[TowerTalk] COAX LENGTH

[Pat Barthelow]

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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