REFLECTOR: Transponder Signal Strength Anomaly -
Recommendationsfor Replacement
Jim Agnew
jim_agnew_2 at yahoo.com
Tue Mar 15 07:13:42 CST 2005
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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James F. Agnew
Jim_Agnew_2 at Yahoo.Com
Tampa, FL
Velocity 173 Elite Aircraft Completed & Flying
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