Transponder antenna replacement questions

[LANCECASPER]

My transponder antenna was cracked when I bought the airplane four years ago and it looks worse now so I'm going to replace it (Comant CI 105-16).



1. I noticed that it has some black RTV? at the base of the BNC connector to seal up the opening. Is there anything special about that sealant or will black RTV suffice?




2. I took the antenna off without verifying whether the angled edge or the straight end goes toward the front of the airplane. The instructions on the new one with probably show, but thought I would check in here first.

[47U]

22 minutes ago, LANCECASPER said:

My transponder antenna was cracked when I bought the airplane four years ago and it looks worse now so I'm going to replace it (Comant CI 105-16).
1. I noticed that it has some black RTV? at the base of the BNC connector to seal up the opening. Is there anything special about that sealant or will black RTV suffice?
2. I took the antenna off without verifying whether the angled edge or the straight end goes toward the front of the airplane. The instructions on the new one with probably show, but thought I would check in here first.

Presumably, the Comant comes with a base gasket?  And mounted on a belly panel?  I don’t think any RTV is required.  

I think convention has the slanted side forward, but, being a Mooney…. why not go with the straight end forward.  This is the (Mooney) way.

[LANCECASPER]

6 minutes ago, 47U said:

Presumably, the Comant comes with a base gasket?  And mounted on a belly panel?  I don’t think any RTV is required.  

I think convention has the slanted side forward, but, being a Mooney…. why not go with the straight end forward.  This is the (Mooney) way.

No base gasket on the antenna when i took it off. This is presumably the first time it's been off of this 2008 airplane. Just a little what looks like RTV around the BNC connector on the inside of the belly pan. 

I figured slanted side forward, but yes it is a Mooney . . lol.

[Aerodon]

I think it is missing the ground plane, my plan is to use some adhesive backed copper foil and make sure there is a ground wire to the airframe somewhere?

Aerodon

 

[N201MKTurbo]

18 minutes ago, Aerodon said:

I think it is missing the ground plane, my plan is to use some adhesive backed copper foil and make sure there is a ground wire to the airframe somewhere?

Aerodon

 

Having the ground plane grounded to the airframe doesn’t really matter, but having it grounded to the transmission line ground is very important.

Without a proper ground plane, the antenna will use the outside of the coax as the other element, which will make the antenna very directional.

Is it possible that the panel the antenna is mounted to is carbon fiber, and the sealant is conductive and coupling the ground to the carbon fiber?

https://geraldartner.com/files/Artner2017EuCAP.pdf

https://shop.boeing.com/cpd/bdi_pr2200b1%3D5n#UkRfQkRJ

 

 

[LANCECASPER]

1 hour ago, N201MKTurbo said:

Having the ground plane grounded to the airframe doesn’t really matter, but having it grounded to the transmission line ground is very important.

Without a proper ground plane, the antenna will use the outside of the coax as the other element, which will make the antenna very directional.

Is it possible that the panel the antenna is mounted to is carbon fiber, and the sealant is conductive and coupling the ground to the carbon fiber?

https://geraldartner.com/files/Artner2017EuCAP.pdf

https://shop.boeing.com/cpd/bdi_pr2200b1%3D5n#UkRfQkRJ

 

 

Yes I believe it is carbon fiber. 

Would something like this work?

https://www.amazon.com/Sciplus-Electrically-Conductive-Soldering-Wire/dp/B000Z9H7ZW/ref=sr_1_3?dib=eyJ2IjoiMSJ9.UR8fFjy6Vhj5xcjIDLKYU5corRdnorhp2iis3FuQ64Wz0x8JaF0vjktsnyHT0MfrnxeiIQeX9dakn0upTlPQZEGIff3s8I1WUppb4xedr-Onhv9l-fnXVHlcyIN34l-RdwBut3dYCWQVg1Z1gEL8JdR0xPxPdFYkWPVS45PB4UEEnrDnj4HWU_EfIUU95EK2304HKGO3LQOi-oqHM2-iXlt0BITdPpB4BqI3rd8jcug.4XEqF4ICzKbhXL0mZSM_NSESbIXSEvKRTE22euqunKQ&dib_tag=se&hvadid=176357318768&hvdev=c&hvexpln=0&hvlocphy=9028236&hvnetw=g&hvocijid=9973164865548719596--&hvqmt=e&hvrand=9973164865548719596&hvtargid=kwd-322856970440&hydadcr=25300_9775880&keywords=electrically+conductive+sealant&mcid=c10e626ad1923fff9c68c0c94806dd51&qid=1763339959&sr=8-3

 

 

[1980Mooney]

5 hours ago, LANCECASPER said:

2. I took the antenna off without verifying whether the angled edge or the straight end goes toward the front of the airplane. The instructions on the new one with probably show, but thought I would check in here first.

