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Re: [TowerTalk] Measuring electrical length of coax...

To: k6rix@earthlink.net, towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring electrical length of coax...
From: TexasRF@aol.com
Date: Thu, 14 Jul 2005 11:55:10 EDT
List-post: <mailto:towertalk@contesting.com>
 
In a message dated 7/14/2005 10:12:08 A.M. Central Standard Time,  
k6rix@earthlink.net writes:

Hello  John, how do you test your lines and what equipment do you use?

I've  experimented with the MFJ 259B for stubs and phasing lines, but if it 
is  critical, I'll order lines from M2.  I'd sure like a good method that I  
could employ myself.  Also, do you short the end of the coax or leave  it 
open.  The MFJ book lists both methods.

>...but I even  measure the actual phase shift for each delay line of solid 
>dielectric  coax, if the application is critical.
>
>And with foam coax it is  MANDATORY to TUNE (trim) each delay line to the 
>correct electrical  length.
>
>--John  W0UN

Dino...k6rix@earthlink.net  


_______________________________________________

See:  http://www.mscomputer.com  for "Self Supporting Towers", "Wireless  
Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with  any 
questions and ask for Sherman,  W2FLA.

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If you connect a Tee to the 259 analyzer with a 50 ohm Dummy load to one  
side of the tee connector, you can then connect your coax to be tested to the  
remaining side of the tee connector.
 
If the coax is open at the far end, you will see a null in the swr reading  
at each frequency that is a half wave multiple of the coax length.
 
If the coax is shorted on the far end you will see a null in the swr  reading 
at each frequency that is an odd multiple of a quarter wavelength of the  
coax length.
 
If you want cables to be the same electrical length then the swr null  should 
occur at the same frequency for each cable.
 
If you want the cables a multiple of half wave length then the cables are  
trimmed for a null at the actual operating frequency.
 
Most tee connectors will add about one inch to the cable length so if you  
are a perfectionist you will want to adjust your test frequency to compensate  
for the length difference.
 
If you don't have a 50 ohm dummy resistor, one is easily made with a PL259  
and three 150 ohm half watt resistors connected in parallel inside. Use minimum 
 lead lengths to reduce the series inductance. The PL259 load is not 
wonderful at  144MHz and higher but is just fine for lower frequencies.
 
Happy tuning!
 
73/k5gw
_______________________________________________

See: http://www.mscomputer.com  for "Self Supporting Towers", "Wireless Weather 
Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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