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Re: [TowerTalk] coax 'sweet lenght'

To: "FireBrick" <>,"TowerTalk List" <>
Subject: Re: [TowerTalk] coax 'sweet lenght'
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Sun, 10 Oct 2004 11:13:02 -0400
List-post: <>

> I remember discussion that there was a 'sweet length' for
rg8, rg13 coax lengths.
> The length was a multiple that gave the best match, etc.
> Or is this the Ham version of 'urban legend'???

If the SWR meter's design impedance or null impedance isn't
the same as the impedance of the line, SWR can change with
line length or as you move the meter along the line. That's
because the normalized impedance of the measurement is
different than the line impedance.

If the line is acting like part of the actual radiator and
radiating a substantial portion of applied power, changing
line length (not position of the meter, unless the meter
alters the ground path) can change SWR reading because it
actually DOES change the real SWR.

A large change in transmission line loss can also cause SWR
to be gradually changed along the line, but it is a smooth
slope and does not have peaks and valleys every 1/4 wl.

So as Dave says, it is a myth that a line feeding a single
antenna  has a specific "sweet length". If the system is
behaving properly, SWR is constant.

73 Tom


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