What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

But then why does the concept of "mismatch loss" exist in
reference to antennas? I have quickly calculated that if a
transmitter outputs 100 watts, and the TX antenna has an impedance
that will cause a VSWR of 10:1 -- using lossless transmission line --
that the mismatch loss in this "lossless" system would be 4.81dB!
(Reflected power 66.9 watts, RL -1.74).

Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

Confused!

-Bill

billcalley wrote:
Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

If the system is Z0-matched, e.g. antenna tuner, there is
a mismatch gain at the tuner that offsets the mismatch
loss at the load so, in a lossless system, nothing is
lost. Wave cancellation toward the source is balanced
by constructive interference toward the load.
--
73, Cecil, w5dxp.com

On Mar 29, 1:34 pm, Cecil Moore wrote:
billcalley wrote:
Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

If the system is Z0-matched, e.g. antenna tuner, there is
a mismatch gain at the tuner that offsets the mismatch
loss at the load so, in a lossless system, nothing is
lost. Wave cancellation toward the source is balanced
by constructive interference toward the load.
--
73, Cecil, w5dxp.com

Now my head *really* hurts! This is a VERY confusing subject, to
say the least. (And I also thought antenna tuners actually had a
*loss* due to their limited Q...I think I'm going to change careers
now and just become a pet groomer; or perhaps simply give up
completely and work at Radio Shack).

-Bill

billcalley wrote:
Now my head *really* hurts! This is a VERY confusing subject, to
say the least. (And I also thought antenna tuners actually had a
*loss* due to their limited Q...I think I'm going to change careers
now and just become a pet groomer; or perhaps simply give up
completely and work at Radio Shack).

Real-world antenna tuners do have a loss but we previously
specified a lossless system. Of course, real world tuners
and transmission lines suffer losses but we all just live
with those losses while striving to minimize them. The point
is that an antenna tuner reflects most of the reflected
energy back toward the load thus accomplishing a mismatch
gain that offsets some, if not most, of the mismatch loss.
High SWR transmission lines are indeed lossier than flat
matched transmission lines of the same material.
--
73, Cecil, w5dxp.com

"billcalley" wrote in message
oups.com...
What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

That's basically true but ignores the stress that can be placed on the
output circuit of the transmitter. Why do you think VSWR shut-down circuits
are so popular? One can see rather high voltages or currents that are
potentially damaging to the transistors and capacitors.

Now, since "lossless" is an abstraction and all materials have voltage and
current limits, just make this easy on yourself and always strive for a VSWR
of 2:1 or less. It simply works better and is more reliable.

Charles Schuler wrote:
Now, since "lossless" is an abstraction and all materials have voltage and
current limits, just make this easy on yourself and always strive for a VSWR
of 2:1 or less. It simply works better and is more reliable.

But renders many all-HF-band dipoles useless. :-) I regularly
run up to an 18:1 SWR on my 450 ohm ladder-line. Owen's
transmission line calculator says I'm losing about 0.8 dB
in 100' of line under those conditions on 40m. IMO, it's
a small price to pay for all-HF-band operation.
--
73, Cecil, w5dxp.com

"Cecil Moore" wrote in message
...
Charles Schuler wrote:
Now, since "lossless" is an abstraction and all materials have voltage
and current limits, just make this easy on yourself and always strive for
a VSWR of 2:1 or less. It simply works better and is more reliable.

But renders many all-HF-band dipoles useless. :-) I regularly
run up to an 18:1 SWR on my 450 ohm ladder-line. Owen's
transmission line calculator says I'm losing about 0.8 dB
in 100' of line under those conditions on 40m. IMO, it's
a small price to pay for all-HF-band operation.

How many Ham transmitters have a balanced output?

How are you feeding a balanced line?

If you are using an antenna tuner with unbalanced in (50 ohms) and balanced
out (variable impedance), you should be OK in most situations.

"Charles Schuler" wrote in message
. ..

"billcalley" wrote in message
oups.com...
What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

That's basically true but ignores the stress that can be placed on the
output circuit of the transmitter. Why do you think VSWR shut-down
circuits are so popular? One can see rather high voltages or currents
that are potentially damaging to the transistors and capacitors.

Now, since "lossless" is an abstraction and all materials have voltage and
current limits, just make this easy on yourself and always strive for a
VSWR of 2:1 or less. It simply works better and is more reliable.

If a tuner is placed directly after the TX and properly adjusted the TX
will always see a 50 ohm load and the shutdown circuit will always be
happpy. Again as long as the TX sees a match there is no unusual stress
placed on it. Remember that before the invention of coax cable SWR was
rarely considered. Instead the tx was tuned for proper established
operational parametrs and all was right with the world.

1:1 SWR CAN MEAN YOUR COAX IS FULL OF WATER.

Jimmie

Charles Schuler wrote:
How are you feeding a balanced line?

http://www.w5dxp.com/notuner.htm

If you are using an antenna tuner with unbalanced in (50 ohms) and balanced
out (variable impedance), you should be OK in most situations.

No tuner! I don't like tuner losses. The feedpoint impedance
is always between 35 ohms and 85 ohms resistive. My choke
has an impedance in the thousands of ohms.
--
73, Cecil http://www.w5dxp.com

billcalley wrote:
What I gleaned from the excellent answers for the original "VSWR
Doesn't Matter?" thread is that high VSWR doesn't really matter in a
lossless transmission line environment between a transmitter's antenna
tuner and the antenna, since any reflected RF energy will simply
continue to "bounce" back and forth between the tuner's output
impedance and the antenna's input impedance until it is, finally,
completely radiated from the antenna without loss.

But then why does the concept of "mismatch loss" exist in
reference to antennas? I have quickly calculated that if a
transmitter outputs 100 watts, and the TX antenna has an impedance
that will cause a VSWR of 10:1 -- using lossless transmission line --
that the mismatch loss in this "lossless" system would be 4.81dB!
(Reflected power 66.9 watts, RL -1.74).

Since mismatch loss is the "amount of power lost due to
reflection", and is as if an "attenuator with a value of the mismatch
loss where placed in series with the transmission line", then I would
think that VSWR would *definitely* matter, and not just for highly
lossy lines either. But here again, I'm probably not seeing the
entire picture here. What am I missing??

Your "amount of power lost due to reflection" statement would be true if
the line were connected to something resistive at the line's
characteristic impedance. With a properly tuned tuner, that's not the
case -- instead, the impedance looking into the tuner will also reflect
power, and in a way that makes it all work out so that the power all
ends up being radiated, which is what you wanted in the first place.

--

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

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

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html