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Comm Antenna

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Dan Reeves

Joined: 10 Jan 2006
Posts: 16

Posted: Tue Sep 25, 2007 6:05 pm Post subject: Comm Antenna

I recently purchased a Comant CI 122 bent whip comm antenna.  If I check the continuity between the base plate and the outer case (ground) of the BNC connector, all is well.  However if I check the continuity between the center pin of the BNC connector and the metal whip/rod/antenna I get no needle deflection.  Do I have a defective antenna?

   

  Thanks

  Dan Reeves

  RV-7A



         [quote][b]

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wrmaxwell(at)bigpond.com
Guest

Posted: Tue Sep 25, 2007 6:58 pm Post subject: Comm Antenna

Not familiar with the Comant CI 122 Dan but it may  be capacitor coupled inside the base of the antenna.  Maybe best checked  with Comant?

  

 Bill 

 [quote]   ---

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jetboy

Joined: 22 Jul 2006
Posts: 233

Posted: Wed Sep 26, 2007 2:22 am Post subject: Re: Comm Antenna

Dont know about Comnant but D&M com antennas have a 12.5 ohm resistor connecting the BNC to the antenna rod. This is for matching purposes to achieve acceptable performance over a wide frequency range.



A VSWR check will tell whether its OK or not



Ralph

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_________________
Ralph - CH701 / 2200a

Rhino

Joined: 01 Aug 2007
Posts: 56
Location: Xenia, Ohio

Posted: Wed Sep 26, 2007 4:14 am Post subject: Re: Comm Antenna

 	  Dan Reeves wrote: 	
		  I recently purchased a Comant CI 122 bent whip comm antenna.  If I check the continuity between the base plate and the outer case (ground) of the BNC connector, all is well.  However if I check the continuity between the center pin of the BNC connector and the metal whip/rod/antenna I get no needle deflection.  Do I have a defective antenna? 	


 	  Quote: 	
		  Why is there a short from center pin to ground on some antennas ?

Some antennas contain an impedance matching transformer, which is a series of windings that short to ground. By measuring resistance between center pin to ground on an antenna that has this configuration, you are measuring DC continuity on the primary side of the winding, and it will appear as a short. However, RF frequencies have a high impedance path to ground, and it is with this signal that these types of antennae operate. 	
http://www.comant.com/faqs.html#question23

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nuckolls.bob(at)cox.net
Guest

Posted: Wed Sep 26, 2007 4:48 am Post subject: Comm Antenna

At 06:59 PM 9/25/2007 -0700, you wrote:



 	  Quote: 	
		  I recently purchased a Comant CI 122 bent whip comm antenna.  If I check 

the continuity between the base plate and the outer case (ground) of the 

BNC connector, all is well.  However if I check the continuity between the 

center pin of the BNC connector and the metal whip/rod/antenna I get no 

needle deflection.  Do I have a defective antenna?



Thanks

Dan Reeves

RV-7A


	


    There are a variety of matching techniques used to

    make the antenna rod appear as idea a load as possible

    to the 50 ohm feedline. There is no "best" way to do

    it and some techniques may involve the use of capacitors

    that will show no DC continuity.



    The only antenna that can be investigated with an ohmmeter

    is the DIY 1/4-wave rod antennas described in the 'Connection.

    These have no matching sections and always show open circuit.

    As soon as you purchase someone's ready-to-fly antenna assembly,

    ohmmeter readings at the connector are essentially meaningless.



    A true test of the antenna's load characteristics must

    be accomplished with an instrument specific to the task

    like:



http://www.aeroelectric.com/Pictures/Tools/SWR/



    This collection of pictures suggests a variety of

    reliable and not-so-reliable test tools for VHF

    antennas.



    As an aside, antennas are sort of like bolts. If they

    ever worked, then they're probably good. There's nothing

    in them that wears out or has a propensity for fialure.



    Bob . . .

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rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Wed Sep 26, 2007 5:41 am Post subject: Comm Antenna

At 06:47 9/26/2007, you wrote:

 	  Quote: 	
		      As an aside, antennas are sort of like bolts. If they

    ever worked, then they're probably good. There's nothing

    in them that wears out or has a propensity for fialure.



    Bob . . .


	


Perhaps active antennae should be excepted here; that is those with 

electronics that require power via the cable.  LORAN and GPS both 

come to mind.  With passives, and no visible damage, I would agree.



Ron Q.

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list(at)toddheffley.com
Guest

Posted: Thu Sep 27, 2007 4:15 am Post subject: Comm Antenna

 	  Quote: 	
		   However if I check the continuity between the center pin of the BNC  

 connector and the metal whip/rod/antenna I get no needle deflection.


	


Dan.



The antenna is coupled with a "transformer"  and the center pin does  

not connect to the whip.



This is normal with a good quality com antenna.



todd

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sarg314(at)comcast.net
Guest

Posted: Thu Sep 27, 2007 4:59 pm Post subject: Comm Antenna

Dan:

    I have the same antenna and made the same observation, however the 

antenna works fine.  It appears to be a case of capacitive coupling.



Dan Reeves wrote:



 	  Quote: 	
		   I recently purchased a Comant CI 122 bent whip comm antenna.  If I 

 check the continuity between the base plate and the outer case 

 (ground) of the BNC connector, all is well.  However if I check the 

 continuity between the center pin of the BNC connector and the 

 metal whip/rod/antenna I get no needle deflection.  Do I have a 

 defective antenna?

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