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Open in Coax

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Subject: Open in Coax
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Date: Fri, 11 Oct 1996 09:14:00 EDT
      ************** From: Keith B. Ballow ******************                   
      *** ID L3RA ** Printer Z126 N1 ** Ph. 903-836-6478  ***                   
SUBJECT: Open in Coax                                                           


You can build a Time Domain Reflectometer that will work reasonably well if you 
also have access to an O-scope.  I do not have the information with me here at  
work, but perhaps someone on the reflector can fill in the blanks.              
Basically you build a circuit that produces a 1 Mhz squarewave.  With a Coax    
Tee, feed the squarewave to the coax under test and sample the wave with an     
O-scope on the Tee.  A good length of coax will display a flat trace on the     
scope.  An impedance bump will show up on the trace as an abrupt up or down     
movement on the scope trace.  The position on the scope trace will be relative  
to the location of the short or open in the coax.                               
A 150 foot length of coax with a short at 75 feet will show a bump midway of    
the scope trace.  You may have to add 100 feet of known good coax between the   
TDR and the coax under test for a length reference.                             
I have used this on our repeater system which has about 300 feet of heliax      
with a splice at the 150 foot point.  Once when the splice connection was       
loose the bump in the scope trace was profound enough to send us up the tower.  
Tightened up the connector and the bump disappeared.                            

Good luck....                                                                   

Keith Ballow                                                                    

Keith B. Ballow                                                                 
Sr. I&C Technician Martin Lake Plant                                            

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