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Transponder Antenna

 
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Tundra10



Joined: 14 Jun 2010
Posts: 99
Location: Scarborough, Ontario

PostPosted: Mon Dec 31, 2018 9:05 pm    Post subject: Transponder Antenna Reply with quote

I am installing a Garmin 335 transponder, using an existing antenna
that has been on the aluminium airplane for decades. It is a blade
style.

I was surprised to measure with the ohm meter between the cable shield
and the air frame and found infinite resistance. I would have
expected the antenna to have a very good connection to the aluminium
skin.

Or is this some kind of dipole antenna and the measurement to be expected ?

I don't want to power on the transponder until I know the antenna is correct.

Thanks and Happy New Year !

Jeff Page
Dream Aircraft Tundra #10


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Kellym



Joined: 10 Jan 2006
Posts: 1598
Location: Sun Lakes AZ

PostPosted: Mon Dec 31, 2018 9:12 pm    Post subject: Transponder Antenna Reply with quote

Have you measured between the ground side of the antenna BNC connector
and airframe ground? Or is that what you meant?

On 12/31/2018 10:03 PM, Jeff Page wrote:
Quote:


I am installing a Garmin 335 transponder, using an existing antenna that
has been on the aluminium airplane for decades.  It is a blade style.

I was surprised to measure with the ohm meter between the cable shield
and the air frame and found infinite resistance.  I would have expected
the antenna to have a very good connection to the aluminium skin.

Or is this some kind of dipole antenna and the measurement to be expected ?

I don't want to power on the transponder until I know the antenna is
correct.

Thanks and Happy New Year !

Jeff Page
Dream Aircraft Tundra #10







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Tundra10



Joined: 14 Jun 2010
Posts: 99
Location: Scarborough, Ontario

PostPosted: Tue Jan 01, 2019 12:08 am    Post subject: Re: Transponder Antenna Reply with quote

This is a Cessna 172, so the aluminum skin is ground.
During the measurement I made, the cable was not connected to the transponder, but was connected to the antenna.
I measured from the shield of the connector at the transponder end, and the frame of the airplane.
Unless this is an unusual antenna, I expect this to be a zero ohm reading.
If the blade transponder antennas are standard, then either the cable is bad, or the antenna is poorly mounted, and I will have to pull up the carpet and inspection panels to look at it.
I didn't want to do that, only to find out the transponder antenna design is unusual.

Thanks !

Jeff Page
Dream Aircraft Tundra #10


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gilles.thesee(at)free.fr
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PostPosted: Tue Jan 01, 2019 4:43 am    Post subject: Transponder Antenna Reply with quote

Le 01/01/2019 à 09:08, Tundra10 a écrit :

Quote:
Quote:

I measured from the shield of the connector at the transponder end, and the frame of the airplane.
Unless this is an unusual antenna, I expect this to be a zero ohm reading.
If the blade transponder antennas are standard, then either the cable is bad, or the antenna is poorly mounted, and I will have to pull up the carpet and inspection panels to look at it.
I didn't want to do that, only to find out the transponder antenna design is unusual.

Jeff,

It would be interesting to know whether the previous transponder was working.
What are the dimensions of your blade XPDR antenna ?
If it is in the 3.23" skin to top ball park, then the antenna is "usual".

I notice you mentioned the antenna has been sitting on the airplane skin for decades. Maybe it is a good time for dissassembling, ensuring clean metal contact, and changing the aging coax ?

Season's greetings
--
Best regards,
Gilles
http://contrails.free.fr
http://lapierre.skunkworks.free.fr


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Kellym



Joined: 10 Jan 2006
Posts: 1598
Location: Sun Lakes AZ

PostPosted: Tue Jan 01, 2019 5:44 am    Post subject: Transponder Antenna Reply with quote

It depends on the vintage Cessna. Many from the era before TCAs did not
come with a transponder, only those that were sold with an IFR radio
package were likely to have one. If the blade has been on the airplane a
long time, I'd put odds as more than 50% that the cable is bad. Not to
mention that it is very likely RG-58 or similar variety cable for an old
mode C transponder. You really want RG-400 for an ADS-B transponder. So
I would say it is very worth the effort to remove the old cable, check
the antenna and the cable. If you can find a homebuilder with the
crimper and RG-400, it would be cheaper than ordering the proper length
pre-made cable or having an avionics shop make one up. Expect at least
$3.00 per foot or more for the RG-400. It wasn't that many years ago
that some BNC connectors were either compression or soldered assemblies.
I had one that was produced in the '70s that gave me no amount of grief
on a com radio until I finally spotted that the soldered shield
connection on one end was a cold solder joint. My last aircraft had
transponder cable with compression BNC connector that often came apart
when belly panel had to be removed for annual. Fixed that with a crimp
on connector.

On 1/1/2019 1:08 AM, Tundra10 wrote:
Quote:


This is a Cessna 172, so the aluminum skin is ground.
During the measurement I made, the cable was not connected to the transponder, but was connected to the antenna.
I measured from the shield of the connector at the transponder end, and the frame of the airplane.
Unless this is an unusual antenna, I expect this to be a zero ohm reading.
If the blade transponder antennas are standard, then either the cable is bad, or the antenna is poorly mounted, and I will have to pull up the carpet and inspection panels to look at it.
I didn't want to do that, only to find out the transponder antenna design is unusual.

Thanks !

