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roll my own transponder antenna?

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messydeer

Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

Posted: Sun Aug 04, 2013 7:33 pm Post subject: roll my own transponder antenna?

Hey 



I rolled my own comm antenna a couple years ago. Antenna worked okay, but the location caused RFI, so I bought one and mounted it in a different location. This homemade com antenna could be cut down and used for my transponder antenna.



But, how long do I cut it? Also heard about not being able to check swr in the xpdr freq range, so if I mess up, it could ruin my unit. ACS sells the antenna for $18, so if there's any doubt, I'd go that way.



Thanks,

Dan

_________________
Dan

messydeer

Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

Posted: Mon Aug 05, 2013 6:23 am Post subject: Re: roll my own transponder antenna?

Well, that cheap TED xpdr antenna is backordered. I came up with a few different lengths, googling. The diagram below shows it's ~2.5" from the ground plane (aluminum skin). Also read on the sequair site it would be 3.1", but that could be for a non-aluminum plane. 



http://www.seqair.com/skunkworks/Antennas/TpxAntProbs/TpxAntProbs.html



Thanks,

Dan



PS: Does that 5/16" ball at the end effect the antenna length? Does it reduce drag??

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Bob McC

Joined: 09 Jan 2006
Posts: 258
Location: Toronto, ON

Posted: Mon Aug 05, 2013 7:55 am Post subject: roll my own transponder antenna?

Dan;



I believe the 3.1" dimension on the Sequoia site is the overall length of

the rod not the portion beyond the ground plane, and yes, this is for a

wooden aeroplane with a relatively small ground plane. The article also

speaks to a larger ground plane having some effect on the length.



Bob McC.

[quote] --

_________________
Bob McC
Falco #908
(just starting)

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 06, 2013 3:34 am Post subject: roll my own transponder antenna?

At 09:23 AM 8/5/2013, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: "messydeer" <messydeer(at)yahoo.com>



 Well, that cheap TED xpdr antenna is backordered. I came up with a few different lengths, googling. The diagram below shows it's ~2.5" from the ground plane (aluminum skin). Also read on the sequair site it would be 3.1", but that could be for a non-aluminum plane. 



  http://www.seqair.com/skunkworks/Antennas/TpxAntProbs/TpxAntProbs.html 

 	


    There's a host of observed effects not all of which

    are significant. When you see an SWR plot like this . . .



 [img]cid:.0[/img] 



    all those peaks and valleys are artifacts of effects

    OTHER than the antenna itself. The plots achieved

    later



 [img]cid:.1[/img]



    were MUCH smoother and had a a deep 'dip' toward

    1:1 SWR at the frequency of interest.



    The article properly identifies the need for TIGHT

    electrical connection between the various components,

    preferably soldered.

  

    The 2.9" figure for overall length above the ground

    plane is fine. The article fails to mention that the

    'resonant ground plane' has a radius equal to the

    height of the antenna or a diameter of 5.8".  Alternatively

    you go with the non-resonant ground plane (like a belly

    skin) were exact dimensions are LARGE compared to

    2.9" but operationally insignificant.



    The thing about every antenna needing to be tailored

    to an installation is bogus. Early transponders used

    a one-tube transmitter consisting of an pencil triode

    as an oscillator operating at a frequency controlled

    by a resonant cavity.



   

 [img]cid:.2[/img]    



 

   These transponders could be 'pulled' off-frequency by

   the effects of antennas. Modern, solid state transponders

   are not so afflicted.



   If you're building for a non-metalic airplane, 2.9"

   is the magic number for antenna length and radius

   of a ground plane. For a metal airplane, go for 2.9"

   antenna on an 'infinite' ground plane.



   In any case, metallic joints should be electrically

   'tight' . . . solder is good. You can pick up one

   of these for about $20 shipping included.



 [img]cid:.3[/img]



  http://tinyurl.com/lulfq5z



   You would mount this on a 2.9" radius disk for use

   in wood or plastic airplane.



 

   Bob . . .

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JOHN TIPTON

Joined: 17 Sep 2006
Posts: 239
Location: Torquay - England

Posted: Tue Aug 06, 2013 3:44 am Post subject: roll my own transponder antenna?

Would two units be a good idea, as a 'dipole' - one to  ground (sleeve) lead



 [quote]   ---

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 06, 2013 4:01 am Post subject: roll my own transponder antenna?

At 06:43 AM 8/6/2013, you wrote:

  	  Quote: 	
		   Would two units be a good idea, as a 'dipole' - one to ground (sleeve) lead 	


   Sure, it's been done.



  http://tinyurl.com/n5ac7zt



 

   this antenna has limited applicability in

   wood/plastic airplanes only and mounted

   on the side of the fuselage. Check out

   the SWR plot.



 

   

   The large area, small aspect ratio antenna elements

   account for the broad-band performance.



 



 

 

   Bob . . .    [quote][b]

dee.whittington(at)gmail.
Guest

Posted: Tue Aug 06, 2013 10:03 am Post subject: roll my own transponder antenna?

