[Top] [All Lists]

Re: [TowerTalk] RE: [Antennas] coax 'sweet lenght'

To: Tom Rauch <>
Subject: Re: [TowerTalk] RE: [Antennas] coax 'sweet lenght'
From: "Tower (K8RI)" <>
Date: Mon, 11 Oct 2004 10:36:45 -0400
List-post: <>
Thanks Tom,

I like that plain language description with the examples. I've known it works that way for years, but I never bothered to put any figures with it.

Roger (K8RI)

Tom Rauch wrote:


SWR is constant except for a small change from line loss,
and reactance doesn't "fool" a properly calibrated SWR

If the bridge is NOT matched or nulled to the feedline Z0,
the error in calibration moves the bridge off center in the
SWR circle and SWR will change. This effect comes ONLY from
using a mismatched reference, like a 50 ohm reference on 75
ohm cable.

Say I terminate a 75 ohm line in 50 ohms. The SWR along the
75 ohm line will be 1.5:1. 1/4 wl away from the load
impedance will be  1.5 times 75 or 112.5 0J ohms. A 75 ohm
bridge will show 1.5:1 but a 50 ohm bridge will show 2.25:1

Move 1/4 wl further to 1/2 wl  and SWR will still remain 1.5
but now the impedance is 50 0j ohms.
A 50 ohm bridge would show 1:1 but a 75 ohm 1.5:1.

At 1/8th wl the impedance would be 69.2 28.8j which is 1.5:1
at 75 ohms but 1.79:1 at 50 ohms

So you see, no matter where you measure SWR it is unchanged
unless you have used a different reference meter with the
transmission line Z0.

73 Tom

Roger Halstead (K8RI ARRL Life Member)
Worlds Oldest Debonair (N833R S# CD-2)


See: for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.

TowerTalk mailing list

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