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How to mount transponder antenna for proper grounding?

sritchie

Well Known Member
Hey folks,

Simple little question here. I'm going to mount my transponder antenna today and noticed that, like in this picture, there's a rubber gasket that will fit between the antenna and the skin of the airplane when installed:

s-l1000.jpg


Do I need to remove this to get a solid connection between the antenna and the skin for a grounding plane? Or is the idea that the lock washer and nut against the doubler INSIDE the plane provide enough contact for the grounding plane?

My intuition's failing me here. Thanks all!
 
Ground

The lock washer and nut give an adequate ground.

Yes, but you may need to clean the area down to bare metal. If the washer can't cut through the paint, it won't make contact to establish ground. Check with an ohmeter to be certain.
 
Yes, but you may need to clean the area down to bare metal. If the washer can't cut through the paint, it won't make contact to establish ground. Check with an ohmeter to be certain.

Not required or desired. This just creates a future corrosion area.

While I suspect the star washer will provide a DC ground, the antenna does not need or care if there is a DC ground or not. It wants an RF ground. Metal that is clamped together like this provides the coupling needed for an RF ground. Think of the plates in a capacitor. They are not touching but RF is conducted though it. While a simplistic example (and yes I do know how this works and the limitations) perhaps it will help understand.

Nomex suit on for the anticipated flames.

Carl
 
From the "Bible" (43-13, 2B) and what most vendors recommend.

307. ANTENNA BONDING.
a. The electrical bonding of the antenna to the aircraft surface is extremely important. The conductive skin of an aircraft is an electrical part of the antenna system. If an antenna is not properly bonded to the aircraft, its pattern may be distorted and nulls in coverage may appear.

b. The electrical bonding of the antennas to the aircraft skin of a metal aircraft is best accomplished by direct metal-to-metal contact of the antenna base to the skin. A resistance of no more than 0.003 ohms between the antenna base plate and skin should be achieved.
NOTE: To achieve this electrical bonding, the aircraft paint in the mounting area will need to be removed and the surface covered with an oxide film (i.e., aluminum conversion coat) to protect aluminum against corrosion in accordance with MIL-C-5541B.

c. An alternate method for providing electrical bonding to metal aircraft skin is through the antenna mounting screws, which attach to a backing plate inside the aircraft, making electrical contact with the backside of the skin. To ensure good contact, remove any interior paint in the area where the backing plate is placed and coat this area in accordance with MIL-C-5541B to minimize corrosion.
 
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.
 
Yep - that is what is says

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.

And if I was going to write a sailor proof set of instructions, it would sound just about the same.

Carl
 
Carl, while the impedance to ground may be low, capacitive coupling will throw in an undesirable phase shift.
 
It's possible that the gasket will compress, allowing good electrical contact between the rim of the antenna and the skin of the airplane, while still sealing the hole. If so, then the skin should be prepped locally as Walt described.

Dave
 
Carl, while the impedance to ground may be low, capacitive coupling will throw in an undesirable phase shift.

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/B00CJ37IQ...t=&hvlocphy=9008197&hvtargid=pla-312080067105

Carl
 
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/B00CJ37IQ...t=&hvlocphy=9008197&hvtargid=pla-312080067105

Carl

Yes, and the measured SWR of this type of ant. is usually rather poor.
 
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.
 
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?
 
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.
 
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.
 
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.
 
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?
 
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.
 
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:
 
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.
 
You can rarely go wrong when following manufacturer instructions. When all else fails, follow the directions.
 
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.

Yep. There are thousands of composite homebuilts flying that don't have the transponder antenna ground planes (often aluminum discs) bonded to aircraft ground...and their transponders work fine.
 
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