VSWR Calculation - RF Cafe Forums

 Post subject: VSWR Calculation

Posted: Wed Jun 15, 2005 9:34 am 

I am trying to figure out what the output VSWR of a microwave modem assembly will be. There is a Freq Conv with an output VSWR of 2.0:1 then a cable to a switch and another cable going to the egress. Cable VSWR is 1.22:1 and switch VSWR is 1.5:1. If I want to know what the output VSWR at the egress will be, how do I calculate this. This isn't something I can measure on a VNA.
Thanks for any help

 Post subject: VSWR

Posted: Fri Jun 17, 2005 2:54 pm 

You need a lot more information to calculate VSWR. Full S Parameters, magnitude and phase, are used to calculate the cascaded response.

As a rule of thumb, the VSWR due to the interaction of 2 connected devices can vary between the product of the 2 VSWR's max and the quocient of the 2 VSWR's min. For example connecting a 2.0:1 device with a 1.5:1 device can yield a 3.0:1 (2.0x1.5) worst case if the reflected waves add in phase or a 1.33:1 (2.0/1.5) best case if the reflected waves are 180 degrees out of phase. If you are incredibly unlucky, the components you described may yield a worst case VSWR of 4:1 at some frequency.

The cables will effect the VSWR in several ways. Long cables will allow the phase to vary greatly and cause a large variation in VSWR over the bandwidth. It will add ripple to the response. The loss of the cables will actually improve the VSWR by attenuating the refleted waves.

Measuring the VSWR can be done the same way that the frequency converter is measured using load pull techniques.