Transponder antenna - blade vs stub monopole

In article brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?

Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.

One that extends front and back but is more limited to the sides
(perhaps wrapping around inside) probably would work, but would be
a bit reduced from a full ground plane.

Alan

Alan wrote:
In article brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?

Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.


23 inch radius for a quarter-wave antenna? Most installations I've seen
use no more than a 3 inch radius ground plane...

The 23" is for a 1/4 wave antenna at the frequency for the aviation radio
band (123 MHz). The transponder operates at about 1,000 MHz, so a 1/4 wave
is much smaller.

You do not even need to do much calculation, since the radius of the ground
plane should be equal to the height of the antenna. So, the diameter would
be twice the antenna height.

The shape need not be round. It can be square or rectangular. Generlaly, a
slight improvement will be achieved with a larger ground plane.

Also remember, that, even though the transmitter is pulsed at a very short
duration, there could be some possible danger having it close to your body.
If you use the ground plane to "hide" the antenna from you, there would be
little danger from exposure. The jury is still out on this one. I do not
think there is any credible evidence of a danger, but I would not expose
myself unless there was no other choice. Mine is mounted under my seat,
upside down.

Upside down is better because radio waves at this frequency are "line of
sight" and the ground stations interrogating you are all below you.

Do a good job of coax installation. I have seen some crappy jobs. Also
remember that coax attenuation at 1,000 MHz with RG-58 is very high - so
keep the run short.

Colin Lamb

wrong! A ground plane should have a radius of at least 1/4 wavelength of the
antenna's operating frequency, a 23" antenna ground plane then would be
about right for a VHF airband communication antenna (118-136 mhz) and in
gliders this is seldom possible so use what you can get..
Transponder antenna ground planes are typically recommended to be 6"
diameter.
These are ideal....but not always possible so use what you can, too large
isn't a problem but performance can degrade from the ideal as it becomes
smaller.
tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

"Alan" wrote in message
...
In article

brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?

Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.

One that extends front and back but is more limited to the sides
(perhaps wrapping around inside) probably would work, but would be
a bit reduced from a full ground plane.

Alan

I installed a blade antenna in the recommended location for my Discus
2 just behind and to one side of the gear door and an internal ground
plane about 6" square. It works very well in this location (also had
a short coax from the transponder, which is a consideration at these
higher frequencies).

Tim is absolutely right, though. It is physically a right royal pain
in the ass. I don't know anyone who has one in this location who
hasn't broken it as it is perfectly located to snag on the trailer
ramp. I have a short quarter wave monopole as back-up. This will be
my default the next time the blade breaks! I doubt that the extra
drag is worth worrying about.

If you can locate the blade above and behind the cockpit, this is a
much safer location and I have seen a couple of these installations
that also work well.

Mike

Alan wrote:
In article brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?

Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.

That sounds like a dimension for the aircraft communication radio,
working in the 120-130 mhz range. The transponder uses 1090 mhz, about
10 times higher. That indicates a 2.3" radius ground plane would be
adequate, or the 6" diameter Marc points out as commonly used.

--
Eric Greenwell - Washington State, USA
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