Transponder Antenna Location

John

On Jan 16, 7:50 am, jcarlyle wrote:
One thing that concerns me is that the transponder frequency is
centered at 1060 MHz. A microwave oven frequency is close by, centered
at 2450 MHz. The power output of a transponder is about half that of a
microwave oven, but still, I don't want the transponder antenna close
to my body. I'll be mounting my antenna as far back in the tail boom
as I can get it.

-John

On Jan 16, 10:39 am, wrote:

I am going to install a transponder in my Ventus and struggling with
the decision on where to locate the antenna.
Any comments on installing the antenna on the forward portion of the
glare shield...inside the cockpit?
Scott

I'm not sure where the "about half" power ratio you mention is coming
from. Radiated power specs for a microwave oven would claim something
like "1,200 watts", a transponder something like "175 watts" and an
(incorrect) naive ratio there is 1,200/175 = ~7%. That is wrong
because the specs on radiated power for a transponder is pulse power,
while that of a microwave is average power (the microwave oven may
still pulse the RF signal, especially at less than full power
settings). If you want to compare the two devices without worrying
about the duty cycles, etc. and converting to equivalent power a
simpler crude comparison would be to compare the power consumption of
a microwave oven (typically ~1.5 to 2 killowatt) and the typical spec
power consumption of a transponder, say ~5 watts (Becker 175 Watt mode
C). The power consumption of the transponder is obviously dependent on
the interrogation rate. Still instead of "half" as claimed a better
simple guesstimate is a ratio of 5/2,000 = ~.2%. A better analysis of
actual radiated power would show an even smaller ratio (the cavity
magnetron in the microwave oven will be more efficient than the
transponder at converting power into RF signal).

Still I agree with not getting too close to the antenna and I would
not put the antenna close to my eyes, head or other parts I care
about. But "mounting as far back as I can get" may not a good
optimization either. You want to avoid long cable runs and for
standard quarter wave antennas you want a part of the fuselage that
allows the mounting of a ground plane, i.e. a large area of the
fuselage that does not have too an extreme curvature. Carbon fiber
fuselages will shield the pilot very well from RF exposure but you
still need a proper ground plane for the antenna, usually adhesive
metal foil or aluminum sheet inside the fuselage wall and of radius at
least about the length of the antenna. In a fiberglass (RF
transparent) fuselage mounting the antenna inside may be a good idea,
you need to avoid metal parts close to the antenna and provide a good
ground plane.

My general advice on these is to follow the manufactures
recommendation on antenna placement, if they did not have something
specific then look at what manufactures are recommending for similar
gliders.

Personally I'd mount the antenna below (where all manufactures
recommend AFAIK) for better RF exposure to ground based radar. On my
ASH-26E I have the antenna in the factory recommended location below
the fuselage behind and to the side of the gear, rigging and handling
are not an issue.

Darryl