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[TowerTalk] Finding roof beams, determining impedance ofcoax cable.

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Subject: [TowerTalk] Finding roof beams, determining impedance ofcoax cable.
From: (David O Hachadorian)
Date: Thu, 26 Jun 1997 13:31:18 EDT
On Thu, 26 Jun 1997 10:09:07 -0500 "Flanders, Jim" <>

>Determining impedance of unknown coax cable.  (also handy to determine 
>ratio of unmarked balun):
>       It's been a long time since I've done this myself (~50 years), 
>here goes.
>Take a half wave length of cable and terminate it with a non-inductive 
>resistor such as 50 ohms.
>Place a swr bridge at the TX end.  Tx into the cable with just enough 
>power to get a reading on the meter.
>If the swr is 1 to 1 the cable is the impedance of the resistor.  Note 
>that the MFJ antenna analyzer will also
>do this for you, but not all of us have one, and we all should have a 
>swr bridge and a 50 ohm dummy load.

Actually, it should be almost any length of cable EXCEPT a half
wave. A half wave will present the 50 ohm termination,
regardless of the cable's characteristic impedance.

A better way to do the test is to hook up the 50 ohm resistor to
a random length of cable and then vary the transmitter frequency
over a wide range of frequencies. If the swr is flat over a wide
range of frequencies, then it is 50 ohm cable. If it's not 50
ohm coax, experiment with various 2 watt carbon terminating
resistors until you find the proper one that makes the swr curve

Dave Hachadorian, K6LL

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