Jeff Page
Dream Aircraft Tundra #10




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Tundra10



Joined: 14 Jun 2010
Posts: 99
Location: Scarborough, Ontario

PostPosted: Wed Jan 02, 2019 2:52 pm    Post subject: Transponder Antenna Reply with quote

Yesterday, I pulled up the carpet and open the inspection panel to get
at the antenna connector. No connection there between the aluminium
airframe and the connector shield.

I unscrewed the antenna and am surprised how well the old Narco AT50
transponder worked. It is doubtful there ever was a ground connection.
The antenna was mounted with a cork gasket and all the original
paint was still on the antenna and the plane.

The antenna turned out to be a Dorne & Margolin DMNI70-2. The nice
label was conveniently located hidden inside against the airplane.

I cleaned up the base of the antenna with Scotchbrite and removed the
paint around the screw holes on the airplane. Alumiprep and Alodine
treated the bare aluminium.

For good measure I will also fabricate a new RG-400 cable.

Thanks for everyone's comments and advice.

Jeff Page
Dream Aircraft Tundra #10


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Kellym



Joined: 10 Jan 2006
Posts: 1598
Location: Sun Lakes AZ

PostPosted: Wed Jan 02, 2019 3:36 pm    Post subject: Transponder Antenna Reply with quote

The cork gasket should not have mattered. The ground connection should
be across the screws and nuts or nut plates. At least that is how most
antennas are designed. Those old Narco transponders were pretty tolerant
of marginal coax connections. You do want the base of the antenna to
aircraft skin joint to be sealed. Often done with narrow bead of clear
or white RTV after the antenna is mounted.

On 1/2/2019 2:42 PM, Jeff Page wrote:
Quote:


Yesterday, I pulled up the carpet and open the inspection panel to get
at the antenna connector.  No connection there between the aluminium
airframe and the connector shield.

I unscrewed the antenna and am surprised how well the old Narco AT50
transponder worked. It is doubtful there ever was a ground connection.
 The antenna was mounted with a cork gasket and all the original paint
was still on the antenna and the plane.

The antenna turned out to be a Dorne & Margolin DMNI70-2.  The nice
label was conveniently located hidden inside against the airplane.

I cleaned up the base of the antenna with Scotchbrite and removed the
paint around the screw holes on the airplane.  Alumiprep and Alodine
treated the bare aluminium.

For good measure I will also fabricate a new RG-400 cable.

Thanks for everyone's comments and advice.

Jeff Page
Dream Aircraft Tundra #10






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KCHD
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Tundra10



Joined: 14 Jun 2010
Posts: 99
Location: Scarborough, Ontario

PostPosted: Thu Jan 03, 2019 1:52 pm    Post subject: Re: Transponder Antenna Reply with quote

One thing I find odd about the blade transponder antenna, is that the entire body is a chunk of aluminum. There is a horizontal band with a plastic? covering. So it seems the radiating element is pretty shielded inside.

It is a Dorne & Margolin DM NI 70-2 if anyone is familiar with it.

Photo attached.

Jeff Page
Dream Aircraft Tundra #10


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PostPosted: Fri Jan 04, 2019 5:42 am    Post subject: Transponder Antenna Reply with quote

At 03:52 PM 1/3/2019, you wrote:
Quote:
--> AeroElectric-List message posted by: "Tundra10" <jpx(at)qenesis.com>

One thing I find odd about the blade transponder antenna, is that the entire body is a chunk of aluminum. There is a horizontal band with a plastic? covering. So it seems the radiating element is pretty shielded inside.

It is a Dorne & Margolin DM NI 70-2 if anyone is familiar with it.

That whole chunk of aluminum above
the plastic insulator IS the antenna.



Bob . . .


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Tundra10



Joined: 14 Jun 2010
Posts: 99
Location: Scarborough, Ontario

PostPosted: Fri Jan 04, 2019 7:33 am    Post subject: Re: Transponder Antenna Reply with quote

This would be easier for me to understand if the plastic insulator went all the way around.

It only goes halfway, so the aluminum at the tip of the antenna is the same solid chunk of aluminum as at the base, which is connected to the shield of the cable and the aluminum skin of the aircraft.

Jeff Page
Dream Aircraft Tundra #10


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PostPosted: Fri Jan 04, 2019 9:57 am    Post subject: Transponder Antenna Reply with quote

At 09:33 AM 1/4/2019, you wrote:
Quote:
--> AeroElectric-List message posted by: "Tundra10" <jpx(at)qenesis.com>

This would be easier for me to understand if the plastic insulator went all the way around.

It only goes halfway, so the aluminum at the tip of the antenna is the same solid chunk of aluminum as at the base, which is connected to the shield of the cable and the aluminum skin of the aircraft.

If only one side of the blade is insulated
then the antenna's feed-point is at the
center of the insulated side with
some form of matching not unlike
a 'gamma match' on a 1/4 wave vertical.

This allows the whole antenna to be formed
from a single piece of metal for mechanical
robustness, total ground at DC, yet presents
a 50 ohm load to the transmission line.

Antenna design and fabrication often seems
like magic . . . looking through the myriad
of patents can leave one in awe for those
to have mastered the art! Just did a quick
search on 'blade antennas' hoping that the
antenna under discussion might pop up . . .

https://tinyurl.com/ya24kfgu

good luck with that!

I think I've got some data on that design
buried around here somewhere. If I run
across the article, I'll post it.


Bob . . .


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