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-   -   How to mount transponder antenna for proper grounding? (https://vansairforce.net/community/showthread.php?t=151289)

rvbuilder2002 07-11-2017 12:47 PM

Quote:

Originally Posted by Carl Froehlich (Post 1187050)
No. Does not work that way at all.

Antennas with no DC connection to ground have been around for decades and I have used many of them HF and up. Here is a simple example of an antenna that is magnetally held to your metal car - so there is paint and a protective plastic cover between the antenna base and the car metal: https://www.amazon.com/dp/B00CJ37IQK...a-312080067105

Carl

Yes, and the measured SWR of this type of ant. is usually rather poor.

flightlogic 07-11-2017 04:45 PM

Ground it to bare metal. I make my living doing this. Coupling theory will result in very poor VSWR. I measure with mili-ohm meter. Our composite helos get metal mesh and bonding straps to ground also. Transponders are not cheap. Don't trust off the wall theories.

sritchie 07-11-2017 07:19 PM

Quote:

Originally Posted by flightlogic (Post 1187115)
Ground it to bare metal. I make my living doing this. Coupling theory will result in very poor VSWR. I measure with mili-ohm meter. Our composite helos get metal mesh and bonding straps to ground also. Transponders are not cheap. Don't trust off the wall theories.

Okay, so to clarify it sounds like I can:

* leave the rubber gasket on
* make sure that my doubler isn't primed, ie down to the bare metal, at the point where the lock washer makes contact
* probably NOT prime the doubler in this case, so that I get a good electrical connection between the doubler and the rest of the aircraft skin?

Sounds like that's going to achieve the best results here, while allowing that rubber gasket to provide some friction.

Does this sound like a good distillation?

flightlogic 07-11-2017 08:39 PM

Sounds good. If you really want to, you can seal the base after installation but most don't need to. Depends on your environment. The transponder is a high power pulse type transmiiter. You should still have it tested and logged in airframe logs. Good luck.

BobTurner 07-12-2017 01:05 AM

Quote:

Originally Posted by flightlogic (Post 1187173)
Sounds good. If you really want to, you can seal the base after installation but most don't need to. Depends on your environment. The transponder is a high power pulse type transmiiter. You should still have it tested and logged in airframe logs. Good luck.

Not 'should' have it tested; you must have it tested, every two years.
You can prime the doubler, as long as you keep paint out of the rivet holes (the rivets will make the electrical connection to the skin) and where the coax braid goes to ground.

rv7charlie 07-12-2017 06:44 AM

The ground path for that antenna is: threads of the body of the antenna>nut>toothed washer>mounting surface. The reason for the toothed washer is both for locking, and (hopefully) to bite through any insulating coating on the aluminum mounting surface. Paint a piece of scrap aluminum, drill it for the antenna, mount & torque the nut, then remove & look at the painted surface that was under that toothed washer.

Worrying about the rubber washer is getting off in the weeds, from an electrical standpoint; that side of the mount doesn't really participate in the grounding of the antenna. As others pointed out, it *is* intended to keep water out of the joint.

You can use a dielectric paste (helps keep out moisture/corrosion on the back side) on the nut/washer side of the joint, but unless you live near the ocean it's probably overkill.

slngsht 12-22-2017 05:50 PM

I have zero experience with this... searching to learn. Could we not run a ground wire with eye connector to the thread of the antenna?

BobTurner 12-22-2017 10:18 PM

Quote:

Originally Posted by slngsht (Post 1227298)
I have zero experience with this... searching to learn. Could we not run a ground wire with eye connector to the thread of the antenna?

No. You want the coax?s braid-shield electrically attached to a ?ground plane? (usually the aircraft skin around and under the antenna) with as short a connection as possible. Lengths matter. The ground plane does not actually have to be attached to aircraft ground, although it usually is.

GalinHdz 12-23-2017 06:40 AM

Quote:

Originally Posted by Mel (Post 1187019)
I agree 100% with Walt. This is exactly how we were taught in "Antennas and Transmission Lines" in the USAF.

Granted this was 53 years ago, but these "basic rules" haven't changed.

+1

The laws of Physics have not changed since, forever! Because something "works" doesn't mean it is correct.

:cool:

slngsht 12-23-2017 06:43 AM

Quote:

Originally Posted by BobTurner (Post 1227335)
No. You want the coax?s braid-shield electrically attached to a ?ground plane? (usually the aircraft skin around and under the antenna) with as short a connection as possible. Lengths matter. The ground plane does not actually have to be attached to aircraft ground, although it usually is.

Thank you for the clarification.


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