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Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?

To: towertalk@contesting.com
Subject: Re: [TowerTalk] RG-149: 50 ohm/70 ohm - does it matter?
From: Steve Hunt <steve@karinya.net>
Date: Sun, 15 Dec 2013 00:07:06 +0000
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Correct!

These "couplers" typically measure the voltage at a point on the line (V1=Vfwd+Vref) and the current (I=Ifwd-Iref). Given a value for Zo they then generate a voltage V2=I.Zo which is equal to Vfwd-Vref. Adding V1+V2 then gives 2.Vfwd, and subtracting gives 2.Vref.

SWR is then derived from Vfwd/Vref. It is independent of the source impedance.

Steve G3TXQ

On 14/12/2013 23:33, Michael Tope wrote:
Yes, but it is normalized to a specific impedance in the sense that the VSWR bridge is calibrated for a certain Zo. If I put 100 ohm transmission lines on either side of a VSWR bridge calibrated for 50 ohms and I terminate the load side 100 ohm transmission line with 100 ohms and drive the source side with a 100 ohm source, the bridge will still read 2:1 VSWR even though both the input and output lines are matched (i.e. no standing wave on the line).

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