[Top] [All Lists]

[TowerTalk] 75 ohm feedline

To: <>
Subject: [TowerTalk] 75 ohm feedline
From: (David Robbins)
Date: Wed, 09 Jul 1997 00:25:52 +0000
L. B. Cebik wrote:
> scenario:  cut your line with precision to keep the equipment adjustments
> flat and minimal or use transformers with attention to proper wx treatment
> for your area and climate for the same purpose, with comparable
> losses--which are negligible in either case relative to all the other loss
> sources that might enter such systems.  No wonder folks have claimed
> success for both ways of doing things.

just another thing to throw a monkey wrench in the works...

do some rough calculations:
compare wavelengths at the two busiest frequencies on 80m in contests
300/3.5mhz = 85.7m
300/3.8mhz = 78.9m

difference = 6.8m 
shorten by velocity factor 6.8*.66(?)=4.5m

so the wavelength changes by 4.5m in the hardline (even assuming the
velocity factor i know off hand) from 3.5 to 3.8 mhz.  this ammounts to
an 8% or an almost 30 degree change for a one wavelength line.  go to 2
and the shift is almost 60 degrees.

so cutting to a half wavelength multiple is not so easy, unless you are
to a narrow bandwidth.  similar effects can be calculated for other
bands, just
remember that the same physical length 'contains' more wavelengths as
you go up
in frequency.

i use random lengths of 75 ohm hardline (cut to fit and never measured)
with no
matching transformers.  i do have a couple pieces of rg-11 that i use to
out some bands inside the shack where its easy to play around with
them.  and 
the losses i have with 50 ohm loads plus home made plumbing fitting
is so low i could not measure it for sure with amateur grade meters at
hf.  i 
could see a measureable loss on 2m, but no more than i could attribute
to the
predicted losses from the cable itself. 

David Robbins K1TTT (ex KY1H)   or

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>