Frank wrote:
On Apr 14, 4:15 pm, 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

A dipole and a monopole with a proper groundplane both have
approximately the same pattern, both above and below the plane of the
groundplane. The whole idea of a 'groundplane' is to produce a
virtual mirror image of the monopole. Monopole/groudplane
combinations are particularly convenient for metal-skinned aircraft,
where the requisite groundplane is essentially free. A dipole
arrangement might be much simpler than a monopole/groundplane for a
glider.

The patterns are different, and the bigger the ground plane, the bigger
the difference. The monopole with a large ground plane radiates only on
the side with the antenna. The "mirror image" you mention is just a
convenient way to think about how a conducting surface affects the
electric field. In practice, it blocks the field. Look up "Faraday
cage", as an example.

If a metal airplane needs a good signal on the top and the bottom, it
uses two antennas, one on each side. In Tony's wood and fabric glider,
the dipole is an good choice.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* Sections on Mode S, TPAS, ADS-B, Flarm, more

* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org