mikerv6a(at)ao-cs.com
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 Posted: Thu Aug 07, 2014 8:29 pm Post subject: Half wave copper foil dipole transponder antenna |
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Transponder uplink frequency is 1030 MHz, downlink frequency is 1090 MHz.
So wouldn't we be building a transponder dipole with each leg about
2.75 inches (or so) long? The 6.25 inch per leg dimension would fit
an operating frequency close to 450 MHz.
Mike RV6A Corvallis, OR
| Quote: | At 02:23 PM 8/5/2014, you wrote:
>
>
>I hope this isn't a duplicate, but I was unable to find sufficient
>info on this topic in the archives.
>
>I have successfully built and tested several copper foil/tape comm
>antennas for my Cozy MkIV and would like to build a transponder
>antenna the same way.
>
>Will the following work for a Mode S transponder:
>
>1. Vertically oriented 1/2 wave dipole.
yes . . .
>2. 1/2" copper tape with ferrites
not particularly useful at VHF, useless
at uhf
>3. Each leg 6.25" long
Yeah . . . about that. Suggest you look over the
Bob Archer SA-005 descriptions and pictures
on the 'net.
http://tinyurl.com/mdho4z8
Bob uses double sided ECB material to craft an
antenna and matching section all in one piece.
As you can see . . . his 'dipole' is rather
wide . . . which yields a low SWR over a rather
wide frequency range . . . not that you NEED
the bandwidth . . . but the wider elements makes
their length less critical.
Here's Bob's narratives on transponder
antennas.
http://tinyurl.com/lwgonmx
Suggest you consider a dipole of at last 1/2"
wide material . . . 1" wouldn't hurt. Assembled
onto a sheet of plexiglas and attached to the
feedline with a Pawsey stub balun.
http://tinyurl.com/lnjhmem
http://tinyurl.com/lxzlpky
The 'thing' about ferrite beads strung onto the
feedline has been popularly circulated through
many venues but there are caveats . . .
First, the torroid material must be a pretty
good performer at the frquency of interest . . .
NO torroid materials I'm aware of are suited
for service at 1000mHz.
Second, the inductance presented by a coil of
wire on a core varies as the SQUARE of the
turns on the core. Effective use of torroid
cores as de-coupling baluns calls for MULTIPLE
turns on one core. This picture shows 7 passes
of wire through the core for an effective
inductance 47x that of a single pass.
[]
Hence, one would need to string 47 single
cores onto the coax as a string of beads to
equal the effectiveness of one core wound with
seven turns . . . but generally useful only
at 200mHz and below.
There used to be a commercial vhf comm antenna
offered that used the multi-turn torroid de-coupling
philosophy. I think it was called the "airwhip" . . .
don't find anything about the company now . . .
it's been a few years.
Now that I have ready access to an EMC lab,
any antenna you'd like to fabricate as
a shippable assembly could be mailed to me
for a quick look-see in the lab.
Bob . . .
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