GPS anetenna location OK in baggage area of a fiblerglass glider?

[Dirk_PW[_2_]]

I'm doing a little housekeeping with instrument panel this winter with the intent to eventually install an ADS-B solution (whatever that may be). The panel is quite cluttered right now. I'm considering one option of moving the FLARM GPS antenna (and eventually a ADS-B GPS antenna) back behind my head somewhere in the baggage area. I'm curious if anyone has done this and has had any problem acquiring a GPS signal (on a fiberglass glider). I'm currently having no problems acquiring a signal with the GPS antenna under the fiberglass front panel. Thanks.

[Paul Remde]

Hi Dirk,

My experience is that GPS antennas work fine through fiberglass and plastic.. The don't work well through metal or carbon fiber.

Best Regards,

Paul Remde
____________________

On Sunday, February 25, 2018 at 8:44:09 PM UTC-6, Dirk_PW wrote:
> I'm doing a little housekeeping with instrument panel this winter with the intent to eventually install an ADS-B solution (whatever that may be). The panel is quite cluttered right now. I'm considering one option of moving the FLARM GPS antenna (and eventually a ADS-B GPS antenna) back behind my head somewhere in the baggage area. I'm curious if anyone has done this and has had any problem acquiring a GPS signal (on a fiberglass glider). I'm currently having no problems acquiring a signal with the GPS antenna under the fiberglass front panel. Thanks.

[Darryl Ramm]

On Sunday, February 25, 2018 at 6:44:09 PM UTC-8, Dirk_PW wrote:
> I'm doing a little housekeeping with instrument panel this winter with the intent to eventually install an ADS-B solution (whatever that may be). The panel is quite cluttered right now. I'm considering one option of moving the FLARM GPS antenna (and eventually a ADS-B GPS antenna) back behind my head somewhere in the baggage area. I'm curious if anyone has done this and has had any problem acquiring a GPS signal (on a fiberglass glider). I'm currently having no problems acquiring a signal with the GPS antenna under the fiberglass front panel. Thanks.

Many (most?) gliders, including carbon fuselage ones have a RF transparent area over the luggage space where you can mount GPS antennas if you want.

With the electronics mounted close by this is not usually an issue. But I'd avoid long runs of RF coax for an GPS antenna. You need to work out the lengths of all connections, power, data and any RF connections involved and what a reasonable maximum can be. Reducing space in the luggage area and putting stuff where it obstructs or can be damaged when rigging/inserting wing pins can be an issue. I'd personally do nothing that reduced available space there.

How many GPS antennas do you need? I'd be looking at keeping ADS-B antennas on a dedicated connection. Possibly combining other antennas though a RF splitter if absolutely required to save space.

[Dirk_PW[_2_]]

The only reason the thought crossed my mind is because the stock FLARM GPS antenna wire is long enough to make the run, with distance to spare. In fact, I'm thinking about putting it BEHIND the luggage area (i.e. behind the spar) where it is completely out of the way (I didn't shorten the stock GPS antenna wire, but thought about it many times). Right now this is the only GPS antenna I have, but I will add an additional antenna for ADS-B at some point (maybe after the Reno show if anyone is offering any discounts - hint, hint for anyone listening). With just two antennas it doesn't make sense to try to put in splitter. Mounting will be the next issue. I was thinking about a couple of simple fiberglass cubby holes against the inside roof of the glider for the antennas to slide into (separate by some small distance).

[OHM Ω http://aviation.derosaweb.net]

Two questions;

> Dirk_PW - What glider do you own? Do you know if it is fiberglass or carbon fiber? They all look the same on the outside, you know! As Paul says, fiberglass will allow GPS reception but carbon will not.

> Darryl - I was unaware that "many" (!) CF gliders have this RF transparent area behind the pilot's head. That would be great as an optional area for mounting antennas as behind our panels things are getting pretty full these days. Any idea which brands/models have this feature? Schleicher ASW-27 by any chance?

PS - Darryl mentions concern about too long of a length of the coax on the GPS antenna. I would suspect, and hope, that the manufacturer wouldn't sell a GPS antenna with an overly long coax. The proof would be in the pudding I suppose - before you go to all the trouble of routing the coax fore to aft, test the functionality with all the coax laying out on the floor. BTW, shortening the tiny GPS coax, and fitting a new tiny connector to it, is much more difficult than a simple BNC.

Best of luck, John Ω

Thanks.

[Dan Marotta]

This may or may not be related, but...

