[[email protected]]

I found this cool website dealing with signal loss on different types
of RF cables:

http://www.ocarc.ca/coax.htm

I have a question. What does Load SWR stand for, and what does it
mean? Thanks in advance!

[TimPerry[_2_]]

wrote:
I found this cool website dealing with signal loss on different types
of RF cables:

http://www.ocarc.ca/coax.htm

I have a question. What does Load SWR stand for, and what does it
mean? Thanks in advance!

SWR means standing wave ratio, more usually expressed as VSWR voltage
standing wave ratio.

it indicates a condition where the transmitted RF power is not completely
utilized by the load (antenna).

excessive VSWR in a transmission line is often harmful to a transmitter.

[Scott Dorsey]

wrote:
I found this cool website dealing with signal loss on different types
of RF cables:

http://www.ocarc.ca/coax.htm

I have a question. What does Load SWR stand for, and what does it
mean? Thanks in advance!

There is a nice discussion of transmission line theory in Horowitz and Hill.

A basic oversimplification: you can think of the characteristic impedance
of a medium as being like the refractive index for light. When light goes
through a window, the refractive index is a little different than that of
air, so some of it is reflected back to the source.

Likewise, when there is an impedance discontinuity in an electrical connection,
some of the signal is reflected back to the source. Audio guys don't care
so much about this because they are using cables that are much shorter than
a quarter-wavelength but it still can be important for audio on very long
cable runs. The echo you sometimes hear on telephone circuits is the
result of impedance matching errors.

In any case, if you run a 50 ohm cable up to a 52 ohm antenna, SOME of
the signal will be reflected back to the source.

The reflection coefficient gamma is (Zload - Zsource) / (Zload + Zsource)
which would be 2/102= 0.02.

This means if we ran a hundred watts forward power into the antenna, we
would get two watts reflected power back.

And the VSWR of the line is 1+ |gamma| / 1- |gamma| which would be
1.02/0.98 or a VSWR of 1.04

RF stuff is very different then audio, but it all follows certain rules
and the rules apply all the time. So once you learn the rules, you can
predict any behaviour.

A lot of people get hung up on the "characteristic impedance" of a cable.
It's not that the cable has any particular resistance across it or anything...
a 50 ohm cable is one that will not produce any reflection with a 50 ohm
load at the end of it. They should probably call it something else, but
it's been called 'characteristic impedance' for more than seventy years now
and it's too late to change it.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."