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VHF Nav/GS Duplexer

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email(at)jaredyates.com
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Posted: Fri Aug 05, 2011 7:40 am Post subject: VHF Nav/GS Duplexer

I've been wondering lately about how I should provide glideslope input for my GNS430. I saw several duplexing modules at the aeromart, but many of them had a threaded connection instead of the usual BNC turn and lock.   Are those threaded connectors available, or should I look for a unit with BNC all around?



The other option that I've considered is making or buying a dedicated antenna, though I'm not sure where I would put it. Would the elements need to be parallel to the wingspan?

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kuffel(at)cyberport.net
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Posted: Fri Aug 05, 2011 10:52 am Post subject: VHF Nav/GS Duplexer

Jared,



<<  I've been wondering lately about how I should provide glideslope input 

for my GNS430. I saw several duplexing modules at the aeromart, but many of 

them had a threaded connection instead of the usual BNC turn and lock.   Are 

those threaded connectors available, or should I look for a unit with BNC 

all around? >>



The threaded connectors are available (see Mouser.com) but come in several 

different types.  You really need to know exactly what you need to get the 

right one.  Much better is to just get the BNC type.  Chief seems to offer 

the lowest price for the Comant CI-507 at about $130.



<<  The other option that I've considered is making or buying a dedicated 

antenna, though I'm not sure where I would put it. Would the elements need 

to be parallel to the wingspan?  >>



I put mine at the center top of my windshield, forward of a cross tube. 

Don't even notice it is there when seated.  While certainly optimum to have 

the dipole parallel to the wing, it shouldn't be critical.  Even end-on 

(parallel to the fuselage) where dipoles have a null there should be enough 

signal bouncing around to be functional.  Try it and see.



Or better yet, for $6.00 in parts plus a small box and short piece of 

coaxial cable make your own diplexer.  See the attached diagram.



Tom Kuffel

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nuckolls.bob(at)aeroelect
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Posted: Fri Aug 05, 2011 5:29 pm Post subject: VHF Nav/GS Duplexer

At 10:00 AM 8/5/2011, you wrote:

  	  Quote: 	
		  --> AeroElectric-List message posted by: Jared Yates <email(at)jaredyates.com>



 I've been wondering lately about how I should provide glideslope input for my GNS430. I saw several duplexing modules at the aeromart, but many of them had a threaded connection instead of the usual BNC turn and lock.   Are those threaded connectors available, or should I look for a unit with BNC all around? 	


   The ones that are threaded but same size as BNC are

   probably TNC connectors . . . there threaded cousin

   to the bayonet lock BNC. The connectors are readily

   available. 



   The diplexers are offered by several companies. Here's

   one by RA Miller



 [img]cid:.0[/img]



   and offered by Aircraft Spruce for under $100

 

  http://www.aircraftspruce.com/catalog/avpages/av570coup.php



   

  The DIY splitter suggested by Jim Wier was posted by

  Tom. This can be even simpler to make than what Jim

  suggested. I've uses a BNC "T" adapter

 [img]cid:.1[/img]



  which feeds the VOR signal through the top

  of the 'tee'. Then there's an RF game to be

  played off the branch with the 10pF capacitor

  and a couple of BNC chassis connectors



 [img]cid:.2[/img]



  and some long 4-40 screws. You end up with

  a more compact and robust version of Jim's

  coupler with a minimum of disruption of the

  VOR feedline characteristics.



  I've got a program working out at Hawker-Beech.

  I'll see if I can get a prototype over to the

  RF lab and optimize the dimesions of the

  10 pF capacitor wiring. 



  Jim called out exactly 1" overall lead length

  for the capacitor. That's because the capacitor

  COMBINED with the inductance of the leads forms

  a series resonant turned circuit that picks off

  the glide slope signal for routing it to the

  GS receiver.



  Once those dimensions are developed, I can

  publish a good DIY project. Or you can build

  what Jim has already suggested. It works too.



 

  	  Quote: 	
		  The other option that I've considered is making or buying a dedicated antenna, though I'm not sure where I would put it. Would the elements need to be parallel to the wingspan? 	


