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coax antenna cable length

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mdnanwelch7(at)hotmail.co
Guest

Posted: Wed Jan 26, 2011 6:36 pm Post subject: coax antenna cable length

Guys,

  

   I have seen it suggested that when cutting off an antenna cable

 for your com radio, nav radio, etc., that you are supposed to cut

 the cable at EVEN multiples of one length of a dipole mast.

  

   In the example of a com radio, one article states the dipole length

 of a com radio to be 20".  Therefore, you should double this, and 

 use this new length to calculate your final minimum cable length.

  

 Like this;  dipole = 20",   so our increment would be 40".  Now,

 beginning at the back of the radio (not at a junction of a pigtail),

 start counting off 40" increments until we have enough cable to

 comfortably reach the actual antenna.  Meaning; only cut the cable at

 40", 80", 120", 160", 200", etc.

   If, after we've done an excellent routing of the cable, we find our

 antenna is not located at a 40" increment, then we should proceed 

 to the next full length of our 40" increment prior to cutting the cable.

  

   Here's my question;  I read from a prominent, nationally known,

 avionics guru that this is not all that necessary.  If I recall correctly,

 he said he has done SWR tests, and the cable length (cut off at even

 multiples) had little effect on the antennas performance.  In other 

 words, it didn't really matter what the cable length was, i.e.., 180"

 or 205", etc.

   If I'm a little fuzzy on my facts here, it's because this information

 is from several years ago.  But, after all these years, I really would 

 like to now the facts!!

  

   Is it really all that critical what the cut-off measurement is for a coax

 cable for aircraft radios?

  

 Thanks,   Mike Welch

  

  

 		 	   		    [quote][b]

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nuckolls.bob(at)aeroelect
Guest

Posted: Wed Jan 26, 2011 7:19 pm Post subject: coax antenna cable length

At 09:32 PM 1/26/2011, you wrote:

 	  Quote: 	
		  Guys,



   I have seen it suggested that when cutting off an antenna cable

for your com radio, nav radio, etc., that you are supposed to cut

the cable at EVEN multiples of one length of a dipole mast.




	


   <snip>



 	  Quote: 	
		     Is it really all that critical what the cut-off measurement is for a coax

cable for aircraft radios?


	


    No.

   Bob . . .

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Thu Jan 27, 2011 3:32 pm Post subject: coax antenna cable length

That’s the first I’ve heard of anything like this.  As Bob has said, the longer the feed cable (coax) the closer the match to the radio will be to 50 Ohms but you will then also have to contend with line losses.

 

The question than has to be what to do with the access cable?  If you coil it up you can be opening a whole new can or worms.  Re-routing it can also cause problems.  Best to decide on the best possible routing and install leaving just enough cable for drip loops.

 

Noel

 

From: owner-aeroelectric-list-server(at)matronics.com [mailto:owner-aeroelectric-list-server(at)matronics.com] On Behalf Of Mike Welch

Sent: January 26, 2011 11:02 PM

To: aeroelectric-list(at)matronics.com

Subject: coax antenna cable length

 

Guys,

 

  I have seen it suggested that when cutting off an antenna cable

for your com radio, nav radio, etc., that you are supposed to cut

the cable at EVEN multiples of one length of a dipole mast.

 

  In the example of a com radio, one article states the dipole length

of a com radio to be 20".  Therefore, you should double this, and 

use this new length to calculate your final minimum cable length.

 

Like this;  dipole = 20",   so our increment would be 40".  Now,

beginning at the back of the radio (not at a junction of a pigtail),

start counting off 40" increments until we have enough cable to

comfortably reach the actual antenna.  Meaning; only cut the cable at

40", 80", 120", 160", 200", etc.

  If, after we've done an excellent routing of the cable, we find our

antenna is not located at a 40" increment, then we should proceed 

to the next full length of our 40" increment prior to cutting the cable.

 

  Here's my question;  I read from a prominent, nationally known,

avionics guru that this is not all that necessary.  If I recall correctly,

he said he has done SWR tests, and the cable length (cut off at even

multiples) had little effect on the antennas performance.  In other 

words, it didn't really matter what the cable length was, i.e.., 180"

or 205", etc.

  If I'm a little fuzzy on my facts here, it's because this information

is from several years ago.  But, after all these years, I really would 

like to now the facts!!

 

  Is it really all that critical what the cut-off measurement is for a coax

cable for aircraft radios?

 

Thanks,   Mike Welch

 

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  [quote][b]

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

nuckolls.bob(at)aeroelect
Guest

Posted: Thu Jan 27, 2011 6:34 pm Post subject: coax antenna cable length

At 06:27 PM 1/27/2011, you wrote:

 	  Quote: 	
		  That's the first I've heard of anything like this.  As Bob has said, 

the longer the feed cable (coax) the closer the match to the radio 

will be to 50 Ohms but you will then also have to contend with line losses.



The question than has to be what to do with the access cable?  If 

you coil it up you can be opening a whole new can or 

worms.  Re-routing it can also cause problems.  Best to decide on 

the best possible routing and install leaving just enough cable for drip loops.


	


     I did not intended proffer the idea that one

     would purposely ADD coax to a feedline for

     the purpose of reducing SWR. It is because

     longer feedlines ADD losses that the SWR

     appears to improve while in fact, the

     effects of real SWR are being masked by

     those losses. It was an observation of

     fact, not a recommendation.

   Bob . . .

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Float Flyr

Joined: 19 Jul 2006
Posts: 2704
Location: Campbellton, Newfoundland

Posted: Fri Jan 28, 2011 12:22 pm Post subject: coax antenna cable length

What I was getting at was not to reroute the wire or coil it up to take care

of excess.  Cut it off.  I think you agree as I agree the longer coax

appears to have a better swr but in fact it has greater line losses.

Certainly I agree that no one should ever add length to the feed wire to

achieve a mach.



Noel



--

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_________________
Noel Loveys
Kitfox III-A
Aerocet 1100 Floats

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