VSWR doesn't matter?
 

Bob wrote:
On Mar 12, 1:08 am, "billcalley" wrote:
We are all told that VSWR doesn't matter when using low loss
transmission lines,

No, we are not all told that.

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?

The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage. It's only 50ohm once it becomes a moving
wave in the transmission line.

Bob9



Kewl, then I'll just run a tap directly to the 5000 ohm plates and start
a long chat up ... what the heck is all those pi matching components in
the way of the rf? Probably some loss there! ROFLOL!!!

JS
--
http://assemblywizard.tekcities.com

VSWR doesn't matter?
 

Danny Richardson wrote in
:

(any edition) and put that myth to rest once and for all.

Wishful thinking Danny, myths are the stuff of ham radio, aren't they?

Owen

VSWR doesn't matter?
 

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

VSWR doesn't matter?
 

"Dan Bloomquist" wrote in message
...
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.

As pointed out, VSWR does matter. A lot of bouncing means you heat the
transmission line with the power instead of radiating the power. 'Doesn't
matter', really means it can be tolerated if need be.

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...

Here is what you are missing. In the case of the output, (real/resistive
component of the transmitter), seeing the reflected wave, it is _not_
reflecting that power back up the transmission line as you think it is. It
would go back to that real impedance and heat the transmitter. Here is
what is done with a miss match in the real world.

trans-output - match - line - antenna

The 'match' is where the magic happens. All the energy coming down the
line that got reflected from the antenna 'sees' the 'trans-output -
match' as a perfect reflector and gets bounced back[*]. On the other side
of the match is the trans-output. There the trans-output sees a perfect
impedance, (technically, the conjugate of the trans-output), so that all
the power travels through the match toward the antenna.

The magic is that when the match is tuned, both of the above conditions
are satisfied.

*The reflected wave sees a purely reactive reflector not just because of
the network but also because of the output power of the transmitter.
Without transmitter power the impedance as seen from the load will
dramatically change.

Best, Dan.


Saying that SWR doesnt matter is a rather broad statement(like saying never
or always) but I have know of antenna systems having an SWR of 30:1 and his
was normal. The feedline was balanced line made of 1 inch copper. Of course
an SWR lie this on coax could be fatal to coax and equipment. A more common
example of this is the 1/4 wl matching section on a J-pole antenna. It
matches 50 ohms to a few Kohms so an SWR of 60: 1 or so would not be unusal
here.S oas long as the feedline can handle the current and voltage peaks
without much los it doesnt matter much as long as the source impedance is
matched to the impedance at the input to the transmission line.Im sure there
is a practical limit though.

Jimmie

VSWR doesn't matter?
 

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

VSWR doesn't matter?
 

"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

VSWR doesn't matter?
 

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

VSWR doesn't matter?
 

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

VSWR doesn't matter?
 

The active part of the transmitter output isn't 50 ohm.
That would cause half the power to be lost as heat in
the output stage. It's only 50ohm once it becomes a moving
wave in the transmission line.

Bob9

Kewl, then I'll just run a tap directly to the 5000 ohm plates and start
a long chat up ... what the heck is all those pi matching components in
the way of the rf? Probably some loss there! ROFLOL!!!

JS

It appears that you agree with that part of my post but you are
drawing
an invalid conculsion from it. I never suggested that the passive
matching
network usually found in a transmitter output is unnecessary.

Bob

VSWR doesn't matter?