Bob,



We've mounted the Bob Archer transponder antenna you picture just aft of Bulkhead B on the right side of the fuselage in our Sportsman 2+2. We haven't flown yet (gosh, going on 9 1/2 years of building) but we hope it works well. If we buckle down first flight will be this fall.

 



Dee

N18TA reserved



DeWitt (Dee) Whittington804-677-4849 iPhone

804-358-4333 Home

www.VirginiaFlyIn.org

 Building Glasair Sportsman with 3 partners





 



On Tue, Aug 6, 2013 at 8:00 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

 [quote]  At 06:43 AM 8/6/2013, you wrote:

  	  Quote: 	
		   Would two units be a good idea, as a 'dipole' - one to ground (sleeve) lead 	


   Sure, it's been done.



  http://tinyurl.com/n5ac7zt



 

   this antenna has limited applicability in

   wood/plastic airplanes only and mounted

   on the side of the fuselage. Check out

   the SWR plot.



 

   

   The large area, small aspect ratio antenna elements

   account for the broad-band performance.



 



 

 

   Bob . . .     	  Quote: 	
		  



ist" target="_blank">http://www.matronics.com/Navigator?AeroElectric-List

tp://forums.matronics.com

_blank">http://www.matronics.com/contribution



	




[b]

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Aug 06, 2013 11:36 am Post subject: roll my own transponder antenna?

At 10:33 PM 8/4/2013, you wrote:

 	  Quote: 	
		  



Hey 



I rolled my own comm antenna a couple years ago. Antenna worked 

okay, but the location caused RFI, so I bought one and mounted it in 

a different location. This homemade com antenna could be cut down 

and used for my transponder antenna.



But, how long do I cut it? Also heard about not being able to check 

swr in the xpdr freq range, so if I mess up, it could ruin my unit. 

ACS sells the antenna for $18, so if there's any doubt, I'd go that way.


	


    A comm antenna design is generally more robust at

    the base to accommodate the wind and vibration loads

    of the larger antenna. Further, the value of keeping

    VERY short leads outside the coax world is not so

    critical.  Unless you can measure and trim for low

    VSWR in your modified antenna, there is risk of failing

    to meet design goals.



   Bob . . .

messydeer

Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

Posted: Tue Aug 06, 2013 2:14 pm Post subject: Re: roll my own transponder antenna?

Thanks, Bob 



I bought one of those $20 ones off ebay yesterday. If I get antsy, get my xpdr installed and want it tested for mode C before it arrives in the mail, I'll cut down my homebuilt antenna to 2.9".

_________________
Dan

tim2542(at)sbcglobal.net
Guest

Posted: Tue Aug 06, 2013 2:44 pm Post subject: roll my own transponder antenna?

I probably missed part of the discussion but why are we trying to home brew a transponder antenna when you can buy one for $20?

ACS or Steinair both sell one that works great. http://www.steinair.com/store.cfm?tlcatid=45

Tim



Sent from my iPad



On Aug 6, 2013, at 3:15 PM, "messydeer" <messydeer(at)yahoo.com (messydeer(at)yahoo.com)> wrote:

[quote]--> AeroElectric-List message posted by: "messydeer" <messydeer(at)yahoo.com (messydeer(at)yahoo.com)>



Thanks, Bob 



I bought one of those $20 ones off ebay yesterday. If I get antsy, get my xpdr installed and want it tested for mode C before it arrives in the mail, I'll cut down my homebuilt antenna to 2.9".



--------

Dan





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[b]

messydeer

Joined: 13 Feb 2006
Posts: 214
Location: Bellingham, WA

Posted: Tue Aug 06, 2013 7:34 pm Post subject: Re: roll my own transponder antenna?

I'm disappointed to report that I couldn't solder the coax to the adapter fitting. I got the core soldered okay, but the braid stumped me. Solder would stick to the braid, but not the fitting. Think it's such a big heat sink. I was afraid to hold the iron on the braid too long for fear of melting the insulation. But I guess if it's not hot enough for the solder to stick to the fitting, it's not going to melt the plastic?

_________________
Dan

nuckolls.bob(at)aeroelect
Guest

Posted: Wed Aug 07, 2013 5:46 am Post subject: roll my own transponder antenna?

At 10:34 PM 8/6/2013, you wrote:

 	  Quote: 	
		  



I'm disappointed to report that I couldn't solder the coax to the 

adapter fitting. I got the core soldered okay, but the braid stumped 

me. Solder would stick to the braid, but not the fitting. Think it's 

such a big heat sink. I was afraid to hold the iron on the braid too 

long for fear of melting the insulation. But I guess if it's not hot 

enough for the solder to stick to the fitting, it's not going to 

melt the plastic?


	


   If you're using a modern coax (RG-141/142/400) then

   soldering temperatures will not harm the insulation.

   Sounds like your soldering iron is too small.



   Bob . . .

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