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

[Steve Hunt]

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).