Roger Sparks wrote:

"Mike Coslo" wrote in message
...

Possible Dummy alert here!

I'm trying to understand VSWR a bit more here. As I recently put up my
first non-ladder line antenna, VSWR has suddenly become important to me.

8^)

This isn't so much about the antenna, which is performing okay so far,
if a little high on 80 meters.

More it is about a tuner's habit of showing a higher VSWR as the power
level is increased.

I have read that this is not the case. VSWR does not change with power
level, and that something is wrong with the VSWR meter, but what would
be wrong with the meter, and more importantly, *which* is correct, my
low power or high power, and how do I find out?

My Rig VSWR meter and my tuner meter read fairly close at the low end,
but diverge at the high end. But both read higher VSWR than at low
power. so that isn't a help.

I do have another VSWR meter, but at this point I suspect that adding
another variable will just further confuse me! 8^P

Anyone have an explanation?


- Mike KB3EIA -


Hi Mike,

One possibilty for the higher readings is that the antenna is actually
changing impedance at higher power levels. This happens when a loose
connection changes resistance under higher current conditions, when
vegatation contacts the antenna and changes condutivity under higher current
conditions, and when corona or arcing occurs at higher voltage conditions
but not at lower voltage conditions.

All of these conditions will usually indicate on the VSWR meter as a sudden
jump in VSWR.

73, Roger, W7WKB
Also, remember that diodes in your swr meter are NOT LINEAR, until the
voltage
across them reachs a certain "Point" (below that point, they respond
as a square law device - double the volts, QUADRUPLE the CURRENT! The
diodes are on the sampleing lines of your vswr meter, and measure
forward (current)!, and backwards current! It is as simple as: double
the power- quadruple the current flowing in the sampleing circuit! This
is a common problem with a diode at a lower power than it is designed to
handle-Jim NN7K