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  #1  
Old 07-11-2017, 05:42 AM
sritchie sritchie is offline
 
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Location: Boulder, CO
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Default How to mount transponder antenna for proper grounding?

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:



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!
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  #2  
Old 07-11-2017, 05:49 AM
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Jesse Jesse is offline
 
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The lock washer and nut give an adequate ground.
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  #3  
Old 07-11-2017, 08:50 AM
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wirejock wirejock is offline
 
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Default Ground

Quote:
Originally Posted by Jesse View Post
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.
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  #4  
Old 07-11-2017, 09:26 AM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by wirejock View Post
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
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Old 07-11-2017, 10:01 AM
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Walt Walt is offline
 
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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.
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  #6  
Old 07-11-2017, 10:22 AM
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Mel Mel is offline
 
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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.
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  #7  
Old 07-11-2017, 10:27 AM
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Default Yep - that is what is says

Quote:
Originally Posted by Mel View Post
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
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  #8  
Old 07-11-2017, 12:13 PM
BobTurner BobTurner is offline
 
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Carl, while the impedance to ground may be low, capacitive coupling will throw in an undesirable phase shift.
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  #9  
Old 07-11-2017, 12:18 PM
David Paule David Paule is online now
 
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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
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  #10  
Old 07-11-2017, 12:22 PM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by BobTurner View Post
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/B00CJ37IQK...a-312080067105

Carl
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