VSWR Question
 

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 -

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?

Simple. This has been discussed many times. The diodes are nonlinear and
when on the low end of the voltage across them, they conduct differant.

To get the best indication of swr use the larger ammount of power. You can
use the lowest ammount of power to adjust the tuner for the lowest indicated
SWR . That will not matter. It is just the most accurate swr will be seen
by the meter at a higher power level.

If you use a directional wattmeter that has a wattage scale and calculate
the swr , it should be the same at all power levels provided the meter is
accurate.

A good rundown on VSWR at the ARRL site -- URL:
http://www.arrl.org/tis/info/pdf/9401070.pdf

--
Red Noise
==========================
"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^)

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

The SWR meter's diode(s) forward voltage drop(s) has(have) to be
overcome before the SWR meter reading is accurate. Also, high
power may cause impedance changes due to heating.
--
73, Cecil http://www.qsl.net/w5dxp



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Mike Coslo wrote:
"More it is about a tuner`s habit of showing a higher VSWR as the power
level is increased."

Standing wave ratio (SWR) is often called "voltage standing wave ratio
to distinguish it from the standing-wave ratio expressed as a power
ratio which is (Emax / Emin)squared' (From Terman`s 1955 edition
footnote on page 97).

SWR is a measure of the amplitude ratio of the reflected to incident
waves at the load. It`s a means of expressing the reflection
coefficient.

SWR is independent of power level or of transmission line length on an
ideal line. Just as a speedometer doesn`t require driving a mile or for
an hour for a valid mph reading, an SWR meter doesn`t require any
particular line length to present a valid mismatch reading at the
transmission line load, despite reasonable objections by Reg Edwards.
The degree of mismatch can be indicated by an SWR meter in the usual
case.

Power level is not an SWR factor but often plays a role in the
indication of SWR as well reported by Mike Coslo and his respondents.

Best regards, Richard Harrison, KB5WZI

"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





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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