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Transponder Antenna

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Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Mon Dec 31, 2018 9:05 pm Post subject: Transponder Antenna

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

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Mon Dec 31, 2018 9:12 pm Post subject: Transponder Antenna

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

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Tue Jan 01, 2019 12:08 am Post subject: Re: Transponder Antenna

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

gilles.thesee(at)free.fr
Guest

Posted: Tue Jan 01, 2019 4:43 am Post subject: Transponder Antenna

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

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Tue Jan 01, 2019 5:44 am Post subject: Transponder Antenna

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

 

 

 

 

 Read this topic online here:

 

 http://forums.matronics.com/viewtopic.php?p=486632#486632

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Wed Jan 02, 2019 2:52 pm Post subject: Transponder Antenna

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

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Wed Jan 02, 2019 3:36 pm Post subject: Transponder Antenna

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

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Thu Jan 03, 2019 1:52 pm Post subject: Re: Transponder Antenna

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

Antenna.jpg

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 04, 2019 5:42 am Post subject: Transponder Antenna

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 . . .

Tundra10

Joined: 14 Jun 2010
Posts: 102
Location: Scarborough, Ontario

Posted: Fri Jan 04, 2019 7:33 am Post subject: Re: Transponder Antenna

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

nuckolls.bob(at)aeroelect
Guest

Posted: Fri Jan 04, 2019 9:57 am Post subject: Transponder Antenna

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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