RadioBanter - View Single Post

September 27th 05, 04:52 AM

On Tue, 27 Sep 2005 02:54:31 +0000 (UTC),
wrote:

Owen Duffy wrote:
On Tue, 27 Sep 2005 02:25:11 +0000 (UTC),
wrote:

SWR is nothing more than a dimensionless impedance ratio.

The fundamental definition of SWR flows from the behaviour and
properties of RF transmission lines.

When a transmission line is terminated in an impedance other than its
characteristic impedance, there will be both a forward wave and a
reflected wave of such magnitude to resolve the conditions that must
apply at the termination.

The forward wave and the reflected wave sum at all points along the
line having regard for their magnitudes and relative phase to produce
a "standing wave". The Standing Wave Ratio (SWR or VSWR) is defined to
mean the ratio of the maximum to the minimum of the magnitude of the
standing wave voltage pattern along the line.

The SWR on a lossless line can be calculated knowing the complex
characteristic impedance of the line and the complex load impedance.

The SWR on the line does not depend in any way on some unrelated
independent reference resistance as you suggest in your formula.

You seem to be suggesting that your redefined SWR is a really good
(obscure) way to talk about an impedance (independently of a
transmission line) in terms of some standardised reference value, and
you can throw away the fundamental meaning of SWR to support your
SWR(50) concept. In your terms (independently of a transmission line),
for instance, a Z of 60+j10 would be SWR(50)=1.299, and so would an
infinite number of other Zs have SWR(50)=1.299... how is that of
value. To know Z is 60+j10 is to know more than to know SWR(50)=1.299.

Owen
--