   Generally, yes. What kind of airplane are we

   talking about?



   GS signals on approach are VERY strong. The

   antenna can suffer huge indignities of missed

   optimization and still get you down the pipe

   in good shape.



 

        Bob . . .

                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================

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nuckolls.bob(at)aeroelect
Guest

Posted: Fri Aug 05, 2011 7:40 pm Post subject: VHF Nav/GS Duplexer

At 08:50 PM 8/5/2011, you wrote:

 	  Quote: 	
		  Thanks everyone, I'm intrigued by the DIY duplex option, assuming 

that I can find the parts.  I especially like Bob's idea of a design 

that minimizes losses to the VHF signal, since that was my only 

motivation for a separate GS antenna.  The airplane is a Bearhawk, 

which is a tailwheel tube and fabric fuselage with a aluminum skin 

from the instrument panel to the nose bowl.


	


   Okay, Cessna put tens of thousands of GS antennas just

   above the compass centered on the windshield. A simple

   dipole 17" long fed in the center with 50 ohm coax

   will perform quite well.



   We had a fancy, clear plastic molded housing that

   contained a copper foil strip. A short length of RG174

   (itty-bitty 50 ohm coax) fed the center and took the

   feed-line under the windshield and upholstery trim. I think

   it spliced onto RG58 to take the signal on to the

   GS receiver.



   If you can manage RG400's larger diameter, it would

   be the material of choice for getting the GS signal

   down to your panel mounted receiver.



   I proposed a methodology for fabricating a GS antenna

   in the chapter on antennas . . . but forget the toroid

   cores . . . they add no observable value.

        Bob . . .

                    ////

                   (o o)

    ===========o00o=(_)=o00o=========

    < Go ahead, make my day . . .   >

    < show me where I'm wrong.      >

    =================================

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 06, 2011 5:25 am Post subject: VHF Nav/GS Duplexer

At 08:00 AM 8/6/2011, you wrote:

 	  Quote: 	
		  Would an antenna like that need a balun,


	


   Technically, it is "best"  . . . practically,

   you wouldn't see any difference in performance

   if it did not have one.



 	  Quote: 	
		    and what kind of gap would be acceptable between the two elements?


	


    . . . minimum practical. Actually, you can fabricate

   a nifty antenna just from coax cable.



   In this case, you clean off the outer jacket of your

   feedline about 11". If it's a double shielded coax,

   strip of the outer layer of shield too.



   Pull center conductor out of the remaining shield

   as illustrated here:



http://www.aeroelectric.com/articles/shldwire/shldwire.html



   Stretch the center conductor and remaining

   braid out into a dipole and trim each to 9.5"

   per side.



   Glue to 1/4" wood or plastic dowel 17" long. Just

   spot it in a few places to fixture it on the dowel.

   Then put pieces of heat shrink over each

   leg of the dipole to "dress it up".



   This technique eliminates the process-sensitive

   and never pretty junction between antenna elements

   an the feedline. It's all one piece.



   Bob . . .

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harley(at)AgelessWings.co
Guest

Posted: Sat Aug 06, 2011 5:45 am Post subject: VHF Nav/GS Duplexer

Bob...

     

     A little inconsistency here...I think...the length of the antenna     elements and the dowel don't match.

     

     Did you mean to cut the shield and center conductor to 8.5 inches     long each and use a 17 inch dowel, or to use a dowel 19 inches long     for the 9.5" element length? 

     

     Or leave two inches of the center of the coax unattached and hanging     at something like a "Y"? Or one inch of each element hanging over     each end of the dowel?

     

     Harley

     

     [quote]   . . . minimum practical. Actually, you can       fabricate       

         a nifty antenna just from coax cable.       

       

         In this case, you clean off the outer jacket of your       

         feedline about 11". If it's a double shielded coax,       

         strip of the outer layer of shield too.       

       

         Pull center conductor out of the remaining shield       

         as illustrated here:       

       

       http://www.aeroelectric.com/articles/shldwire/shldwire.html       

       

         Stretch the center conductor and remaining       

         braid out into a dipole and trim each to 9.5"       

         per side.       

