"Owen Duffy" wrote
The mechanism is not "that the reflected wave must travel back to
the PA anode and will be absorbed there causing overheating".
Take an example of the 50 ohms load discussed, and an electrical half
wave of 70 ohm line connected to a transmitter designed for a 50 ohm
load. The transmitter behaves exactly as if that line were 50 ohms.
Though there is a reflected travelling wave on the line, it does not
travel back to the PA anode where it is absorbed and converted to
heat.
____________

Really, the mechanism is there -- only the unique circumstance you describe
protects the PA from seeing reflected power in such cases. Other line
lengths in this scenario could stress PA components beyond their ratings.

And even if the PA saw no reflected power because of a fortunate length of
transmission line connecting it to a mismatched antenna/load, that reflected
power still exists in the transmission line, and may cause its failure.

Manufacturers of the rigid coaxial line used in broadcast stations (e.g.,
Dielectric) require derating its maximum power rating inversely by the value
of the SWR existing in it . A power derating factor related to SWR also
applies to Andrew Heliax® and RG-type coax line. Deliberately setting up,
or tolerating reflected power on a transmission line is not done without
risk.

RF

Richard Fry wrote:
Really, the mechanism is there -- only the unique circumstance you
describe protects the PA from seeing reflected power in such cases.
Other line lengths in this scenario could stress PA components beyond
their ratings.

Yes, the exposure of the PA to reflected power depends
upon the phase of the reflected wave referenced to the
phase of the source wave. SWR doesn't tell the whole
story because phase is not reported by the SWR
measurement.

What is happening with 1/2WL of Z0=600 ohm feedline
connected to a 50 ohm load on one end and a 50 ohm
source on the other end is interference. Destructive
interference toward the source causes constructive
interference toward the load and the reflected energy
at the source is re-directed back toward the load.

What we want to avoid in our antenna systems is
constructive interference toward the source.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore"
What is happening with 1/2WL of Z0=600 ohm feedline
connected to a 50 ohm load on one end and a 50 ohm
source on the other end is interference. Destructive
interference toward the source causes constructive
interference toward the load and the reflected energy
at the source is re-directed back toward the load.

What we want to avoid in our antenna systems is
constructive interference toward the source.
______________

Whether the source sees that reflected power or not, that reflected power is
stressing the transmission line -- regardless of the electrical length of
that line at the operating frequency.

IOW, for these unique conditions the source may be unaffected by the
reflected power in the line, but the line itself is still stressed by that
power -- and possibly to the point of failure.

RF

Richard Fry wrote:
Whether the source sees that reflected power or not, that reflected
power is stressing the transmission line -- regardless of the electrical
length of that line at the operating frequency.

Didn't mean to imply that I was disagreeing with you in
any way. Given the forward and reflected power readings
for any particular line, the stresses can be calculated.
--
73, Cecil http://www.w5dxp.com

On Tue, 5 Sep 2006 06:48:57 -0500, "Richard Fry"
wrote:

"Owen Duffy" wrote
The mechanism is not "that the reflected wave must travel back to
the PA anode and will be absorbed there causing overheating".
Take an example of the 50 ohms load discussed, and an electrical half
wave of 70 ohm line connected to a transmitter designed for a 50 ohm
load. The transmitter behaves exactly as if that line were 50 ohms.
Though there is a reflected travelling wave on the line, it does not
travel back to the PA anode where it is absorbed and converted to
heat.
____________

Really, the mechanism is there -- only the unique circumstance you describe
protects the PA from seeing reflected power in such cases. Other line
lengths in this scenario could stress PA components beyond their ratings.


Richard,

There is not such a mechanism in the general case, the example I gave
shows that the reflected wave does not necessarily travel back to the
source where it is absorbed.

If you re-read my words "Next thing, you will be thinking that the
reflected wave must travel back to the PA anode and will be absorbed
there causing overheating." Remembering the context was a wave on the
dipole reflected from the o/c end, and the word "must" was used to
mean "necessarily".

Sure,transmission lines with VSWR may transform impedance, have higher
losses (if they are long enough), operate at higher voltages in places
(if they are long enough), operate with higher currents in places (if
they are long enough). Nothing I have said is in conflict with that or
suggests otherwise.

Transmitters operated at other than their rated load impedance may
operate at higher voltages, higher currents, different power etc and
may damage components. Nothing I have said is in conflict with that or
suggests otherwise.

But, the mechanism is not that the reflected wave *necessarily*
travels all the way back to the PA anode by virtue of some kind of
momentum (as sometimes expressed by some amateurs).

