REFLECTOR: Transponder Signal Strength Anomaly - Recommendations for Replacement

Mon Mar 14 22:22:54 CST 2005

It seems that all the folks I have heard of trying the transponder antenna
at the end of the strake reported unsatisfactory results.  Not a
sufficiently "low-loss" cable, I suppose.  However, the vast majority of
folks who used a cheap "ball on spike" monopole transponder antenna out the
belly of the plane with a ground plane have had satisfactory results.

I doubt that you will even be able to detect the additional drag caused by a
transponder antenna - why fight it?

Keith

----- Original Message ----- 
From: "John Tvedte" <JohnT at comp-sol.com>
To: <reflector at tvbf.org>
Sent: Monday, March 14, 2005 7:58 PM
Subject: REFLECTOR: Transponder Signal Strength Anomaly -
RecommendationsforReplacement

> 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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