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# Waveguide VSWR - RF Cafe Forums

 The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Below are all of the old forum threads, including all the responses to the original posts. -- Amateur Radio -- Antennas-- Circuits & Components-- Systems-- Test & Measurement

 Post subject: Waveguide VSWR Posted: Mon May 08, 2006 7:47 am
 Captain

Joined: Fri Jul 16, 2004 4:08 pm
Posts: 11
Hello,

I need to desing a waveguide system for Ku band that will have a total length of around 100' and will have a total of about 135 degrees worth of bends between the Tx and the antenna (parabolic dish). Throw in a coupler and a SPDT switch (redundant paths).

My question is how do I calculate the total VSWR for the system given the loss and VSWR of each section? I know I could do worst case, but that's not what I excpect to really show up when testing it. Does anyone know how to go about it? Do you use an RSS or RMS addition from stage to stage? Any assistance will be welcome. I hate to go into these things blind (this is my first WG system).

Thanks guy.

 Post subject: Cascaded VSWR formula Posted: Mon May 08, 2006 3:08 pm
 Captain

Joined: Thu Apr 13, 2006 9:53 am
Posts: 18
Location: Florida
I can't figure out how to write an equation here so I'll "write" it out in the hopes it is clear enough to understand.

The basic formula for cascaded VSWR is: VSWR(total)= 1+gamma(total)/1-gamma(total).

The trick here is to figure out what the total gamma is. In the early nineties I had a supplier who built for us an RF switching/conditioning WRA. In his ATP he had the following formula to calculate the cascaded VSWR through the many switches to the front panel. He got it from a text book. The formula goes like this:

gamma total = the square root of the sum of the squares of the individual gammas minus the product of the square of the individual gammas.

gamma total = SQRT[(gamma_1^2 + gamma_2^2+...gamma_n^2) * (gamma_1^2*gamma_2^2...gamma_n^2)]

Hopes this helps

RFTEJerry

 Post subject: Cascade VSWR Posted: Thu May 18, 2006 3:38 pm
 Lieutenant

Joined: Thu May 18, 2006 1:54 pm
Posts: 1
I found a reference for calculating cascade VSWRS using a linear combination method refer to web site for equation http://www.y1pwe.co.uk. I don't know how much this differs from the one you provided.

I would like to refer to a text reference on this subject and how does this equation differ for when you have two components and the worst VSWR is the multiplication of each VSWR individual VSWR.

Thanks.

 Post subject: WAVEGUIDE VSWR Posted: Fri May 19, 2006 7:36 am
 General

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 304
Location: London UK
 Hi AllBack on line againJerry, I think the formula you mention (in which you have * instead of - ???) is actually the range of variation that Gamma can take.The Gamma will range between a min and max set of values as the relative phasing of the components' gammas change (gamma is a vector). Hence you need to calculate both halves of the expression independently and this gives the extreme limits of gamma.I think.

Posted  11/12/2012