brush it off walking under the wing. Very few people get under that oil covered bottom.
DENNY

Those ball-end-rod txp antennae are very easy to break off.
Don't ask me how I know.

This should solve the head gouging issue. Place it in the belly behind your pod.


http://www.aircraftspruce.com/pages/av/antenna_transponder/antennasystems1.php

I put mine in the belly under the extended baggage and it works fine.

Sent from my SAMSUNG-SM-G900A using SuperCub.Org mobile app

skywagon8a said:

This should solve the head gouging issue. Place it in the belly behind your pod.

http://www.aircraftspruce.com/pages/av/antenna_transponder/antennasystems1.php

Click to expand...

I have the VHF-comm version of this antenna - works well.

So, to follow up on my original question, I mounted a blade type antenna on the bottom of the leading edge skin. This position seems to work fine and the skin is working well as a ground plane.

The reason for the blade rather than post type, which I did have, is because I also installed a Garmin GDL 82 UAT box as an ADS-B Out source. I had a Becker transponder with Mode C and the UAT box works in combination with the transponder and a WAAS GPS.

I haven't flown with it a lot yet but I'm getting an In uplink to my GDL 39 3D and I requested a performance report that shows it is putting out all of the ADS-B required information.

For anyone interested, I mounted the GPS antenna on top of the leading edge skin, close to the top of the windscreen.

How about a pirep on the GDL82?
Both installation and your experience with using it.
That one's on my short list when it's time to ADS/B-up.

hotrod180 said:

How about a pirep on the GDL82?
Both installation and your experience with using it.
That one's on my short list when it's time to ADS/B-up.

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The installation was straight forward. I mounted the UAT box high on the kickpanel on the same side as my existing Becker Mode C transponder. The coax that did go to the antenna, from the transponder, now goes into the box along with a coax out of the box to the blade antenna and a coax in from the new WAAS GPS. They say blade antennas work best with ADS-B.

The box proper has power in and ground. Two options are for a switched ground wire that puts the ADS-B Out into an anonymous mode when squawking 1200. The other option is a warning light if the unit isn't performing properly. Simple stuff to wire.

There is also a provision for a squat switch, that I didn't use. And there are pins for RS-232 WAAS GPS input if you already have a suitable GPS source. Or this unit will also feed position data to a GPS display if needed. None of this was needed for my installation.

The Out data is broadcast on 978 only. Some ADS-B transponders also broadcast on 1090. I think that's only important if you're planning to be above 18,000 feet or flying outside of North America.

The weight penalty is about 2.5 pounds for the box, additional coax and two antennas.

In use it seems to be working fine. As I mentioned I got into the FAA site and requested a performance report. It showed me it was outputting everything correctly. The avionics shop I bought this from ran tests too and it showed the same after they programmed it. And this includes the anonymous mode.

When I'm in an ADS-B tower coverage area I'm getting traffic uplinks to my GDL 39 3D/796.

The short answer it was easy to install and is performing as it should. I put this in an experimental and my avionics shop was willing to sell me the unit and let me install it. And afterwards they programmed it and ran tests.

spinner2 said:

So, to follow up on my original question, I mounted a blade type antenna on the bottom of the leading edge skin. This position seems to work fine and the skin is working well as a ground plane. The reason for the blade rather than post type, which I did have, is because I also installed a Garmin GDL 82 UAT box as an ADS-B Out source. .....
For anyone interested, I mounted the GPS antenna on top of the leading edge skin, close to the top of the windscreen.

Click to expand...

Another thought is that a "stub" or post antenna would probably have a very short life expectancy out in a vulnerable position like that.

Re antenna location(s), I believe the GDL82 manual goes into a lot of detail about it,
and suggested the "GPS-in" antenna not be too close to the windscreen.

I know their G5 manual calls out a spec for the GPS antenna,
can't remember but I imagine that their GDL82 manual probably calls out specs for the "out" antenna.
I'd like to stay with my simple post antenna,
but if I wouldn't be too surprised if I had to pop for a new blade type to meet Garmin's spec.