I mentioned GPS dropouts to my avionics technician since I had several
GPS pucks under the fiberglass glare shield on my Stemme. They were
mounted directly under my Flarm antennae so I moved them to the top of
the glare shied and to the canopy frame up front. This took care of most
of the dropouts but there is still an occasional dropout.

The tech said he's seen this on power planes as well and he thinks it's
a failing GPS antenna.Â* He said that one antenna failing could
(apparently) transmit interference which could take out others.Â* He
suggested disconnecting my antennae one at a time and flying with that
system down and see if there are problems with the others.Â* It might be
less painful simply to get another GPS antenna (they're cheap) and swap
it around the cockpit from system to system and see if that fixes the
problem.Â* ...Or maybe an antenna combiner...

OBTW, there should be no problem with mounting an antenna underneath a
fiberglass cover.

On 2/25/2018 7:44 PM, Dirk_PW wrote:
> I'm doing a little housekeeping with instrument panel this winter with the intent to eventually install an ADS-B solution (whatever that may be). The panel is quite cluttered right now. I'm considering one option of moving the FLARM GPS antenna (and eventually a ADS-B GPS antenna) back behind my head somewhere in the baggage area. I'm curious if anyone has done this and has had any problem acquiring a GPS signal (on a fiberglass glider). I'm currently having no problems acquiring a signal with the GPS antenna under the fiberglass front panel. Thanks.

--
Dan, 5J

[Dan Marotta]

It's not a problem to shorten the coax.Â* Simply open the antenna puck by
peeling off the sticker on the back and removing the screw(s).Â* The coax
cable is soldered conveniently in front of you and it's an easy thing to
remove it, cut and strip it to length, and solder back into place.Â*
Replace the cover and you're done.

Hmmmmmmm...Â* Regarding my previous post - maybe there's a problem with
my Flarm antenna since I shortened the coax.Â* I'll swap that one out
first as a test.

On 2/26/2018 6:59 AM, OHM Ω http://aviation.derosaweb.net wrote:
> BTW, shortening the tiny GPS coax, and fitting a new tiny connector to it, is much more difficult than a simple BNC.
>
> Best of luck, John Ω
>
> Thanks.

--
Dan, 5J

[krasw]

maanantai 26. helmikuuta 2018 16.00.04 UTC+2 OHM Ω http://aviation.derosaweb.net kirjoitti:
> I was unaware that "many" (!) CF gliders have this RF transparent area behind the pilot's head.

Well there is not, this is simply not true. Some level of satellite reception is possible trough canopy to that area. Only RF transparent area is usually at tail where VHF antenna is mounted.

[jfitch]

On Monday, February 26, 2018 at 6:40:27 AM UTC-8, krasw wrote:
> maanantai 26. helmikuuta 2018 16.00.04 UTC+2 OHM Ω http://aviation.derosaweb.net kirjoitti:
> > I was unaware that "many" (!) CF gliders have this RF transparent area behind the pilot's head.
>
> Well there is not, this is simply not true. Some level of satellite reception is possible trough canopy to that area. Only RF transparent area is usually at tail where VHF antenna is mounted.

Every glider I've seen has white glass, not carbon, in the turtle deck. White glass is highly transparent to RF in that frequency range. You can check by putting a very strong light on the outside and see if the inside glows. It will through white glass, not through carbon. Sometimes it is covered with paint which makes it harder to tell (the light will penetrate the outside gel coat).

Within reason, you can have a pretty long coaxial cable on a normal GPS antenna. They have a 26 - 30 dB amplifier in them, so the signal from the antenna to the device is not weak. That is also the reason they can interfere with other GPS antennas and devices - if not well shielded and grounded, they become a transmitter themselves. Make sure if you shorten the cable at the antenna end that you get a good solder joint on the shield (easy to get a cold joint here because of the ground plane in the board). Another way to shorten the cable is to cut the other end and crimp on a new coaxial connector. The Flarm uses an MCX.

[Darryl Ramm]

On Monday, February 26, 2018 at 6:40:27 AM UTC-8, krasw wrote:
> maanantai 26. helmikuuta 2018 16.00.04 UTC+2 OHM Ω http://aviation.derosaweb.net kirjoitti:
> > I was unaware that "many" (!) CF gliders have this RF transparent area behind the pilot's head.
>
> Well there is not, this is simply not true. Some level of satellite reception is possible trough canopy to that area. Only RF transparent area is usually at tail where VHF antenna is mounted.

