K7ITM wrote:

There are several possible reasons for being interested in standing
waves (on transmission lines). Some are valid, some are not, and
you'll even get plenty of, um, discussion about what's valid and what
isn't.

If your goal is to get maximum power delivered to a load, then it's
good to minimize standing waves on the line delivering power to that
load, because a line delivering a particular amount of power to a load
will have greater power lost in the line with greater standing waves.
If the line is being used near its maximum power or voltage rating,
standing waves are a concern because for a given power delivered to the
load, the rms current at current nodes and the peak voltage at voltage
nodes both increase with increased standing waves. And a high standing
wave ratio on a line which is long compared with a wavelength suggests
that the input impedance to the line will vary rapidly with frequency,
whereas a line with low standing wave ratio will present a relatively
constant impedance to the driving source, assuming the load is
reasonably "flat" with frequency.

As an example of this last point, a 30 meter (~100 foot) 50 ohm line
with 0.8 velocity factor and very low loss, delivering power to a 50
ohm load at 450MHz, will present a 50 ohm load to the driving source.
But delivering power to a 200 ohm load, the source will "see" almost
200 ohms at frequencies where the line is an integer number of
electrical half-waves long, and it will "see" just over 12.5 ohms
midway between those frequencies. You get 200 ohms at 440MHz, 12.5
ohms at 442MHz--and reactive in between.

It's possible to use stubs and series line sections to effect an
impedance match between a load and a line. For example, the right
length and impedance series section will give you a match at one
particular frequency, at least, and multiple sections can give you a
"perfect" match at multiple frequencies, with (perhaps) quite
acceptable match over a range of frequencies.

There are lists of analogs among electrical, mechanical, acoustic, and
other media. "electrical hydraulic impedance analog" in a Google
search will give you many hits.

Cheers,
Tom

Thanks for your reply. I have a few questions. When you say "standing
waves", I take it that one can have more than one on the line?

I follow your example, but I may come back to it once I've done the calcs.

How does one know they want to improve their impedance match? Why doesn't
there seem to be a need for this (probably through a balun) on a standard AM
radio with a 1/2 wave line antenna or even some ferrite coil? Is there some
auto-balun that works this all out?

Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
(121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
Obz Site: 39° 15' 7" N, 121° 2' 32" W, 2700 feet

"He who laughs, lasts." -- Mary Pettibone Poole
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