We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

Thanks!

-Bill

billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were
the transmitter output impedance actually at 50 ohms, on the other side
of the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner
loss will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Mon, 12 Mar 2007 01:25:00 UTC, Tim Wescott
wrote:

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.


OK ;-))


--
Jim Backus running OS/2 Warp 3 & 4, Debian Linux and Win98SE
bona fide replies to j dot backus the circle thingy jita dot
demon dot co dot uk

Tim Wescott wrote:
billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were
the transmitter output impedance actually at 50 ohms, on the other side
of the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner
loss will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The
cherry red plates are the reflected energy being absorbed. Transistors
will just turn to smoke under the same conditions.

Dave WD9BDZ

"David G. Nagel" wrote in message
...
Tim Wescott wrote:
billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were the
transmitter output impedance actually at 50 ohms, on the other side of
the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner loss
will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The cherry
red plates are the reflected energy being absorbed. Transistors will just
turn to smoke under the same conditions.

Dave WD9BDZ

Hi david

Wouldnt it be OK to have a high VSWR along the transmission line if the
"tank ckt" can be adjusted to match the load to the transmitter output
impedance? That is, the VSWR along the transmission could concievely be
high, yet, with proper "tank ckt" adjustment that impedance seen by the
output circuit (plate) wouldnt result in a "cherry red plate".
What I am asking is ? is the transmission line VSWR directly related to
"plate reddening"?
I'm more asking than *telling*.

Jerry

Jerry Martes wrote:

Wouldnt it be OK to have a high VSWR along the transmission line if the
"tank ckt" can be adjusted to match the load to the transmitter output
impedance? That is, the VSWR along the transmission could concievely be
high, yet, with proper "tank ckt" adjustment that impedance seen by the
output circuit (plate) wouldnt result in a "cherry red plate".

Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

What I am asking is ? is the transmission line VSWR directly related to
"plate reddening"?

Absolutely not.

I'm more asking than *telling*.

That's the first step in learning.

Roy Lewallen, W7EL

Roy Lewallen wrote:

Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

Well, without a line, you don't have a real component to tune into.
Drawing arcs on a smith chart from an open line with capacitors and
coils will only get you to another purely reactive point.

Best, Dan.

On Mar 12, 11:53 pm, Roy Lewallen wrote:
Yes! All that matters to the transmitter is the impedance it sees. It
doesn't know or care that you've mathematically separated the delivered
power into "forward" and "reverse" components. It doesn't know or care
what the SWR is on the transmission line connected to it, or even if a
transmission line is connected at all.

Think about this - if the transmission line is exactly one-wavelength
long
and lossless, the transmitter sees exactly the same impedance as the
load. At the load, we know reflections occur, but they are same-cycle
reflections so during steady-state with no modulation, exactly the
same
conditions exist at the transmitter as exist at the load if the
transmitter
has the same impedance as the transmission line. So even if we
cannot measure the reflections back into the transmitter, they are
no doubt, there - that is, unless one denies the existence of
reflections
in which case, one needs to explain how standing waves are possible
without reflections in a single-source system.
--
73, Cecil, w5dxp.com

Jerry Martes wrote:
"David G. Nagel" wrote in message
...
Tim Wescott wrote:
billcalley wrote:

We are all told that VSWR doesn't matter when using low loss
transmission lines, since the RF energy will travel from the
transmitter up to the mismatched antenna, where a certain amount of
this RF energy will reflect back towards the transmitter; after which
the RF will then reflect back up to the antenna -- where the energy is
eventually radiated after bouncing back and forth between the
transmitter and antenna. I understand the concept, but what I don't
quite understand is why the reflected RF energy isn't simply absorbed
by the 50 ohm output of the transmitter after the first reflection?
For the RF to bounce back and forth, wouldn't the transmitter's
impedance have to be very, very high (or low) when the reflected RF
energy hit its output stages? I know I'm missing something vital
here...

That's assuming you use an antenna tuner. The tuner will transform the
transmitter's output impedance* just as it transforms the line. Were the
transmitter output impedance actually at 50 ohms, on the other side of
the tuner it would have the same VSWR as the line when everything was
tuned up.

Having said that, the VSWR _does_ matter somewhat when using low loss
lines, both because the line loss is low but not zero, and the tuner loss
will tend to go up as you correct for higher and higher VSWR.

* I am _not_ going to start the Big Transmitter Output Impedance Debate.
sed denizens -- just don't comment on what a transmitter's "actual"
output impedance may be, lest you start a flame war.

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The cherry
red plates are the reflected energy being absorbed. Transistors will just
turn to smoke under the same conditions.

Dave WD9BDZ

Hi david

Wouldnt it be OK to have a high VSWR along the transmission line if the
"tank ckt" can be adjusted to match the load to the transmitter output
impedance? That is, the VSWR along the transmission could concievely be
high, yet, with proper "tank ckt" adjustment that impedance seen by the
output circuit (plate) wouldnt result in a "cherry red plate".
What I am asking is ? is the transmission line VSWR directly related to
"plate reddening"?
I'm more asking than *telling*.

Jerry


Jerry;

The point I was trying to make is that the reflected current is
disapated as heat in the finals if the transmitter isn't matched to the
load.
In a tube radio the tank circuit is the equivilent of an antenna
match/tuner and converts the 2000 or so ohms at the plate to the 50 ohms
of the transmission line and the unknown ohms of the mis matched antenna.


Dave WD9BDZ

David G. Nagel wrote:

If you want a quick lesson in high vswr find a ham with an old tube
transmitter and see if he will hook it up to a mismatched load. The
cherry red plates are the reflected energy being absorbed. Transistors
will just turn to smoke under the same conditions.

Unfortunately, you'd be learning the wrong lesson.

The cherry color is due to the transmitter being loaded with an
impedance it's not designed for, causing the final to run at low
efficiency. You can disconnect the antenna and replace it with a lumped
RC or RL impedance of the same value and get exactly the same result.
Alternatively, you can attach any combination of load and transmission
line which give the same impedance, resulting in a wide variation of
"reflected energy", and get exactly the same result. All that counts is
the impedance seen by the transmitter, not the VSWR on the line or the
"reflected power".

The problem is that the idea of "reflected energy" turning the plates
hot is so easy to understand, that people aren't willing to abandon it
simply because it isn't true.

See http://eznec.com/misc/Food_for_thought.pdf for more.

Roy Lewallen, W7EL