I'm not sure which cereal box you are reading to get all your knowledge, but you might want to consider switching brands.

[krasw]

maanantai 26. helmikuuta 2018 19.49.54 UTC+2 Darryl Ramm kirjoitti:
> On Monday, February 26, 2018 at 6:40:27 AM UTC-8, krasw wrote:
>
> I'm not sure which cereal box you are reading to get all your knowledge, but you might want to consider switching brands.

Please refer to make and model of glider where the fuselage is carbon with glass turtle deck?

[Darryl Ramm]

Name one where it is not. Go do the light test Jon discussed. Every Schleicher fueslage I have looked at is this way. Pretty sure the DG1000 is, ...

You were claiming no glider fuselages have this, having used GPS antennas mounted under the turtleneck in several otherwise carbon fiber glider fuselages I know you are wrong about that.

Here is a game for the upcoming SSA convention... folks there can ask or look to check if the turtledecks in all the gliders on display are RF transparent or not.

Oh the anticipation for the advancement of human (well r.a.s.) knowledge is killing me.... :-)

[Charlie M. (UH & 002 owner/pilot)]

Dude, please don't die.......

While I hope you have some replies copied to a text file so you can, "copy paste" to cover things that have been asked many times (yes, on some car forums I have "canned replies" in a text file, just copy paste as an answer), I hope you stay around.

Maybe that is a kiss of death.......;-)

[krasw]

maanantai 26. helmikuuta 2018 21.12.23 UTC+2 Darryl Ramm kirjoitti:
> Name one where it is not.

Sorry but with "light test" you have no clue of the turtle deck material. Why an earth would factories go trough massive pain and expense of laying up turtledeck using different material instead of just rolling up carbon/kevlar? Instruments and their antennas tend to reside in the instrument panel. Routing GPS antennas to behind pilots back, who does this, and why would factories be interested in this? Please give some reference to glass turtledeck in carbon fuselage, anything will do. RF transparence is different thing, antenna on the hat shelf sees sky trough canopy as I stated earlier.

[[email protected]]

If you are fortunate enough to own an SZD 55-1, mount the FLARM in the tail battery box. Bond the FLARM antenna to the vertical stab just below the metal parts of the horizontal stab. Bond the ADS-B antenna to the vertical stab just above the rudder counter weight. Makes for very short antenna leads. Use the existing battery cables to power the FLARM from a new battery box mounted to the stbd side of the main gear box. Now for the hard part, fish a data cable forward to the FLARM display of your choice.

[[email protected]]

On Monday, February 26, 2018 at 6:40:27 AM UTC-8, krasw wrote:

> Well there is not, this is simply not true.

Read The Fine Manual... From the ASW-27 flight manual:

"Since the whole airframe ex cept for the fin and a small area above the baggage compartment contains carbon fibre which screens electromagnetic radiation, the ELT's antenna must be fitted in the top of the baggage compartment and extend into the canopy area."

On a sunny day, have a friend place their hand on the outside of the fuselage and look inside. If you can see the hand's shadow, it's not carbon.

5Z

[Bob Kuykendall]

On Monday, February 26, 2018 at 12:07:49 PM UTC-8, krasw wrote:

> Sorry but with "light test" you have no clue of the turtle deck material.

Dude, chill. They're trying to talk sense for ya.

* Radio waves and light are both electromagnetic radiation; the only difference between them is frequency. If light goes in through the turtledeck structure, then radio waves will almost certainly to in and out as well.

* It doesn't matter if the turtledeck is fiberglass or Kevlar (aramid) or basalt fiber. If light goes through it, so will radio waves. Light doesn't go through carbon or aluminum, and neither does radio waves. BTW that's one of the reasons UV-curing vinylester resins are kind of a dead-end, they won't play nice with carbon. Someday there might be x-ray-curing vinynlesters, though, so hang on to your lead apron.

* When I designed the HP-24 fuselage, I surveyed the field and found that many if not most manufacturers were using radio transparent fiberglass for the portion over the wing, and I did the same. I also made the rudder out of Kevlar, and developed a tuned dipole com antenna that is a standard part of the rudder kit.

* I specifically decided not to develop an internal transponder antenna for the rudder or fuselage; for that I recommend an external blade antenna. I have on my to-do list the development of a flush 1090 mHz slot antenna, but it's pretty far down the list.

--Bob K.

[Matt Herron Jr.]

