REFLECTOR: Transponder Signal Strength Anomaly - Recommendations forReplacement

John Tvedte JohnT at comp-sol.com
Mon Mar 14 21:58:02 CST 2005


Gary,

	I assume you mean a Bob Archer Sportcraft transponder antenna....?
	 
	The AV22 is a dipole antenna and will require a ground plane - btw, I don't think the antenna is the issue. I agree with your analysis about the engine/firewall blocking the signal.
	 
	I would suggest you consider moving the antenna to the end of the wing strake.  This will of course require removing one wing.  It will also very likely result in too long of coax run (even with RG400).  You can check the db loss requirements of the transponder you are using, and contact the folks below for a low loss cable custom made to the length you require.
	 
	http://www.ecsdirect.com/ecs_home.html
	 
	The factory suggests a dipole (like the AV22 suggested) - personally, if you want to go that route - I'd just use a TED antenna.  If you install per the manual - you should be fine with the coax length - however, I'd use RG400.  Still - I'd suggest putting this in the end of the strake.
	 
	Some info on tuned ground planes from Jim Weir in response to a question I asked:
	-----------------------
	From: Jim Weir (jim at rst-engr.com)
	Subject: Re: tuned ground plane 
	 
	  
	View this article only 
	Newsgroups: rec.aviation.homebuilt
	Date: 2004-11-19 08:22:07 PST 
	 
	A "tuned" ground plane is one that is an odd multiple of a quarter wave at the
	operating frequency.  Odd -- one, three, five, and so on.  In practicality (with
	the possible exception of GPS) the norm is ONE quarter wave, or a quarter-wave
	groundplane.
	Having said that, you must understand that you cannot make a "perfect" circular
	("disk") ground plane for a transponder, as it must operate on two frequencies
	simultaneously -- 1030 and 1090 MHz.  What is quarter wave for one is not
	quarter wave for the other.  While the error is slight and relatively
	insignificant for transponders, it DOES become significant when you start to
	talk about things like the aircraft COM band, where the instantaneous bandwidth
	is on the order of 11%.
	My chosen way to make a transponder ground plane is to start off with a square
	of aluminum using the following calculations:
	w = 11810 / f
	s = 0.487 w
	where "w" is a quarter wave in air at frequency "f"
	and "s" is the length of one side of the square of aluminum.
	Then punch a hole in the exact center of the groundplane for the antenna.
	Then cut the corners of the square to make a regular octagon.
	SOMEWHERE along the periphery of that octagon will be an exact quarter wave at
	both 1030 and 1090 MHz. IF you select w to be halfway between these two
	frequencies (i.e. 1060 MHz.).
	So, in the above scenario, w = 11.14" and s = 5.42".

	Jim
	 
	Some additional comments:
	 
	(A) 'w' is mis-identified.  This is a _full_ wavelength in air, not a quarter.
	    Assumes frequency in mHz,  gives result in inches.
	    only accurate to 4 sig figs -- good enough for most 'practical' work.
	    Use 11811.02 [ (300 million meter/sec) * (inches/meter) /  one million ]
	    for 'high precision' "in vacuum" and then correct for propagation in air 
	    vs. in vacuum (299702547/299792458), giving 11807.45775338017 <grin>
	(B) the 'mysterious' 0.487 is a combination of two things.
	    1) we want a 1/4 wavelength _radius_, so the "diameter" will be 1/2 wave.
	    2) we want the 'minimum' dimension of the octagon to be resonant at 
	       1090 mHz, while we did the 'w' calc at 1060 mHz.
	    3) guess what 1060/1090, _divided_by_2_ is?  <grin>
	  (0.48623853211009+, for those without a calculator handy)
	Thus, from the center, to the _middle_ of any side  is a 1/4 wave at 1090 mHz.
	And, the distance from the center of the octagon, to a 'corner' is enough 
	longer than the distance from the center to the 'middle' of a side (a factor 
	of 1.0823922002+ [sqrt(1**2+(sqrt(2)-1)**2), if anybody cares) that the 1/4 
	wave resonant frequency on the 'diagonal/ is 1007 mHz.
	Thus, 'somewhere' between the middle of a side, and the corner, the length
	will be 'right' for a 1030mHz 1/4 wave.

		-----Original Message----- 
		From: reflector-bounces at tvbf.org on behalf of Gary T. Ciampa 
		Sent: Mon 3/14/2005 8:32 PM 
		To: reflector at tvbf.org 
		Cc: 
		Subject: REFLECTOR: Transponder Signal Strength Anomaly - Recommendations forReplacement
		
		

		All,
		
		I'd like to describe an anomaly that occurs with my current transponder
		and solicit recommendations on a solution.
		<snip>



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