REFLECTOR: Transponder Signal Strength Anomaly
-Recommendationsfor Replacement
John Tvedte
JohnT at comp-sol.com
Tue Mar 15 08:12:15 CST 2005
RG 142 and RG400 have very similar attenuation -
RG142 is approx. 19db @1Ghz / 100 ft.
RG400 is approx. 17db @1Ghz / 100 ft.
RG58/U is approx 16.5 db @ 900Mhz /10 ft - it will be very similar to
the above @ 1Ghz.
Of course this is one spec - the materials these are made of make RG142
and RG400 a better choice for aircraft.
ECS is listed as a specialty coax supplier in the GTX-330 install
manual.
ECS 311601 is good for 17 ft. 8.6 db @1Ghz / 100 ft.
ECS 311501 is good for 21 ft. 7.0 db @1Ghz / 100 ft.
ECS 311201 is good for 27 ft. 5.5 db @1Ghz / 100 ft.
ECS 310801 is good for 41 ft. 3.5 db @1Ghz / 100 ft.
In the GTX-330 example, @ 1090Mhz, the cable attenuation must not exceed
1.5db
Gary - please check the specs on the transponder you are using to
determine it's requirements.
John
-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Jim Agnew
Sent: Tuesday, March 15, 2005 7:14 AM
To: Velocity Aircraft Owners and Builders list
Subject: RE: REFLECTOR: Transponder Signal Strength Anomaly
-Recommendationsfor Replacement
Just use RG-142 coax and you will have no problem placing the Xponder
antenna in the wing.
RG-142 is low loss and NOT inexpensive. Same size as standard coax.
Jim
--- John Tvedte <JohnT at comp-sol.com> wrote:
> Keith,
>
> I agree - about the loss of the cable - which is why I mentioned ECS
direct.
> They can supply cable that will easily meet db loss requirements with
> the length required.
>
> I suggest in this case, in particular, as Gary already has an antenna
> that does not require a ground plane -
>
> I also know Rich Guerra has his transponder antenna at the end of the
> strake
> - works perfectly.
>
> I also mention the dipole antenna - and suggest it will work as
> suggested by the factory.
>
> John
>
> -----Original Message-----
> From: reflector-bounces at tvbf.org on behalf of KeithHallsten
> Sent: Mon 3/14/2005 10:22 PM
> To: Velocity Aircraft Owners and Builders list
> Cc:
> Subject: Re: REFLECTOR: Transponder Signal Strength Anomaly -
> Recommendationsfor Replacement
>
>
>
> 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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>
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James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying
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