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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