

VSWR Calculation  RF Cafe Forums 
jaryi4pora

Post subject: VSWR Calculation
Posted: Wed Jun 15, 2005 9:34 am


Joined: Wed Jun 15, 2005
9:12 am Posts: 1 
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





Joe 
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.


Posted 11/12/2012



