There are a number of people in this group who used dipole antennas.
I've got the L2 model from Advanced Aircraft Electronics in my ASW-19,
it was $99 if I remember right.

Because I have a fiberglass tail boom, all I had to do was glue the
dipole to the supplied strip of balsa, then glue the dipole into the
tail boom in a vertical position. No need for a ground plane at all,
and as you point out it radiates both upwards and downwards.

It seems to work very well, but I couldn't tell you just how much
better it performs than a monopole under the same circumstances. For
me its biggest advantage was the ease of installation.

-John

Tony Condon wrote:
Has anyone used a Dipole antenna on their transponder? I understand that
the dipole gives better coverage above where the "standard" antennas
usually only project down and to the sides. Aircraft Spruce carries
dipoles, they are a little spendy, but might be worth it. Any experience
out there?
-Tony Condon
Cherokee II N373Y