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Transponder antenna ground plane

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kesleyelectric(at)iowatel
Guest

Posted: Sat Oct 16, 2021 8:56 am Post subject: Transponder antenna ground plane

Greetings,

 

I’m preparing to install the ground plane for the transponder antenna on my steel tube fabric covered plane.  It would be more convenient to mount the transponder antenna ground plane on the inside of the fuselage, on the bottom behind the baggage compartment.  This would require a 7/8” thick spacer so the antenna would mount flush with the fabric.  If I machined such a spacer from solid aluminum and made sure of low resistance connections between the spacer and the ground plane, would this adversely affect the antenna performance?

 

Any advice appreciated.

 

Tom Barter 

    	 		 			 				 			 		 		 			

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user9253

Joined: 28 Mar 2008
Posts: 1908
Location: Riley TWP Michigan

Posted: Sat Oct 16, 2021 1:26 pm Post subject: Re: Transponder antenna ground plane

Read this thread:

http://www.matronics.com/forums/viewtopic.php?t=16762861

I would mount the transponder antenna to a 5.3 inch diameter ground plane.

Make sure the outer braid of the coaxial cable is electrically connected to the ground plane.

The antenna ground plane does not need to be electrically connected to the tubular steel airframe.

Wood could be used between the ground plane and airframe steel tubes to support the antenna where desired.

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Joe Gores

Ceengland

Joined: 11 Oct 2020
Posts: 379
Location: MS

Posted: Sat Oct 16, 2021 1:36 pm Post subject: Transponder antenna ground plane

On 10/16/2021 11:55 AM, Tom Barter       wrote:

     

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Greetings,         

          

I’m             preparing to install the ground plane for the transponder             antenna on my steel tube fabric covered plane.  It would be             more convenient to mount the transponder antenna ground             plane on the inside of the fuselage, on the bottom behind             the baggage compartment.  This would require a 7/8” thick             spacer so the antenna would mount flush with the fabric.  If             I machined such a spacer from solid aluminum and made sure             of low resistance connections between the spacer and the             ground plane, would this adversely affect the antenna             performance?         

          

Any             advice appreciated.         

          

Tom             Barter

                

     	
     Have you considered just flipping the assy over, so the xpndr spike     points up? Fabric skin does little to attenuate RF, so if you can     avoid punching another hole, why not? There might be a little     'shadowing' of the signal if you're too close to an aluminum baggage     compartment, but I doubt you'd ever know without taking the plane to     an 'antenna range' with a lot of expensive measuring hardware. I've     heard of people putting them inside cowlings, and the uAvionix tech     recommended that I mount their Echo UAT box, the WAAS GPS, and the     antenna spike all on a plate within the fiberglass wingtip of my     RV6. Obviously, there's much more shadowing by the wing rib than     anything you'd see, and he told me that they have numerous examples     flying that way.

     

     To answer your actual question, extra aluminum on the back side of     the ground plane should have no effect on the antenna's performance.     Think of it as a much fatter, stronger 'bulkhead', as you would see     inside aluminum skinned a/c near the xpndr antenna mount.

     

     FWIW, 

     

     Charlie

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