 

4 hours ago, 47U said:

I think convention has the slanted side forward, but, being a Mooney…. why not go with the straight end forward.  This is the (Mooney) way.

Slanted side facing forward.  From a 2020 topic with informative comments by Clarence / former M20Doc. 
 

 

Edited November 17 by 1980Mooney

[EricJ]

You may be able to tell whether that sealant is conductive with a multimeter.   Just put the two probes in the sealant on opposite sides of the fitting and see what you get.   I'm guessing there's a conductive layer or conductive screen in the fiberglass which will serve as the ground plane, or a reinforcement plate for the antenna or something.  If you're worried that there isn't sufficient ground plane, +1 that a layer of copper or similar tape on the inside will do the job as long as it is electrically connected to the shield part of the fitting.   The bolt studs are probably part of the shield, so that may work, too.   I doubt it would have made it out of the factory or had very good performance if there wasn't sufficient ground plane there already.

[N201MKTurbo]

4 hours ago, LANCECASPER said:

Yes I believe it is carbon fiber. 

Would something like this work?

https://www.amazon.com/Sciplus-Electrically-Conductive-Soldering-Wire/dp/B000Z9H7ZW/ref=sr_1_3?dib=eyJ2IjoiMSJ9.UR8fFjy6Vhj5xcjIDLKYU5corRdnorhp2iis3FuQ64Wz0x8JaF0vjktsnyHT0MfrnxeiIQeX9dakn0upTlPQZEGIff3s8I1WUppb4xedr-Onhv9l-fnXVHlcyIN34l-RdwBut3dYCWQVg1Z1gEL8JdR0xPxPdFYkWPVS45PB4UEEnrDnj4HWU_EfIUU95EK2304HKGO3LQOi-oqHM2-iXlt0BITdPpB4BqI3rd8jcug.4XEqF4ICzKbhXL0mZSM_NSESbIXSEvKRTE22euqunKQ&dib_tag=se&hvadid=176357318768&hvdev=c&hvexpln=0&hvlocphy=9028236&hvnetw=g&hvocijid=9973164865548719596--&hvqmt=e&hvrand=9973164865548719596&hvtargid=kwd-322856970440&hydadcr=25300_9775880&keywords=electrically+conductive+sealant&mcid=c10e626ad1923fff9c68c0c94806dd51&qid=1763339959&sr=8-3

 

 

I’ve never used that stuff, so I can’t help much. I would check the service manual and if there isn’t anything in there, I would contact Mooney.

[LANCECASPER]

On 11/16/2025 at 9:55 PM, EricJ said:

You may be able to tell whether that sealant is conductive with a multimeter.   Just put the two probes in the sealant on opposite sides of the fitting and see what you get.   I'm guessing there's a conductive layer or conductive screen in the fiberglass which will serve as the ground plane, or a reinforcement plate for the antenna or something.  If you're worried that there isn't sufficient ground plane, +1 that a layer of copper or similar tape on the inside will do the job as long as it is electrically connected to the shield part of the fitting.   The bolt studs are probably part of the shield, so that may work, too.   I doubt it would have made it out of the factory or had very good performance if there wasn't sufficient ground plane there already.

I tested with a multimeter this morning, no conductivity, so I am going to use a little bit of black RTV, which is what it looks like they used just to seal up the gap between the BNC  and the hole drilled. Thanks for the help

[PT20J]

The Mooney belly is fiberglass. My antenna is mounted with the sloping side forward as is usual practice, but electrically it doesn't matter. Mine has an aluminum ground plane on the inside attached only by the two mounting studs. Use star washers under the nuts for good electrical conductivity. I haven't removed it, so I don't know if it has RTV around the hole or not. I also never measured the ground plane, but from memory it's about 8" square. Exact size should not be important.

[N201MKTurbo]

4 minutes ago, PT20J said:

The Mooney belly is fiberglass. My antenna is mounted with the sloping side forward as is usual practice, but electrically it doesn't matter. Mine has an aluminum ground plane on the inside attached only by the two mounting studs. Use star washers under the nuts for good electrical conductivity. I haven't removed it, so I don't know if it has RTV around the hole or not. I also never measured the ground plane, but from memory it's about 8" square. Exact size should not be important.

The radius of the ground plane needs to be greater than the height of the antenna. More than that has very little advantage.

