Standing Waves (and Impedance)
 

I've just begun (and I do mean begun!) a little refresher reading on the
subject of transmission lines from a time *long* ago. I'm trying to make
some sense out of why the emphasis of standing waves. The idea is familiar.
Is it that somehow knowing something about the standing wave on the line
that one can construct some sort of stub to smooth out the input impedance?
If so, wouldn't the stub need to be tuned depending on the length of a
cable? Can this be done somehow by the xmitter?

BTW, is there any analog of electrical impedance in hydraulics or other
areas where waves are widely studied?

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
--

Web Page: home.earthlink.net/~mtnviews

Standing Waves (and Impedance)
 

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

Standing Waves (and Impedance)
 

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
--

Web Page: home.earthlink.net/~mtnviews

Standing Waves (and Impedance)
 

W. Watson wrote:
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?

Standing waves are created by two coherent traveling waves moving
in opposite directions in a transmission line. In a conventional
system of source, transmission line, and load, one of the traveling
waves moves from the source toward the load and is called the forward
wave. The other traveling wave moves from the load toward the source
as a reverse or reflected wave. The reflected wave is usually the
result of a load being mismatched to a transmission line. If no
mismatch exists, no standing waves are created and the system is
considered to be "flat", i.e. one forward traveling wave.

How does one know they want to improve their impedance match?

For a transmitted signal, we establish a Z0-match to our transmitter
often at the input of an antenna tuner. When reflected energy is
eliminated on the coax between the tuner and transmitter, we know
we have a Z0-match by the SWR meter reading of 1:1. We also use our
antenna tuners to tune for maximum received signal on our S-meters.
At the Z0-match point, maximum available energy is transferred.

If you know the input impedance to a receiver, you can match your
antenna system to it to achieve maximum available energy transfer
from the antenna.
--
73, Cecil http://www.qsl.net/w5dxp

Standing Waves (and Impedance)
 

On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson"
wrote:

I'm trying to make
some sense out of why the emphasis of standing waves.

Here is the short version:
A matched transmission line behaves like the theory books say it does.
The rated power from the transmitter goes through the transmission
line with the lowest possible loss to the antenna where it is radiated
just like the book says.

A mismatched transmission just MIGHT work OK. If there is any
possibility of generating interference, especially TVI, it will. The
currents and voltages on a mismatched line are extreme... There MIGHT
even be some sparks. Power loss will be at its worst for a given line.
RF finds its way every where. Getting zapped once in a while
eventually grows old to everyone. I remember the good old days when
desk mikes were the only way to go. If you got too close, you got an
RF zap on your lip. Solid state rigs don't tolerate a high SWR. They
either protect themselves by reducing power or they require a lot of
maintenance.

You can learn to tolerate high SWR's, but I find it worthwhile to try
to keep things matched. The energy has to go somewhere, I prefer it
leave here through the antenna...
John Ferrell W8CCW

Standing Waves (and Impedance)
 

On Wed, 21 Dec 2005 20:18:05 GMT, John Ferrell
wrote:

On Tue, 20 Dec 2005 03:41:41 GMT, "W. Watson"
wrote:

I'm trying to make
some sense out of why the emphasis of standing waves.

Here is the short version:
A matched transmission line behaves like the theory books say it does.
The rated power from the transmitter goes through the transmission
line with the lowest possible loss to the antenna where it is radiated
just like the book says.

It is true that reducing SWR for a given line does reduce the loss if
the line is long enough. (There are some scenarios where a short line
with high VSWR has less loss than matched line of the same length.)

But is matched line the real goal?

If low loss is the goal, there are often cost effective lower loss
solutions possible with lower loss line operated at high VSWR.

A mismatched transmission just MIGHT work OK. If there is any
possibility of generating interference, especially TVI, it will. The

Why? How is TVI "generated" by line mismatch?

Owen
--

Standing Waves (and Impedance)
 

John Ferrell wrote:
You can learn to tolerate high SWR's, but I find it worthwhile to try
to keep things matched. The energy has to go somewhere, I prefer it
leave here through the antenna...
John Ferrell W8CCW

I assume that you realize there is a high SWR on a standing-
wave antenna, like a resonant 1/2WL dipole? :-)
--
73, Cecil http://www.qsl.net/w5dxp

Standing Waves (and Impedance)
 

Cecil Moore wrote:

W. Watson wrote:

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?


Standing waves are created by two coherent traveling waves moving
in opposite directions in a transmission line. In a conventional
....

How does one know they want to improve their impedance match?

....
antenna tuners to tune for maximum received signal on our S-meters.
At the Z0-match point, maximum available energy is transferred.

If you know the input impedance to a receiver, you can match your
antenna system to it to achieve maximum available energy transfer
from the antenna.
Thanks.
A standing wave is the sum of an incident added to the reflective wave.
Isn't it possible to send two incident waves down an xline with different
frequences, and produce two different standing waves by having some
multiplicative relationship between the two incident waves and the xline length?

Not a bad explanation from Wikipedia:

SWR has a number of implications that are directly applicable to radio use.

1. SWR is an indicator of reflected waves bouncing back and forth within
the transmission line, and as such, an increase in SWR corresponds to an
increase in power in the line beyond the actual transmitted power. This
increased power will increase RF losses, as increased voltage increases
dielectric losses, and increased current increases resistive losses.
2. Matched impedances give ideal power transfer; mismatched impedances
give high SWR and reduced power transfer.
3. Higher power in the transmission line also leaks back into the radio,
which causes it to heat up.
4. The higher voltages associated with a sufficiently high SWR could
damage the transmitter. Solid state radios which have a lower tolerance for
high voltages may automatically reduce output power to prevent damage. Tube
radios may arc. The high voltages may also cause transmission line
dielectric to break down and/or burn. Abnormally high voltages in the
antenna system increase the chance of accidental radiation burn if someone
touches the antenna during transmission.

Standing Waves (and Impedance)
 

W. Watson wrote:
A standing wave is the sum of an incident added to the reflective wave.
Isn't it possible to send two incident waves down an xline with
different frequences, and produce two different standing waves by having
some multiplicative relationship between the two incident waves and the
xline length?

Sure, it's possible but one wonders about the application.

2. Matched impedances give ideal power transfer; mismatched
impedances give high SWR and reduced power transfer.

A middle ground - Conjugately matched impedances give ideal power
transfer in the presence of high SWR. A feedline doesn't have to
be flat to be "matched". All that is required is that maximum
available power (actually energy) be transferred.
--
73, Cecil http://www.qsl.net/w5dxp

Standing Waves (and Impedance)
 

I was simply sharing my experiences from the past.
If you have followed a few of my earlier posts you are aware I am
simply a student who should have been studying this many years ago.

I welcome any corrections.
I never gave the swr on the radiator any thought. That is a good
point.
..

On Wed, 21 Dec 2005 22:18:26 GMT, Cecil Moore wrote:

John Ferrell wrote:
You can learn to tolerate high SWR's, but I find it worthwhile to try
to keep things matched. The energy has to go somewhere, I prefer it
leave here through the antenna...
John Ferrell W8CCW

I assume that you realize there is a high SWR on a standing-
wave antenna, like a resonant 1/2WL dipole? :-)
John Ferrell W8CCW