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



 
 
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  #1  
Old December 27th 19, 07:34 PM posted to rec.aviation.soaring
[email protected]
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Default Locating Transponder Antenna on top of the fuselage instead of under it.

Hi all,
Planning to install a Transponder Antenna on my Ventus CM.
Schempp Hirth indicates the possibilty of installing on top of the fuselage , behind the engine compartment doors.
Regarding being seen by ATC and Airlines, would that location perform better/worse/same as the standard location on side of fuselage - above gear doors ?
Dan
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  #2  
Old December 27th 19, 08:19 PM posted to rec.aviation.soaring
Dan Marotta
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

I prefer the bottom location unless you have a diversity transponder
which can use both.* Having the antenna on the bottom gives better view
to ground stations and ATC will inform airliners of your location should
you be of concern.* Practically speaking, however, since gliders spend
so much time in banked turns, some of them steep banks, it's entirely
possible that either antenna location would work.* But I still vote
"bottom".

On 12/27/2019 12:34 PM, wrote:
Hi all,
Planning to install a Transponder Antenna on my Ventus CM.
Schempp Hirth indicates the possibilty of installing on top of the fuselage , behind the engine compartment doors.
Regarding being seen by ATC and Airlines, would that location perform better/worse/same as the standard location on side of fuselage - above gear doors ?
Dan


--
Dan, 5J
  #3  
Old December 27th 19, 10:22 PM posted to rec.aviation.soaring
Darryl Ramm
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

On Friday, December 27, 2019 at 11:34:32 AM UTC-8, wrote:
Hi all,
Planning to install a Transponder Antenna on my Ventus CM.
Schempp Hirth indicates the possibilty of installing on top of the fuselage , behind the engine compartment doors.
Regarding being seen by ATC and Airlines, would that location perform better/worse/same as the standard location on side of fuselage - above gear doors ?
Dan


The preferred location should be below. That is more visible to ATC SSR, and indirectly providing ADS-R/TIS-B services to other aircraft. Close by aircraft with TCAS (or ADS-B In if you have ADS-B Out) that are a collision threat to you usually have no problem seeing your glider transponder. And arguably below is more a blind spot for gliders so maybe putting more signal there even if everything else did not matter makes sense (and it should also provide good coverage behind).

"on the side above the gear doors??? I've never seen a transponder antenna installed "on the side". Many are behind the gear doors and offset slightly from the centerline. The antenna needs to be mostly vertical since transponders, SSR, and TCAS use vertical polarization. Yes aircraft bank, but you need to start with the antenna somewhat vertical, slightly off centerline is fine.

Pay attention to the proper ground plane installation with any of the standard external transponder 1/4 wave antenna. That is very important.

---

Interesting things may happen in Canada and other countries with Aerion satellite based 1090ES reception is being used. The satellites (much further away than threat aircraft) need good top antennas. That *might* lead to aircraft (including gliders?) using diversity (top and bottom) transponder antennas at some future time... not supported by the transponders we use today (GA works so well with just a bottom antenna it's not been needed until...)..











  #4  
Old December 28th 19, 01:22 AM posted to rec.aviation.soaring
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

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
  #5  
Old December 28th 19, 02:05 AM posted to rec.aviation.soaring
George Haeh
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

I had a standard length PowerFLARM and ADS-B antenna through a hole in the glareshield of my ASW-27. After upgrading to dual long PF antennas that I placed on opposite sides of the canopy with Velcro, I placed an internal L2 transponder antenna in the glareshield hole reaching to just below the canopy favoring visibility to TCAS above and satellites. Being a long way from any big airports, I haven't verified with ATC, but suspect I'm visible to ground installations unless close in above or pointed directly away when my body and carbon fiber will be in the way.
  #6  
Old December 28th 19, 04:28 AM posted to rec.aviation.soaring
Darryl Ramm
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

On Friday, December 27, 2019 at 6:05:20 PM UTC-8, George Haeh wrote:
I had a standard length PowerFLARM and ADS-B antenna through a hole in the glareshield of my ASW-27. After upgrading to dual long PF antennas that I placed on opposite sides of the canopy with Velcro, I placed an internal L2 transponder antenna in the glareshield hole reaching to just below the canopy favoring visibility to TCAS above and satellites. Being a long way from any big airports, I haven't verified with ATC, but suspect I'm visible to ground installations unless close in above or pointed directly away when my body and carbon fiber will be in the way.


