# Re: [SI-LIST] : LOSS IN HIGH VSWR TX LINE (STUB Q) (CAN YOU SOLVE THIS ?)

From: Raymond W. Waugh ([email protected])
Date: Thu Apr 12 2001 - 08:16:42 PDT

Steve...

You can represent the shunt short circuited stub as a lumped element
inductor with a lossy series resistance. Calculating this shunt impedance
is straightforward, using the R, G, L and C of the transmission line -- the
attached equations not only predate E-M simulators, they predate computers.

Ray

-----Original Message-----
From: Steve Rogers <[email protected]>
To: '[email protected]' <[email protected]>
Date: Thursday, April 12, 2001 12:31 AM
Subject: [SI-LIST] : LOSS IN HIGH VSWR TX LINE (STUB Q) (CAN YOU SOLVE THIS
?)

Hi all,

A problem......

Consider a shorted length of transmission line used as a synthetic inductor
(tx line stub). What loss (theoretically) will the stub incur (what is its
Q).

I am sure this problem is easy to crack with E-M simulation but is their an
intuitive, not too math' loaded approach?

An ac voltage applied to the line will give the normal forward travelling
wave which will be totally reflected at the short. The reflected wave will
travel back along the line and be re-reflected by the impedance mismatch at
the generator. The process will continue ad infinitum (multiple reflection).
We may nearly calculate the loss of the line if the generator impedance is
known and assume the line loss is that for a matched line. The multiple
reflections will give rise to an equation not unlike the standard feedback
equation. The result of this anlysis however will be incorrect (We have
assumed the line is matched). The line will have standing waves of V and I.
The loss on the line will therefore vary along the line length and be I
squared R at current maxima and V squared G and voltage maxima. How do we
deal with this????

I have seen curves in a number of books giving loss increase factor versus
VSWR. So either somone has made some measurements or done some sums. If it
be the latter then what are the sums? Going by the age of some books I have
seen these curves in I doubt if E-M tools were around!!!!

There's probably a really simple way to attact this problem but I don't know
anyone yet who has suggested one..

All suggestions welcome

Regards,

Steve

.

Regards

Steve Rogers

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