Darryl,

Thanks for the power calculation. My microwave says 350w, so I just
did the 175/350 ratio. It appears I left out a few
considerations...grin

My glider is a fiberglass ASW-19, and Schleicher has not responded to
my request for guidance on antenna installation. I did see their write-
up for the ASH-26, but I want an internal installation. The DG service
info note 53/04 issue 3 shows a diagram for an internal mounting, but
it neglects to show the ground plane that is required with the type of
antenna they use.

I'm thinking of using an Advanced Aircraft Electronics L-2 transponder
antenna. It's a dipole, doesn't require a ground plane, claims to have
more gain than normal antennas, and can be installed by simply gluing
it onto a vertical surface.

One pilot I'm aware of who used the L-2 antenna made a simple stand
out of 1/4" balsa wood, with a 6" vertical run and a small cruciform
base. He glued the L-2 to the vertical member, and glued the cruciform
base to the bottom center of his tail boom, as far back as he could
reach. The cable he used was RG-213, to minimize losses.

-John


On Jan 17, 2:39 am, Darryl Ramm wrote:
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