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[TowerTalk] Finding roof beams, determining impedance of coax cable.

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Subject: [TowerTalk] Finding roof beams, determining impedance of coax cable.
From: (Flanders, Jim)
Date: Thu, 26 Jun 1997 14:05:12 -0500
I stand corrected the way K6LL replied.  Use anything but a half wave.
Like I said in the beginning, it's been 50 years since I did this 
After WWII we had lots of war surplus coax with all sorts of impedance.
I was trying to remember how my dad (W1KTQ now K4RQQ)showed me.
The bottom line is that I got a constructive discussion going here. 
end result is that we will all know how to do it many ways.  As long as
everyone stays constructive, guys like me won't worry about sticking 
their neck out. 

Jim Flanders
-----Original Message-----
From:   Edward W. Sleight []
Sent:   Friday, June 27, 1997 3:06 AM
To:     Flanders, Jim
Subject:        Re: [TowerTalk] Finding roof beams, determining impedance of 
coax cable.

Enjoyed your discussion of the "stud" finders Jim. Use the electronic
type also.

But, your bit on determining the impedence of an unknown cable is a wee
tad off.

ASSUMMING that you meant a half wave ELECTRICALLY, the line will repeat
its input impedence ( and with a non inductive resistor, X will be 0
so you can just as well say input resistance, at the other end of the
line without regard to the impedence of the actual cable itself.

With a random length of line ( such as anything from a few feet to a
half wave physically, the result on the SWR  ( or other meter used )
will be useless because of the characteristics of the SWR bridge.

The only way I have ever known( assumming it isn't marked on the cable 
is to use the ratio of the diameter of the inner conductor to that of
the outer diameter, and compensate by applying the approriate factor
of the the dialectric material. You can determine this by working the
formula backward using known factors, ie: if you have a piece of
marked 50 ohm coax with a foam dialectric, work the formula by just
using the diameters of the inner and outer, and then compensate
by dividing 50 ohms by the figure you get.



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