On Monday, February 26, 2018 at 6:28:31 AM UTC-8, Dan Marotta wrote:
> This may or may not be related, but...
>
> I mentioned GPS dropouts to my avionics technician since I had several
> GPS pucks under the fiberglass glare shield on my Stemme. They were
> mounted directly under my Flarm antennae so I moved them to the top of
> the glare shied and to the canopy frame up front. This took care of most
> of the dropouts but there is still an occasional dropout.
>
> The tech said he's seen this on power planes as well and he thinks it's
> a failing GPS antenna.Â* He said that one antenna failing could
> (apparently) transmit interference which could take out others.Â* He
> suggested disconnecting my antennae one at a time and flying with that
> system down and see if there are problems with the others.Â* It might be
> less painful simply to get another GPS antenna (they're cheap) and swap
> it around the cockpit from system to system and see if that fixes the
> problem.Â* ...Or maybe an antenna combiner...
>
> OBTW, there should be no problem with mounting an antenna underneath a
> fiberglass cover.
>
> On 2/25/2018 7:44 PM, Dirk_PW wrote:
> > I'm doing a little housekeeping with instrument panel this winter with the intent to eventually install an ADS-B solution (whatever that may be). The panel is quite cluttered right now. I'm considering one option of moving the FLARM GPS antenna (and eventually a ADS-B GPS antenna) back behind my head somewhere in the baggage area. I'm curious if anyone has done this and has had any problem acquiring a GPS signal (on a fiberglass glider). I'm currently having no problems acquiring a signal with the GPS antenna under the fiberglass front panel. Thanks.
>
> --
> Dan, 5J

Check for electrical noise on your mains. In my case it was caused by switching power supplies for the Oudie and Colibri II. I was seeing occasional dropouts in my IGC traces. Liberal use of ferrites stopped the problem cold.

[[email protected]]

I fly out west with a Powerflarm and a garmin 196 with remote antennae.The antennae are seperated by at least a foot or so under the glare sheild. At first and for years I had the 196 feeding my SN10B and without fail at exactly 00:00 UTC, I was usually on final glide or a long way from home, I would get about minute or three of dropouts. These would last for 5 to 30 seconds but very annoying. They would not show up on the 196, but I would get the warning on the SN10.If looking at the signal on the 196 it was always good and it would be updating its position constantly. After switching to the powerflarm as a gps source it has stopped. Still dont know why it was happening.

CH

[jfitch]

On Monday, February 26, 2018 at 12:07:49 PM UTC-8, krasw wrote:
> maanantai 26. helmikuuta 2018 21.12.23 UTC+2 Darryl Ramm kirjoitti:
> > Name one where it is not.
>
> Sorry but with "light test" you have no clue of the turtle deck material. Why an earth would factories go trough massive pain and expense of laying up turtledeck using different material instead of just rolling up carbon/kevlar? Instruments and their antennas tend to reside in the instrument panel.. Routing GPS antennas to behind pilots back, who does this, and why would factories be interested in this? Please give some reference to glass turtledeck in carbon fuselage, anything will do. RF transparence is different thing, antenna on the hat shelf sees sky trough canopy as I stated earlier.

An ASH26E has an all carbon fuselage but a fiberglass turtle deck. As does every other AS glider I have ever seen (20, 24, 25, 29, 31, etc.). Every Ventus and Discus I have ever seen has a fiberglass turtle deck. Ditto every DG and Lak. Rather, please point to a composite glider that does not. In a Pilatus, Schweizer, or Blanik you will have some trouble as they are aluminum.

[krasw]

tiistai 27. helmikuuta 2018 8.33.22 UTC+2 jfitch kirjoitti:
>
> An ASH26E has an all carbon fuselage but a fiberglass turtle deck. As does every other AS glider I have ever seen (20, 24, 25, 29, 31, etc.). Every Ventus and Discus I have ever seen has a fiberglass turtle deck. Ditto every DG and Lak.

No they don't. I've maintained and repaired a bunch of gliders you mention and I'm familiar with their structure. I've been to LAK factory seeing them lay-up parts. Here is a link to fuselage lay up showing full carbon lay-ups with glass at the tail for antenna:

https://photos.app.goo.gl/icyQoCCntLEBb8TI2

Your list includes gliders that have all-glass-fuselage BTW. Just calling BS, for the last time now.

