On Sunday, December 29, 2019 at 10:11:09 AM UTC-8, 5Z wrote:
On Friday, December 27, 2019 at 5:22:04 PM UTC-8, wrote:
I put mine on a turtle deck on my ASW 27 and it works fine no issues with anyone seeing me or even worrying about it I really like the fact that it's there and was really easy to install compared to down by the gear

Long ago someone (I believe from Arizona) posted here about doing some research by talking to folks very familiar with FAA radar design about putting the transponder antenna on top of the fuselage. The conclusion was that if very near and above the radar site, the signal *might* be blocked. Other than that, it should be fine. Since we do a lot of turning, the antennal will be blocked for times anyway.

Unfortunately "it's more complex" applies to lots of stuff with aircraft surveillance.Â*And I would hope any research folks did thought about some of the things I'll describe here.

My concern with upper surface antennas is more blanketing from the wing, and that includes to distant SSR sites. The fuselage directly blocking the antenna like when near/overhead an SSR is not the main concern I have because already the SSR likely can't see you.Â*(OK sure if the long length of the fuselage points directly at the antenna you might have an issue).

If you are very near, i.e. ~overhead a radar site you may well be in the "cone of silence" of both the primary radar and SSR and likely not seen regardless of transponder antenna location or orientation. SSR and primary radar have a fairly wide cone of silence/shallow viewing angle above the horizon.. Think things like an SSR sees things only below ~30 degrees above the horizontal around the antenna.

Transponder antennas also have a cone of silence pointing out the end of their antenna. Hopefully a bit narrower than the SSR cone.

An antenna mounted on an upper fuselage area on a glider might be helped by diffraction around the glider fuselage, and that weak diffracted part of the signal may be OK when closer to a SSR (or ADS-B ground site) but still within that say 30 degree angle above the horizon. As the transponder antenna looks out more towards a distant SSR antenna the angle shallows and hopefully seeing a bit better signal due to decreased angle, but you can have significant range loss, especially say if relying on a distant (up to ~200NM!) en-route SSR to fill in details). But it's complex and would need modeling to see what happens.

Ah the irony is that very local overhead performance to an SSR antenna may depends on other (possibly quite distant) SSR sites.... SSR cones of silence get somewhat addressed by integrating together data from overlapping radar systems.Â*That is what the FAA Fusion system does, and it also folds in ADS-B data. (And ADS-B ground stations themselves have similar cone of silence issues, and a similar solution of adjacent ground towers partially covering each other).Â*

Any analysis you would want to look at upper and lower antenna performance to local SSR interrogators at around that 30 degree angle, and at more distant SSR sites that are filling in the data for the cone of silence at shallower angle. And my concerns again are more things like wings obscuring line of site, and wanting to make sure that any ground plane and antenna installation is as good as posible to maximize signal quality to/from more distant SSR sites regardless of wether the antenna is top or bottom mounted.

And I sure appreciate Dan posting a followup link to that picture of the antenna stuck out the side of a fuselage. I had never seen that before, much worse than I had imagined, and I sure did not want to seem to be recommending it.. blanketed directly above by the wing, blanketed to the side by the fuselage and side/below by the gear when extended, and ~90 degree crossed signal polarization when the glider is banking left.Â*

One reason for a transponder is to be seen by large aircraft TCAS, it's more likely they will be descending on a collision course, so having the antenna on top is a better idea....

5Z

Like Eric I don't understand why TCAS threats are assumed to come from above. I would suspect an equal distribution, and threats are going to be approaching near the same altitude with a relatively shallow climb or descent angle. In a bad case TCAS IIÂ* is going to command an RA which could put you relatively close laterally but above or below the threat aircraft... and I would assume those RA directions are equally distributed as well. And you've already executed the RA command before the angle above/below the threat gets very steep. I think there are studies/modeling of aircraft showing TCAS performance with single antennas if I can find them I'll point them out (but the system exists as it does with all light aircraft largely having single lower antennas).

With it being hard to guess/know how well things are really working one thing I have thought would be neat is a transponder antenna performance test tool similar to FLARM range tool or OGN/KTrax used ground and/or airborne 1090 MHz/ADS-B receivers. (Mark Hawkins are you looking for a project? :-))