[LANCECASPER]

4 hours ago, PT20J said:

The Mooney belly is fiberglass. My antenna is mounted with the sloping side forward as is usual practice, but electrically it doesn't matter. Mine has an aluminum ground plane on the inside attached only by the two mounting studs. Use star washers under the nuts for good electrical conductivity. I haven't removed it, so I don't know if it has RTV around the hole or not. I also never measured the ground plane, but from memory it's about 8" square. Exact size should not be important.

Mine does not have an aluminum plate underneath, but the instructions say that "if the skin is not metallic but made from a composite material, a metal mesh such as Astrostrike material just below the composite material skin may be used as a ground plane." (https://www.astrosealproducts.com/expertise)

The closer that I look at the backside of the belly pan it looks like that's what they have done. They also had a black rtv/silicone material around the gap where the hole was drilled and the BNC connector fits. It feels "harder" than typical rtv., so to make sure I'm going clean off any of that around the hole so I get a clean edge and then use a little of this conductive product to seal that gap: https://www.amazon.com/dp/B0FHW1G85J?ref=ppx_yo2ov_dt_b_fed_asin_title

It also says that "antenna lightning protection can be improved by grounding the coaxial shield with an appropriate metal mounting clamp at any convenient location before the receiver is connected to the CI 105–16 antenna". I am guessing they have done that since it is in the instructions. 
It was time for replacement:
 

[CCAS]

4 hours ago, LANCECASPER said:

It was time for replacement:

My antenna is in about the same shape as your old one. Where did you end up sourcing your new replacement? 

[Marc_B]

I had my transponder antenna go out and both Garmin and my shop thought it's "usually not the antenna" but it was.  Fortunately I had an orphaned antenna beside it on the belly maybe from DME previously?  Switched it and moved on.  But the Garmin antenna shows an "optional antenna kit" which looks like just a grounding plane and mounting hardware.

FWIW, my antenna did have a ground plane but didn't have any sealant/RTV.

[1980Mooney]

6 hours ago, LANCECASPER said:

Did the antenna blade separate from the base when you were dismantling it?  Or was it fractured before you removed it?   It looks like a lateral fracture - as if something hit it from the side.  Or some ham handed line crew or mechanic pushed on it sideways. Possibly an errant tug slipped under the plane And whacked it?  

[PT20J]

One more thing: Do not power up the transponder with the antenna removed as running without a load causes energy to be reflected back into the final amplifier and can damage it. The old tube types may survive this but the newer solid state transponders claim to be more sensitive. I always pull the transponder breaker when I have the belly panel removed.

[LANCECASPER]

2 hours ago, CCAS said:

My antenna is in about the same shape as your old one. Where did you end up sourcing your new replacement? 

Aircraft Spruce   https://www.aircraftspruce.com/catalog/avpages/comant10516.php

[LANCECASPER]

57 minutes ago, 1980Mooney said:

Did the antenna blade separate from the base when you were dismantling it?  Or was it fractured before you removed it?   It looks like a lateral fracture - as if something hit it from the side.  Or some ham handed line crew or mechanic pushed on it sideways. Possibly an errant tug slipped under the plane And whacked it?  

It was cracked when I bought the airplane. I noticed it on the pre-buy four years ago when I was crawling around underneath it. But I noticed last week that the crack was now much worse than cosmetic, so I ordered a new one. I had to pull it off to see which model it was and once I did it was in two pieces.

[Schllc]

I have changed two of these in the past, both times they came with gaskets, and the ones i removed had them as well, no sealant or adhesive.  

I reinstalled the same way and never had any issues

[LANCECASPER]

22 hours ago, Schllc said:

I have changed two of these in the past, both times they came with gaskets, and the ones i removed had them as well, no sealant or adhesive.  
I reinstalled the same way and never had any issues

The one I removed was original and had never been off and had no gasket. There was still the tape line from when the airplane was painted. and no paint under the antenna, as you would expect.

Also no gasket was included with the new antenna. 

All I did was put new star washers under the nuts on the studs and then put a small amount of sealant around the BNC connector just like the factory had in the original. I used the sealant that is conductive so that the BNC connector and the embedded mesh in that small area of the belly pan can make contact. 

[Jerry 5TJ]

If the belly is non-conducive fiberglass then you could install the transporter antenna on the aluminum airframe under the belly skin.  

[LANCECASPER]

1 minute ago, Jerry 5TJ said:

If the belly is non-conducive fiberglass then you could install the transporter antenna on the aluminum airframe under the belly skin.  

After I cleaned out the old sealant that had been around the BNC connector and looked closely  I could see strands of the mesh that's embedded in that area. It has worked well this way since 2008 so I didn't want to re-invent the wheel.