[krasw]

tiistai 27. helmikuuta 2018 8.33.22 UTC+2 jfitch kirjoitti:
> An ASH26E has an all carbon fuselage but a fiberglass turtle deck. As does every other AS glider I have ever seen (20, 24, 25, 29, 31, etc.). Every Ventus and Discus I have ever seen has a fiberglass turtle deck. Ditto every DG and Lak. Rather, please point to a composite glider that does not. In a Pilatus, Schweizer, or Blanik you will have some trouble as they are aluminum.

Here is few photos from LAK factory, fuselage and tail lay-up (glass around antenna in tail):

https://imgur.com/a/NoHcw

Your list includes gliders with all-glass fuselage which is bit confusing.

Just calling BS, for the last time.

[Tango Whisky]

Le mardi 27 février 2018 11:46:42 UTC+1, krasw a écritÂ*:
list includes gliders with all-glass fuselage which is bit confusing.
>
> Just calling BS, for the last time.

From the Schleicher homepage
https://www.alexander-schleicher.de/werk/mitarbeiter/
Photos #1 and #10 show the glass part of the turtle deck.

[[email protected]]

On Monday, February 26, 2018 at 2:12:23 PM UTC-5, Darryl Ramm wrote:
> Name one where it is not. Go do the light test Jon discussed. Every Schleicher fueslage I have looked at is this way. Pretty sure the DG1000 is, ...
>
> You were claiming no glider fuselages have this, having used GPS antennas mounted under the turtleneck in several otherwise carbon fiber glider fuselages I know you are wrong about that.
>
> Here is a game for the upcoming SSA convention... folks there can ask or look to check if the turtledecks in all the gliders on display are RF transparent or not.
>
> Oh the anticipation for the advancement of human (well r.a.s.) knowledge is killing me.... :-)

'27 and '28 lamination schemes show no carbon in a portion of the turtle deck area. Low loads and it is easy to shape carbon layer during lamination. My ELT antennas are in this area for that reason.
UH

[Tango Whisky]

Le mardi 27 février 2018 14:47:14 UTC+1, a écritÂ*:
> '27 and '28 lamination schemes show no carbon in a portion of the turtle deck area. Low loads and it is easy to shape carbon layer during lamination.. My ELT antennas are in this area for that reason.
> UH

Actually, the ELT antennas are the reason why there is a glass part in the turtle deck. Otherwise you would have to put these antennas somewhere else, and you would need a cable to connect the antenna to the ELT. And that would be real stupid.

[Ross[_3_]]

Not sure how many Discus 2 and Ventus 2 fuses I made, but none of them have a glass turtle deck.
Radio antenna located in the fin which is glass, except the Nimbus 4 where it is located in the rudder.
This however doesn't help your issue

[Matt Herron Jr.]

On Tuesday, February 27, 2018 at 7:16:13 AM UTC-8, Ross wrote:
> Not sure how many Discus 2 and Ventus 2 fuses I made, but none of them have a glass turtle deck.
> Radio antenna located in the fin which is glass, except the Nimbus 4 where it is located in the rudder.
> This however doesn't help your issue

Has anyone tried putting a transponder antenna like the T2glued to the inside the turtle deck? does the antenna require a downward view? 250 watts of RF too close to your head? This would make install on an ASW27 MUCH simpler than a fin or stinger on the outside behind the gear, not to mention more aerodynamic.

[Bob Kuykendall]

On Tuesday, February 27, 2018 at 4:20:51 PM UTC-8, Matt Herron Jr. wrote:

> Has anyone tried putting a transponder antenna like the T2glued to the inside the turtle deck? does the antenna require a downward view? 250 watts of RF too close to your head? This would make install on an ASW27 MUCH simpler than a fin or stinger on the outside behind the gear, not to mention more aerodynamic.

There is little if any scientific evidence one way or the other. But the general consensus seems to be that a 1/3 horsepower gigahertz ping next to the noggin is a lot more likely to be bad than good. You might hit the jackpot and mutate into a master of clairvoyance and psychokinesis, but the odds aren't in your favor on that.

And, yeah the view down seems to be more important than the one up.

One possibility might be a slot antenna, which are said to work well in the gigahertz range. I have it on my to-do list to try it, but it's pretty far down the list:

http://www.antenna-theory.com/antennas/aperture/slot.php

--Bob K.

