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Transponder antenna - blade vs stub monopole



 
 
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  #11  
Old June 14th 08, 07:18 AM posted to rec.aviation.soaring
Marc Ramsey[_2_]
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Posts: 211
Default Transponder antenna - blade vs stub monopole

Alan wrote:
In article brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?


Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.


23 inch radius for a quarter-wave antenna? Most installations I've seen
use no more than a 3 inch radius ground plane...
Ads
  #12  
Old June 14th 08, 09:31 AM posted to rec.aviation.soaring
Bruce
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Posts: 174
Default Transponder antenna - blade vs stub monopole

For what it is worth Schempp-hirth are mounting the transponder antenna in the fin on the Duo Discus. Right next to the
radio aerial. I presume the same applies to the Nimbus 4D that uses essentially the same fuselage.
Apparently there is lots of space for a good dipole antenna, and the foam cored glass structure is not opaque to the
frequencies used.

Nothing external to get damaged but heaven forbid you should ever have to work on it...

Presumably it is not something you could retrofit (for the same reasons you would never want to have to repair it).

Bruce

Tim Mara wrote:
I strongly suggest you DO NOT but the blade antenna's!....
ever if there were some small aerodynamic advantage (I doubt it could be
measured at any glider speeds anyway) but these are very expensive and they
are easily damaged....
I offer blade antennas, the "preferred" simple and cheap stub antenna's and
the composite dipole antenna's..I've had several buyers of the blade
antenna's break them (they aren't typically very happy when this happens).it
doesn't take much effort to damage the composite blade antennas since they
are essentially a tiny wire on a thin metal plate with small (6/32) screws
in a very nice looking shark fin that is mostly filler (putty) with a thin
composite shell..
best regards
Tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

"5Z" wrote in message
...
Any aerodynamic thoughts on the two available choices?

My ASH-26E came with the stub mounted just behind the wheel. Every
now and then it catches on the trailer, so I just straighten it, no
big deal.

An ASW-24B in our club is about to get a transponder and the factory
recommends the installation to be part way up around the fuselage
above the rear gear door hinge - looks like just behind the oxygen
bottle.

The blade looks cool, but is it really any better aerodynamically than
a $20 (http://www.aircraftspruce.com/catalog/avpages/
ted_transponder.php) stub?

-Tom



  #13  
Old June 14th 08, 02:33 PM posted to rec.aviation.soaring
COLIN LAMB
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Posts: 94
Default Transponder antenna - blade vs stub monopole

The 23" is for a 1/4 wave antenna at the frequency for the aviation radio
band (123 MHz). The transponder operates at about 1,000 MHz, so a 1/4 wave
is much smaller.

You do not even need to do much calculation, since the radius of the ground
plane should be equal to the height of the antenna. So, the diameter would
be twice the antenna height.

The shape need not be round. It can be square or rectangular. Generlaly, a
slight improvement will be achieved with a larger ground plane.

Also remember, that, even though the transmitter is pulsed at a very short
duration, there could be some possible danger having it close to your body.
If you use the ground plane to "hide" the antenna from you, there would be
little danger from exposure. The jury is still out on this one. I do not
think there is any credible evidence of a danger, but I would not expose
myself unless there was no other choice. Mine is mounted under my seat,
upside down.

Upside down is better because radio waves at this frequency are "line of
sight" and the ground stations interrogating you are all below you.

Do a good job of coax installation. I have seen some crappy jobs. Also
remember that coax attenuation at 1,000 MHz with RG-58 is very high - so
keep the run short.

Colin Lamb


  #14  
Old June 14th 08, 02:46 PM posted to rec.aviation.soaring
Tim Mara
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Posts: 375
Default Transponder antenna - blade vs stub monopole

wrong! A ground plane should have a radius of at least 1/4 wavelength of the
antenna's operating frequency, a 23" antenna ground plane then would be
about right for a VHF airband communication antenna (118-136 mhz) and in
gliders this is seldom possible so use what you can get..
Transponder antenna ground planes are typically recommended to be 6"
diameter.
These are ideal....but not always possible so use what you can, too large
isn't a problem but performance can degrade from the ideal as it becomes
smaller.
tim
Please visit the Wings & Wheels website at www.wingsandwheels.com

"Alan" wrote in message
...
In article

brianDG303 writes:
Is there a downside to mounting on a ground plane inside the fuselage
of a non-carbon glass glider?