       

         Glue to 1/4" wood or plastic dowel 17" long. Just       

         spot it in a few places to fixture it on the dowel.       

         Then put pieces of heat shrink over each       

         leg of the dipole to "dress it up".       

       

         This technique eliminates the process-sensitive       

         and never pretty junction between antenna elements       

         an the feedline. It's all one piece.       

       

         Bob . . .       

       

     [b]

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email(at)jaredyates.com
Guest

Posted: Sat Aug 06, 2011 5:47 am Post subject: VHF Nav/GS Duplexer

Awesome, that sounds like the best solution so far!  I have some hefty spanwise steel structure up at the top of the windshield.  In this application should I be worried about being close to those?



 On Sat, Aug 6, 2011 at 9:20 AM, Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

[quote] --> AeroElectric-List message posted by: "Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)>

 



 At 08:00 AM 8/6/2011, you wrote:

  	  Quote: 	
		   Would an antenna like that need a balun,

 	
 



   Technically, it is "best"  . . . practically,

   you wouldn't see any difference in performance

   if it did not have one.

 

  	  Quote: 	
		    and what kind of gap would be acceptable between the two elements?

 	
 



    . . . minimum practical. Actually, you can fabricate

   a nifty antenna just from coax cable.

 

   In this case, you clean off the outer jacket of your

   feedline about 11". If it's a double shielded coax,

   strip of the outer layer of shield too.

 

   Pull center conductor out of the remaining shield

   as illustrated here:

 

 http://www.aeroelectric.com/articles/shldwire/shldwire.html

 

   Stretch the center conductor and remaining

   braid out into a dipole and trim each to 9.5"

   per side.

 

   Glue to 1/4" wood or plastic dowel 17" long. Just

   spot it in a few places to fixture it on the dowel.

   Then put pieces of heat shrink over each

   leg of the dipole to "dress it up".

 

   This technique eliminates the process-sensitive

   and never pretty junction between antenna elements

   an the feedline. It's all one piece.

 

   Bob . . .



 

 

 ====================================

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 ====================================

 http://forums.matronics.com

 ====================================

 le, List Admin.

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

 ====================================

 

 

 

 



[b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 06, 2011 7:55 am Post subject: VHF Nav/GS Duplexer

At 08:41 AM 8/6/2011, you wrote:

  	  Quote: 	
		  Bob...

 

 A little inconsistency here...I think...the length of the antenna elements and the dowel don't match.



 Did you mean to cut the shield and center conductor to 8.5 inches long each and use a 17 inch dowel, or to use a dowel 19 inches long for the 9.5" element length? 



 Or leave two inches of the center of the coax unattached and hanging at something like a "Y"? Or one inch of each element hanging over each end of the dowel? 	


    No, stretch the whole thing out on a dowel of appropriate

    length . . . in this case the 9.5" was a typo, should

    be 8.5" and 17" overall. Thanks for the heads-up!

  



 

        Bob . . .

                    //// 

                   (o o) 

    ===========o00o=(_)=o00o========= 

    < Go ahead, make my day . . .   > 

    < show me where I'm wrong.      > 

    =================================



     [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Sat Aug 06, 2011 7:59 am Post subject: VHF Nav/GS Duplexer

At 08:43 AM 8/6/2011, you wrote:

 	  Quote: 	
		  Awesome, that sounds like the best solution so far!  I have some 

hefty spanwise steel structure up at the top of the windshield.  In 

this application should I be worried about being close to those?


	




  Technically, yes. Ignore them for now. Space away as far

  as sensibilities and craftsmanship will allow and give

  it a try. Close proximity of the conductive structure will

  electrically lengthen the antenna . . . so after installation

  the purist would trim to new resonance after installation.



  In this case, I don't think it's going to make much operational

  difference.

        Bob . . .

                    ////

                   (o o)

    ===========o00o=(_)=o00o=========

    < Go ahead, make my day . . .   >

    < show me where I'm wrong.      >

    =================================

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