Saandy , 4Z5KS wrote:
...if already going into it, a little bit of history.
when the cows had bigger heads and the air was greener, there was no
such thing as coax. what we used was the ubiquitous ladder wire, with
an unknown impedance and with a frequency response depending on what
the Gods ate at lunch! the VSWR story was not invented yet.
What we did was one of two things: either tune the system for maximum
current in the line or else used a light bulb in conjunction with a
small light bulb and tuned for maximum brilliance. in neither case was
SWR involved in the mess.
the whole SWR uproar began after WW2 with the advent of coax and the
new fangled theories. that was also the time when all kind of
directional couplers came up.in due time a few wise guys developed all
kinds of theories on the subject, and manged to convey the impression
that SWR is king! nothing further from truth. what's really true is
that reflections can cause the apparent impedance at the network's
input to differ from Zo. SO WHAT? if you can adjust your matching
network between the transmitter and the line for a match what do you
care?
actually the hitch is that, with a high SWR on the line, the losses go
up. if the cable can take it, without melting no harm's done: whatever
remains will get radiated. this was the good pint of open feeders: the
losses were very low. an SWR fo 10 and more was insignificant from the
losses' point ov view.
Guys, leave it alone! Just make sure that the SWR is a reasonable
value, something that the transmitter can handle and leave it at that.
Saandy 4Z5KS

Some of what you say is very true. Especially in the world of HAM
radio. How ever, this being an open forum, truth is of great
importance. Your thesis on the "whys" of the importance of vswr
measurements are incorrect in some areas.
Yes, importance grew with the advent of coaxial lines simply because of
the relatively small distances between inner and outer conductors. That
part is true.

However, until you've seen a 6" universal coaxial transmission line with
no insulators or inner conductor remaining over a 350" run, I guess you
can't really appreciate the need for monitoring and maintaining good
"system" vswr characteristics.

Now to the measuring of said vswr. It can be done. In the broadcast
world it's accomplished through the measurement of "Return Loss". By
measuring the system return loss at the line input (generator end) and
deducting twice the line attenuation, we get an indication of the load
return loss thesis value is easily converted to vswr.

Return Loss:
This is the dB value of absolute reflection coefficient.
It is rather curious concept of transmission engineering.
This loss value becomes 0 for 100% reflection and becomes infinite for
an ideal connection.

RL = 20log((VSWR+1) / (VSWR-1))


Voltage Standing Wave Ratio (VSWR):
This is the ratio of maximum voltage to minimum voltage
in standing wave pattern.
It varies from +1 to infinite.

VSWR = (1+(10^RL/20)) / ((10^RL/20)-1)

These are good and valid measurements which should be performed at
initial installation of the system and periodically verified throughout
the system life.

Return Loss/VSWR is only one of many measurements that should be
periodically done. DC measurements such as megger. LO-Ohms are also
very important.




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