Uh this seems pretty bad advice. And there are great reasons why glider manufacturers do not have anything like this as a recommended installation.

You are bathing all the other avionics with high-energy RF pulses from the transponder. An external quarter wave antenna with a ground plane helps get that energy away from the other avionics and pilot. A L2 type wave antenna for mounted inside a non-conductive fuselage should normally be mounted back in the fuselage away from the pilot and avionics. And things like PowerFLARM 1090ES In are designed to work by seeing somewhat reduced power leakage of your transponder signal, not being blasted by the direct high-energy signal.

You may be operating the FLARM antennas closer to a transponder antenna than a good idea (~30cm minimum recommended by FLARM... but that's when both externally mounted, this may be worse mounted inside). I'm not even sure something as close as 30cm is a good idea externally. Does ownship PowerFLARM transponder identification work OK? Or do you have to disable PCAS etc.? FLARM range checks all look good? How do you know if the Transponder is working OK until you check/ask?. You also might be flying in a remote enough area that you get few transponder interrogations, so things seem to work OK. A close encounter with a TCAS equipped aircraft might result in a chirp of ~1kHz transponder interrogations. Even if things are are working OK now who knows what that will do to your electronics, PowerFLARM, traffic display, etc. when you might need to most rely on it.

You likely have lots of conductive objects within the antenna RF near field near field (including the Schleicher panel support brass rod, all the usual wiring and conductive instrument cases). That can significantly affect the radiation pattern in ways unlikely to be obvious.

And you are possibly exposing yourself to excessive RF radiation.

In the USA I would expoect that an A&P doing a transponder RF check would not sign off on such an install.

  #7  
Old December 28th 19, 08:26 AM posted to rec.aviation.soaring
Tango Whisky
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

My transponder hs 250 W output, no way I would want that any place near my head or my balls.
  #8  
Old December 28th 19, 09:02 AM posted to rec.aviation.soaring
Darryl Ramm
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

On Saturday, December 28, 2019 at 12:26:15 AM UTC-8, Tango Whisky wrote:
My transponder hs 250 W output, no way I would want that any place near my head or my balls.


I don't want your balls near my transponder either. :-)

To be fair it's a low average duty factor (1%) so you don't see all that power. But it's enough for some folks to worry about that folks do pay attention. RF exposure to low-power TABS and ADS-B systems for example designed to be close to wetware are a part of regulatory discussions.
  #9  
Old December 28th 19, 04:14 PM posted to rec.aviation.soaring
Dan Marotta
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

And yet, how many folks spend a major part of their lives with a radio
transmitter (cell phone) strapped to the side of their head or in their
pocket?

On 12/28/2019 2:02 AM, Darryl Ramm wrote:
On Saturday, December 28, 2019 at 12:26:15 AM UTC-8, Tango Whisky wrote:
My transponder hs 250 W output, no way I would want that any place near my head or my balls.

I don't want your balls near my transponder either. :-)

To be fair it's a low average duty factor (1%) so you don't see all that power. But it's enough for some folks to worry about that folks do pay attention. RF exposure to low-power TABS and ADS-B systems for example designed to be close to wetware are a part of regulatory discussions.


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Dan, 5J
  #10  
Old December 28th 19, 04:55 PM posted to rec.aviation.soaring
jfitch
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Default Locating Transponder Antenna on top of the fuselage instead ofunder it.

On Saturday, December 28, 2019 at 12:26:15 AM UTC-8, Tango Whisky wrote:
My transponder hs 250 W output, no way I would want that any place near my head or my balls.


The normal location (recommended by many manufacturers) just ahead of the gear doors is much closer to your balls than an L2 in the nosecone (or the instrument pod), if that worries you. Probably less than 2' away. I always wear tin foil underwear to match my tin foil hat. But my transponder antenna is in the nosecone and works perfectly as tested by external radiated power and FCC ADS-B out reports.
 




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