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

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.

Yep, but there is a carbon shell between antenna andthe pilot's privates.

On Friday, December 27, 2019 at 2:34:32 PM UTC-5, 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

After reading lots of comments I am still wondering why not just mount on the bottom of the fuselage in the conventional manner?
UH

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

UH, as I said in my earlier post it was way easier to install on the turtle deck than down by the gear. In my 27, it took less than 4 hours from start to finish including the transponder and power routing. The longest portion was the routing of the antenna cable up through the tunnel to the panel, not alot of room in there I didnt crimp on the bnc to that end until after I fished it up to the panel. I mounted the blade as far back as I could possibly reach in the fuselage which is close to the junction of the flap and wing.

CH

jfitch wrote on 12/29/2019 10:16 PM:
And uh.... not relevant... the pilot is inside his carbon fiber partial faraday cage cockpit, and (agreeing with Eric) the pattern from the 1/4 wave antenna with groundplane shields the pilot from the antenna.

Over a beer some time: details of teaching assistants punking male students in their undergrad physics lab classes into wearing aluminium foil "mini skirts" during a_very_ low power microwave experiment. Oh so well done.
You'd need to show me the data on that. A minimally conductive carbon fuselage with some huge holes in it, a very imperfect 1/4 wave vs. twice the distance. We're talking r^3 here. I'm not actually sure what measurements might show, and I wouldn't bet my glider on one answer over the other.

Not r^3, but only r^2 for antenna field strength reduction. It's an area thing,
not a volume thing.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

On Monday, December 30, 2019 at 5:47:51 AM UTC-8, wrote:
On Friday, December 27, 2019 at 2:34:32 PM UTC-5, 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

After reading lots of comments I am still wondering why not just mount on the bottom of the fuselage in the conventional manner?
UH

I was assuming with a CM there may be issues, never looked inside one but is there space given the engine bay and other junk?

Anyhow clearly the right thing to do is buy a new glider with transponder antenna in the tail. "It's for collision avoidance safety honey. You want me to be safe? Right?" :-)

I tried that argument earlier today Darryl.

Got shot down quickly

Lou

O
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.

Transponder rod antennas spec drag at .41 lbs at 250 MPH and blade antennas at .09 lbs at 250 MPH. That seems to be a big difference but anyone have a comment to what this measures in the gliding world?

On Monday, December 30, 2019 at 2:52:49 PM UTC-8, wrote:
O
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.

Transponder rod antennas spec drag at .41 lbs at 250 MPH and blade antennas at .09 lbs at 250 MPH. That seems to be a big difference but anyone have a comment to what this measures in the gliding world?

The simplest answer: parasitic drag ~ V^2. So at (100mph/250mph)^2 * 0.4 = ~0.07 lb. = ~1 oz.

I suspect this has been flogged to death on r.a.s. before.

"Honey I need a new glider with a tail mounted transponder antenna because it's lower drag. And I'll win more contests. Honey?" :-)