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Tube and Fabric Ground Plane

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junk(at)jaredyates.com
Guest

Posted: Mon Dec 28, 2009 6:51 am Post subject: Tube and Fabric Ground Plane

Dear group,



I'm building a Bearhawk and would like to find the best way to mount the ELT

antenna, especially regarding the ground plane.  I'm using the Ameriking 450

and the whip antenna that came with it.  I can use the aluminum wing as a

ground plane for the VHF com, but I was thinking that it would be better to

keep the ELT antenna on the fuselage.  This would allow me to reduce the

length of the wire run and thus reduce the chances of crash damage.  It

would also allow me to provide enough distance between the com and ELT to

prevent resonance on the emergency frequency.



Some builders have mounted the antenna inside of the 4130 tube fuselage,

which has the best crash resistance; but, I would think that the fuselage

would be a great faraday cage and would thus limit the transmission

considerably.



For these reasons I'd prefer to mount the antenna on the top of the fuselage

behind the cabin.  In this particular fuselage there are non-structural

stringers that protrude about 3 inches above the primary structure, so I was

planning to make a bracket to use as a mount for the antenna.  The bracket

would be shaped like an upside down U with the top of the bracket even with

the top of the stringers, so that the antenna would attach at the top of the

bracket and the bottom legs would be welded to either side of a crossing

tube.  How should I provide for a ground plane?  The stringers are aluminum

and about 12 inches apart, and the steel fuselage is about 3" below the base

of the antenna.  Would it be a good idea to use either of those as part or

all of the ground plane, or would it be better to make something out of

copper foil and try to insulate that ground plane from the airframe parts?

Any thoughts?



Here's a picture of how the fuselage is built:

http://jaredyates.com/temp/groundplane.jpg



Thanks

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Dec 28, 2009 10:05 am Post subject: Tube and Fabric Ground Plane

At 08:31 AM 12/28/2009, you wrote:

 	  Quote: 	
		  

Some builders have mounted the antenna inside of the 4130 tube fuselage,

which has the best crash resistance; but, I would think that the fuselage

would be a great faraday cage and would thus limit the transmission

considerably.


	


    Yes. Don't do this . . .

 	  Quote: 	
		  For these reasons I'd prefer to mount the antenna on the top of the fuselage

behind the cabin.  In this particular fuselage there are non-structural

stringers that protrude about 3 inches above the primary structure, so I was

planning to make a bracket to use as a mount for the antenna.  The bracket

would be shaped like an upside down U with the top of the bracket even with

the top of the stringers, so that the antenna would attach at the top of the

bracket and the bottom legs would be welded to either side of a crossing

tube.  How should I provide for a ground plane?


	


    Make sure you have good electrical connection to the bracket

    that is welded to the tube. This can be accomplished by mate

    up pressures of attaching hardware even when intermediate layers

    include insulating materials.



 	  Quote: 	
		    The stringers are aluminum

and about 12 inches apart, and the steel fuselage is about 3" below the base

of the antenna.  Would it be a good idea to use either of those as part or

all of the ground plane, or would it be better to make something out of

copper foil and try to insulate that ground plane from the airframe parts?

Any thoughts?


	


    Foil ground plane strips on the inside of the fabric

    would be useful but probably wouldn't increase

    performance greatly. The BIG design problem is

    one of mechanical robustness. Getting your antenna

    mounted to solid structure through your proposed

    bracket will go a long way  toward a low maintenance

    and adequately performing system. Of course the feedline

    shield needs to get a good electrical connnection to

    the same bracket.



    Bob . . .

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