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what should antenna resistance read

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CardinalNSB(at)aol.com
Guest

Posted: Thu Aug 17, 2006 8:30 pm Post subject: what should antenna resistance read

If I remove the connector from the back of my radio and measure across the  shield and inner conductor  of the antenna coax (leaving the antenna  connected at the other end, what resistance should I see for:

  

 comm antenna Cessna type

  

 dme short type with ball on end

  

 marker beacon wire type

  

 gps Garmin active type

  

  

 Thank you, Skip Simpson

  [quote][b]

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FLYaDIVE(at)aol.com
Guest

Posted: Thu Aug 17, 2006 11:22 pm Post subject: what should antenna resistance read

In a message dated 8/18/06 12:33:14 AM Eastern Daylight Time, 

CardinalNSB(at)aol.com writes:



 	  Quote: 	
		   If I remove the connector from the back of my radio and measure across the  

  shield and inner conductor  of the antenna coax (leaving the antenna  

 connected at the other end, what resistance should I see for:

  comm antenna Cessna type

  dme short type with ball on end

  marker beacon wire type

  gps Garmin active type

   

   

  Thank you, Skip Simpson

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


Skip:



Don't do it.  The word RESISTANCE is a not the proper term and is very 

misleading.  The proper term is IMPEDANCE and that cannot be read with a VOM. 



If you do try to read a resistance there is no way of knowing what it should 

or should not read.  For example the GPS you said is active, that means it has 

an amplifier built in.  So you will be reading something that relates to the 

output of the amplifier.  This could be a transistor, capacitor, resistive or 

inductor output.  What the value is, is not know or appropriate for a VOM.  

The other antennas are also unknown variables due to the different 

configurations for impedance matching.



Simple answer: Don't do it.  The best you can guess would be IF you see ZERO 

OHMS you may have a SHORT.  But any reading you get is a GUESS and NOT REAL.



Barry

"Chop'd Liver"



"Show them the first time, correct them the second time, kick them the third 

time." 

Yamashiada

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europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Fri Aug 18, 2006 6:52 am Post subject: Re: what should antenna resistance read

 	  CardinalNSB(at)aol.com wrote: 	
		   comm antenna Cessna type

  

 dme short type with ball on end

  

 marker beacon wire type

  

 gps Garmin active type 	


Some of this is simple.  The DME and comm antenna should read infinity (open).  The sled-type marker should read dead short. Active GPS -- whatever.



Fred F.

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sportav8r(at)gmail.com
Guest

Posted: Fri Aug 18, 2006 7:23 am Post subject: what should antenna resistance read

Barry is correct, but I think we can assume the proper DC reading for

the first 3 types listed is infinite resistance.  Nix that, if a

(transformer-type) balun is in the line, however.

Best tool for the readings you're wanting to take is an antenna

analyzer, which will reveal the impedance and also any transmission

line (coax) shorts, opens.   But don't attach any type of measuring

device to an active GPS antenna unless you absolutely know what you're

doing.

-Bill B.

On 8/18/06, FLYaDIVE(at)aol.com <FLYaDIVE(at)aol.com> wrote:

 	  Quote: 	

 In a message dated 8/18/06 12:33:14 AM Eastern Daylight Time,

 CardinalNSB(at)aol.com writes:

 > If I remove the connector from the back of my radio and measure across the

 >  shield and inner conductor  of the antenna coax (leaving the antenna

 > connected at the other end, what resistance should I see for:

 >  comm antenna Cessna type

 >  dme short type with ball on end

 >  marker beacon wire type

 >  gps Garmin active type

 >

 >

 >  Thank you, Skip Simpson

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

 Skip:

 Don't do it.  The word RESISTANCE is a not the proper term and is very

 misleading.  The proper term is IMPEDANCE and that cannot be read with a VOM.

 If you do try to read a resistance there is no way of knowing what it should

 or should not read.  For example the GPS you said is active, that means it has

 an amplifier built in.  So you will be reading something that relates to the

 output of the amplifier.  This could be a transistor, capacitor, resistive or

 inductor output.  What the value is, is not know or appropriate for a VOM.

