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Re: [TowerTalk] Cable TV hardline question

To: "David Robbins K1TTT" <>,"'Tim Holmes'" <>, <>
Subject: Re: [TowerTalk] Cable TV hardline question
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Fri, 29 Oct 2004 12:21:35 -0400
List-post: <>
I'm not saying with luck you can't do use 75 ohm cable at
UHF in 50 ohm systems, but personally I'd never do it
without matching at each end.

> I use it on 6m,2m and 70cm with a ts-2000 and a couple
tm-261's for 2m
> packet and none of them complain.

The impedance range of a 75 ohm cable feeding a 50 ohm load
is from 50 ohms to 112.5 ohms, or a 2.25:1 SWR. Losses help,
eventually pulling it down to a 75 ohm impedance (1.5:1) if
the line is lossy.

A 2:1 mismatch really isn't good for radios that are fixed
tuned, whether they appear to like it or not. FM or
non-linear PAs are less of a problem, but linearity can fall
apart quickly when a PA designed to operate into a 50 ohm
load is misterminated.

 Plug it in and try it, if it isn't
> good enough match for your radio add a short length of 75
ohm to the
> transceiver end, this will transform the impedance some
more, if you
> calculate it so the overall length is a multiple of 1/2
wavelength you
> get back whatever impedance is on the other end more or
less... but I
> have never found that to be really necessary in real

At about six inches per 1/4 wl section on 70cm, you'd go
through a full cycle from 50 to 112 ohms and back with a one
foot change. When you have multiple 1/4 wl sections in
series, the SWR bandwidth decreases. The impedance error
with frequency change of each small section adds. That's why
a MFJ 259 analyzer can measure cable lengths into the
thousands of feet using 10MHz as the test frequency.

I'd be careful using CATV cable in 50 ohms systems where the
cable is multiple wavelengths long.


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