Locating Transponder Antenna on top of the fuselage instead of under it.

In the absence of antenna diversity, I'm pretty sure all general
aviation factory built aircraft have the transponder antenna on the
bottom of the aircraft.Â* I would think the manufacturers must know
something.

On 12/30/2019 12:13 AM, Darryl Ramm wrote:
On Sunday, December 29, 2019 at 10:43:44 PM UTC-8, wrote:
Well, Here is a picture of Schlicher's recommended location for the Antenna-same as Schempp Hirth-Above landing gear doors.
Interesting.

https://www.alexander-schleicher.de/...reinbau-27.pdf
Dan
Dang. Well I don't like that either :-)

My ASH-26E has a more central transponder antenna location behind the gear (and offset to avoid the fuselage join seam and a fuel pump on one side) as can an ASH25 and some others, so I guess I've mostly noticed those. And ASH-30/31 etc. owners all seem to be ordering the tail mounted antenna (which is what I'd do on a new glider).

Again, if I was looking at your glider, based on gut feel (and some background in microwave engineering), I personally would install the upper antenna, I wonder if SH came out with that to improve on the lower location, or to make the install easier or both....

And since you asked about different style 1/4 wave dipoles. The rods, blades etc. should all behave the same from a RF viewpoint. They are effectively the same internally. Rods/single attach point style antennas may be easier to attach/mount on curved surfaces, but have higher drag than a blade if that small drag worries you. I've also seen A&Ps recommend the rod style so that if you do manage to crunch it will bend and hopefully do less damage to the fuselage/finish.





--
Dan, 5J

On Monday, December 30, 2019 at 6:44:50 AM UTC-8, Dan Marotta wrote:
In the absence of antenna diversity, I'm pretty sure all general
aviation factory built aircraft have the transponder antenna on the
bottom of the aircraft.Â* I would think the manufacturers must know
something.

When transponders first came out, the only receiver/interrogator was ground based. So anything certificated prior to TCAS, would definitely have the antenna on the bottom.
Then TCAS came along, and now ADS-B. These will interrogate/receive from any direction. So to be visible from any direction, one should probably have an antenna on top as well as bottom... And satellite based ADS-B likely demands it.
I'm sure Darryl has all the gory technical details :-) But my guess is that unless told otherwise, and considering cost of certification, etc., putting the antenna "where it's always been" is why it's on the bottom. :-)

5Z