 The other antennas are also unknown variables due to the different

 configurations for impedance matching.

 Simple answer: Don't do it.  The best you can guess would be IF you see ZERO

 OHMS you may have a SHORT.  But any reading you get is a GUESS and NOT REAL.

 Barry

 "Chop'd Liver"

 "Show them the first time, correct them the second time, kick them the third

 time."

 Yamashiada

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europa flugzeug fabrik

Joined: 13 Feb 2006
Posts: 65
Location: North Coast, USA

Posted: Fri Aug 18, 2006 12:19 pm Post subject: Re: what should antenna resistance read

 	  sportav8r(at)gmail.com wrote: 	
		  Barry is correct, but I think we can assume the proper DC reading for the first 3 types listed is infinite resistance.  Nix that, if a (transformer-type) balun is in the line, however. 	


For marker, we have to define "wire type."  If suspended between two insulators, then it should be infinite.  However, a 1/4-wave monopole antenna -- comm and DME --  does not require a balun.  I mention this only because if a low resistance (probably dead short) is found, I wouldn't assume a transformer balun there, but rather check for a short somewhere, like in the coax.



Fred F.

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N6030X(at)DaveMorris.com
Guest

Posted: Fri Aug 18, 2006 3:27 pm Post subject: what should antenna resistance read

There are some antenna types that will present a dead short at DC and 

yet the proper impedance at RF.  I don't know that any of your 

antennas are of that type, but beware that just because you read a 

dead short with a DC voltmeter does not necessarily mean 

anything.  On the other hand, if you disconnect your antenna from its 

coax and still read a dead short between the coax center and braid, I 

would look for bad connectors.



Dave Morris



At 03:19 PM 8/18/2006, you wrote:

 	  Quote: 	
		  

<n3eu(at)comcast.net>

sportav8r(at)gmail.com wrote:

 > Barry is correct, but I think we can assume the proper DC reading 

 for the first 3 types listed is infinite resistance.  Nix that, if 

 a (transformer-type) balun is in the line, however.



For marker, we have to define "wire type."  If suspended between two 

insulators, then it should be infinite.  However, a 1/4-wave 

monopole antenna -- comm and DME --  does not require a balun.  I 

mention this only because if a low resistance (probably dead short) 

is found, I wouldn't assume a transformer balun there, but rather 

check for a short somewhere, like in the coax.



Fred F.





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brian

Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA

Posted: Sat Aug 19, 2006 8:04 pm Post subject: what should antenna resistance read

On Aug 17, 2006, at 9:28 PM, CardinalNSB(at)aol.com wrote:



 	  Quote: 	
		   If I remove the connector from the back of my radio and measure  

 across the shield and inner conductor  of the antenna coax (leaving  

 the antenna connected at the other end, what resistance should I  

 see for:



 comm antenna Cessna type


	


infinite ohms or something close to zero ohms. If there is any sort  

of matching network in the antenna then the antenna will probably  

appear to be at DC ground.

 	  Quote: 	
		  

 dme short type with ball on end


	


infinite ohms.



 	  Quote: 	
		  

 marker beacon wire type


	


infinite ohms or close to zero ohms. The wire type of MB antenna has  

a tap some distance down the antenna but one end of the antenna will  

be connected to the airframe. This will make it appear as a dead  

short at DC.



 	  Quote: 	
		  

 gps Garmin active type


	


Something less than infinite ohms, probably something like a couple  

hundred ohms, but make sure that the ohmmeter lead that is positive  

goes to the center pin. Most inexpensive ohmmeters have the *black*  

lead be positive. Check it with another meter to be sure.



Brian Lloyd                         361 Catterline Way

brian-yak AT lloyd DOT com          Folsom, CA 95630

+1.916.367.2131 (voice)             +1.270.912.0788 (fax)



I fly because it releases my mind from the tyranny of petty things . . .

— Antoine de Saint-Exupéry

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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)

I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery

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