Since the ground plane should extend at least 23 inches in each
direction around the antenna, it is probably hard to get that inside
the glider.

One that extends front and back but is more limited to the sides
(perhaps wrapping around inside) probably would work, but would be
a bit reduced from a full ground plane.

Alan



  #15  
Old June 14th 08, 03:46 PM posted to rec.aviation.soaring
Mike the Strike
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Posts: 952
Default Transponder antenna - blade vs stub monopole

I installed a blade antenna in the recommended location for my Discus
2 just behind and to one side of the gear door and an internal ground
plane about 6" square. It works very well in this location (also had
a short coax from the transponder, which is a consideration at these
higher frequencies).

Tim is absolutely right, though. It is physically a right royal pain
in the ass. I don't know anyone who has one in this location who
hasn't broken it as it is perfectly located to snag on the trailer
ramp. I have a short quarter wave monopole as back-up. This will be
my default the next time the blade breaks! I doubt that the extra
drag is worth worrying about.

If you can locate the blade above and behind the cockpit, this is a
much safer location and I have seen a couple of these installations
that also work well.

Mike
  #16  
Old June 15th 08, 01:23 AM posted to rec.aviation.soaring
COLIN LAMB
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Posts: 94
Default Transponder antenna - blade vs stub monopole

There is a unique antenna that works fine as a transponder antenna - with no
blade or stub to break off or impede airflow. It is called an annular slot
antenna. Many antenna engineering textbooks contain information on the,
but they are not common. The entire antenna is flush with the aircraft skin
and they are vertically polarized. They would work with fiberglass
aircraft.

I did find a photograph and description of a commercial one at
www.mircrowaveeng.com Look for the L-Band Annular Slot Data Sheet (8 of
10). I suspect this particular unit may be beyond the budget of the average
sailplane owner, but you can see how what they look like.

It would be a good candidate for a slippery sailplane that does not want
stuff hanging out.

Colin Lamb


  #17  
Old June 15th 08, 01:39 AM posted to rec.aviation.soaring
COLIN LAMB
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Posts: 94
Default Transponder antenna - blade vs stub monopole

Whoops, I misspelled the website. It is:

www.microwaveeng.com

Colin Lamb


  #18  
Old June 15th 08, 10:41 AM posted to rec.aviation.soaring
[email protected]
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Posts: 20
Default Transponder antenna - blade vs stub monopole

On 15 Jun, 01:39, "COLIN LAMB" wrote:
Whoops, I misspelled the website. *It is:

www.microwaveeng.com

Colin Lamb


The CAA made some comments about slot antennas in their study on low
power SSR transponders. See page 9 of:

http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf

John Galloway
  #19  
Old June 15th 08, 02:46 PM posted to rec.aviation.soaring
COLIN LAMB
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Posts: 94
Default Transponder antenna - blade vs stub monopole

John gets a gold star. Excellent report. About the only question not
answered is the comparative aerodynamic losses for a glider with the
different antennas.

Colin


  #20  
Old June 15th 08, 07:26 PM posted to rec.aviation.soaring
Martin Gregorie[_3_]
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Posts: 28
Default Transponder antenna - blade vs stub monopole

On Sun, 15 Jun 2008 02:41:42 -0700, jpg797 wrote:

On 15 Jun, 01:39, "COLIN LAMB" wrote:
Whoops, I misspelled the website. *It is:

www.microwaveeng.com

Colin Lamb


The CAA made some comments about slot antennas in their study on low
power SSR transponders. See page 9 of:

http://www.caa.co.uk/docs/810/Study%...er%201%202.pdf

It looks, from that, as if the optimum for those of us with glass
fuselages would be a vertical dipole mounted inside the rear fuselage. A
14.5cm dipole sounds small enough to fit fairly easily.

However, I have two questions for the experts, as I don't understand RF
radiation patterns or coax losses.

- my glider (Std Libelle) uses pushrods for the rudder and elevator
linkages. IIRC these run along the bottom of the fuselage. If the
dipole was mounted vertically in the upper part of the fuselage
would the control rods wreck the radiation pattern?

- with transponder in the panel and the antenna behind the oxygen
cylinder mount this would put the antenna at the end of 3 - 3.5m
or co-ax. Would that be an acceptable installation?

I hope I never have to fit a transponder, but I'd like to understand this
type of issue in case it becomes a requirement here or I take the glider
some place where one is needed.


--
[email protected] | Martin Gregorie
gregorie. |
org | Zappa fan & glider pilot


 




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