[TowerTalk] COAX LENGTH

[Woods, Eric D (edwoods)]

I think you can do this maybe for one band, but don't depend on the primary 
constants (R, L, G, C) to be non-variable with frequency.  In other words, 
you can reproduce the load impedance with an N X 1/2  wavelength electrical 
feedline at one particular frequency (+/- some kinda bandwidth) but don't 
expect it to do the same thing when you double the frequency.

Eric, K6GV
edwoods@pacbell.com

Here come the Smith - Chart people......
 ----------
From: Pat Barthelow
To: towertalk
Subject: [TowerTalk] COAX LENGTH
Date: Monday, July 07, 1997 11:56AM

Checking Coax Length Checks:

          I have a large amount of CATV Coax which I want to use for
long runs to HF antennas at the N6IJ antenna farm.  In light of the
issues brought up here regarding variablility of Velocity factor in
foamed CATV hardline,  could I trim the Hardline to achieve a 1/2
wave multiple, in the following way?

1. Attach a 100 ohm non inductive resistor to the antenna end of
the coax.

2. Measure the feedpoint impedance/reactance at the "shack" end
of the coax, using an MFJ or similar analyzer, at the frequency
desired?

3. Trim length until feedpoint impedance is 100 ohms, + 0j....

Would this work to get the 1/2 wavelength multiple desired?  I could
get it close by reading the foot markers printed on the outside of the
coax, to make the first cut....leaving plenty of "extra" feet from the
computed value to allow for Vel Factor variations....

Also, without a balun at the load end, would the RF current path
down the outside of the coax skew the reading?

Should I use a higher impedance load, say 400 ohms to develop a
faster "skew rate" of the impedance variation with length, to get a
more accurate measurement?

73, DX, de Pat, AA6EG/N6IJ, The Contest Station from the Government...
aa6eg@tmx.com

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