[jfitch]

On Tuesday, February 27, 2018 at 4:20:51 PM UTC-8, Matt Herron Jr. wrote:
> On Tuesday, February 27, 2018 at 7:16:13 AM UTC-8, Ross wrote:
> > Not sure how many Discus 2 and Ventus 2 fuses I made, but none of them have a glass turtle deck.
> > Radio antenna located in the fin which is glass, except the Nimbus 4 where it is located in the rudder.
> > This however doesn't help your issue
>
> Has anyone tried putting a transponder antenna like the T2glued to the inside the turtle deck? does the antenna require a downward view? 250 watts of RF too close to your head? This would make install on an ASW27 MUCH simpler than a fin or stinger on the outside behind the gear, not to mention more aerodynamic.

Can't you put it in the nose of the 27? I mounted an L2 there on my 26, ahead of the rudder pedals. The nose it also not carbon, it's kevlar and glass.. I believe the antenna wants to be vertical for the correct polarization, that might add to the problems putting it in the turtle deck. I have the L2 transponder antenna on one side and the Flarm A on the other, works fine.

[Darryl Ramm]

On Tuesday, February 27, 2018 at 5:26:55 PM UTC-8, jfitch wrote:
> On Tuesday, February 27, 2018 at 4:20:51 PM UTC-8, Matt Herron Jr. wrote:
> > On Tuesday, February 27, 2018 at 7:16:13 AM UTC-8, Ross wrote:
> > > Not sure how many Discus 2 and Ventus 2 fuses I made, but none of them have a glass turtle deck.
> > > Radio antenna located in the fin which is glass, except the Nimbus 4 where it is located in the rudder.
> > > This however doesn't help your issue
> >
> > Has anyone tried putting a transponder antenna like the T2glued to the inside the turtle deck? does the antenna require a downward view? 250 watts of RF too close to your head? This would make install on an ASW27 MUCH simpler than a fin or stinger on the outside behind the gear, not to mention more aerodynamic.
>
> Can't you put it in the nose of the 27? I mounted an L2 there on my 26, ahead of the rudder pedals. The nose it also not carbon, it's kevlar and glass. I believe the antenna wants to be vertical for the correct polarization, that might add to the problems putting it in the turtle deck. I have the L2 transponder antenna on one side and the Flarm A on the other, works fine..

I think the '27 is different from the '26 there and does not have the clear spots in the nose.

[Darryl Ramm]

On Tuesday, February 27, 2018 at 4:20:51 PM UTC-8, Matt Herron Jr. wrote:
> On Tuesday, February 27, 2018 at 7:16:13 AM UTC-8, Ross wrote:
> > Not sure how many Discus 2 and Ventus 2 fuses I made, but none of them have a glass turtle deck.
> > Radio antenna located in the fin which is glass, except the Nimbus 4 where it is located in the rudder.
> > This however doesn't help your issue
>
> Has anyone tried putting a transponder antenna like the T2glued to the inside the turtle deck? does the antenna require a downward view? 250 watts of RF too close to your head? This would make install on an ASW27 MUCH simpler than a fin or stinger on the outside behind the gear, not to mention more aerodynamic.

Ah no, no and no.

Bad on all counts...

Irradiating yer noggin.

The antenna needs to be as vertical as possible.

You don't want the pilot able to affect it by stacking stuff near it in the luggage area. Like say bottles of RF opaque drinking water.

The carbon fibre spars and rest of the fuselage will block much of the RF. If you don't have a carbon fibre fuselage, go ahead and stick an antenna inside there elsewhere.

Aerodynamics? Uh you flying a world record in an Eta?

But start with... KISS, just follow the glider manufacturers instructions for the transponder antenna install they recommend. The main goal is to have the transponder works as best as it can. Over the last decade or so the major manufacturers have worked this out pretty well. Back in the 1990s some of them had just awful transponder antenna installs.

[OHM Ω http://aviation.derosaweb.net]

At the risk of a hanging and in an effort to circle back to the original question ... a recap;

* Most carbon fiber (CF) gliders have a fiberglass (RF transparent) section in the turtledeck. Evidence, picture on the Schelicher page and 27 manual.. YMMV.
* Yes, this glass area can be used for GPS antennas as they are receiving signals from overhead satellites. Think flight recorders. The GPS antenna should be mounted on the surface of the turtledeck for best coverage (not lower on the shelf which will limit receiving signals from GPS satellites nearer to the horizon).
* Antennas that radiate/receive horizontally (FLARM, Xponder, etc) should not be mounted here as the remainder of the CF fuselage will block much of the signal.

May the rebuttals commence.

- John Ω