In the case raised by the OP, the reflected wave on the dipole and the
forward wave resolve (as in resolution of phasors) to an impedance of
50+j0 (OP's hypothetical example), and the constraints / conditions at
the feedline / feedpoint junction are fully satisfied with no
reflected wave on the feedline. (I used the term resolve, Cecil must
call it destructive interference.) The reflected wave on the dipole
does not have a momentum that *must* carry it to the PA anode to be
absorbed there.

Owen
--

"Owen Duffy" wrote:
But, the mechanism is not that the reflected wave *necessarily*
travels all the way back to the PA anode by virtue of some kind of
momentumne / feedpoint junction are fully satisfied with no
reflected wave on the feedline. (I used the term resolve, Cecil must
call it destructive interference.) The reflected wave on the dipole
does not have a momentum that *must* carry it to the PA anode to be
absorbed there.
_____________

However, any amount of reflected power from the termination at the far end
of a transmission line has a greater chance of damaging tx PA components,
and of stress/failure to the transmission line itself than if the reflected
power from the antenna/load was zero, regardless of the electrical length of
said transmission line.

This reality of physics is not subject to debate, don't you agree?

RF

On Tue, 5 Sep 2006 18:04:36 -0500, "Richard Fry"
wrote:

"Owen Duffy" wrote:
But, the mechanism is not that the reflected wave *necessarily*
travels all the way back to the PA anode by virtue of some kind of
momentumne / feedpoint junction are fully satisfied with no
reflected wave on the feedline. (I used the term resolve, Cecil must
call it destructive interference.) The reflected wave on the dipole
does not have a momentum that *must* carry it to the PA anode to be
absorbed there.
_____________

However, any amount of reflected power from the termination at the far end
of a transmission line has a greater chance of damaging tx PA components,
and of stress/failure to the transmission line itself than if the reflected
power from the antenna/load was zero, regardless of the electrical length of
said transmission line.

If it makes you more comfortable to restrict your solutions to those
with flat lines (VSWR~=1) then that is ok with me, but that does not
invalidate other approaches. Antenna systems incorporating lines with
high VSWR do not necessarily subject the PA components to any
different risk, or transmission lines to excessive stresses, it is a
matter of design... and the design is more complex than buying a tx
intended for 50ohm load, some 50ohm coax and a 50ohm antenna and
plugging them all together.

Owen
--

Richard Fry wrote:

However, any amount of reflected power from the termination at the far
end of a transmission line has a greater chance of damaging tx PA
components, and of stress/failure to the transmission line itself than
if the reflected power from the antenna/load was zero, regardless of the
electrical length of said transmission line.

This reality of physics is not subject to debate, don't you agree?

Suppose I have a 50 ohm antenna fed with a one wavelength, 50 ohm
transmission line. No reflected power, no damage.

Now I replace the 50 ohm one wavelength line with a 300 ohm one
wavelength line. For 100 watts delivered to the load, the forward power
on the line is 204 watts and the reverse power is 104 watts. All that
reverse power could do a lot of damage, then?

So I guess I should replace the 50 ohm load with a 300 ohm one. Then
there won't be any reflected power, and the transmitter should be ok. Right?

After all, it's physics.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Now I replace the 50 ohm one wavelength line with a 300 ohm one
wavelength line. For 100 watts delivered to the load, the forward power
on the line is 204 watts and the reverse power is 104 watts. All that
reverse power could do a lot of damage, then?

Probably not at that power level but try this:

Let's say the power handling limit for the 300 ohm line is
1000 watts under matched line conditions. We are delivering
1000 watts to the 50 ohm load. The forward power on the 300
ohm line is 2040 watts and the reverse power on the 300 ohm
line is 1040 watts.

Yes indeed, the reflected power can cause the 300 ohm line
to burn up which is the point Richard Fry was trying to make.
--
73, Cecil http://www.w5dxp.com

"Roy Lewallen" wrote
Suppose I have a 50 ohm antenna fed with a one wavelength, 50 ohm
transmission line. No reflected power, no damage.

Now I replace the 50 ohm one wavelength line with a 300 ohm one wavelength
line. For 100 watts delivered to the load, the forward power on the line
is 204 watts and the reverse power is 104 watts. All that reverse power
could do a lot of damage, then?

So I guess I should replace the 50 ohm load with a 300 ohm one. Then there
won't be any reflected power, and the transmitter should be ok. Right?
___________

As you well know, the system change you describe will not remove all
reflected power seen at the output of a tx expecting a 50 ohm load. You
have simply moved the source of the reflection back to the plane of the tx
output connector.

Under your scenario the transmission line is happy, but the tx - not so
much.

RF