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

Owen Duffy wrote:
(I used the term resolve, Cecil must
call it destructive interference.)

When two coherent waves superpose in an out-of-phase
manner, it is called destructive interference. When
two coherent waves superpose in an in-phase manner,
it is called constructive interference.

100V at 0 deg superposed with 50V at 180 deg equals
50V at 0 deg. That's destructive interference.

100V at 0 deg superposed with 50V at 0 deg equals
150V at 0 deg. That's constructive interference.

One can solve a mental example. Assume a 1/4WL 600
ohm open stub made from resistance wire such that
its impedance is 50 ohms. Apply a signal from a
piece of 50 ohm coax and analyze the conditions
at the physical 50/600 ohm impedance discontinuity.
One will get a good idea of what goes on with a
1/2WL dipole at the feedpoint.
--
73, Cecil http://www.w5dxp.com

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

Richard Fry wrote:
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.

Sometimes it helps to understand the situation by mentally
adding one wavelength of lossless feedline to the system.

Roy's suggestion to change to a 300 ohm load only apparently
eliminates the reflections.

XMTR---1/2WL 300 ohm feedline---300 ohm load

Mentally add the one wavelength of lossless 50 ohm line.
Conditions remain the same but the reflections are exposed.

XMTR--1WL 50 ohm feedline--+--1/2WL 300 ohm feedline--300 ohm load

There's a 6:1 SWR on the 1WL 50 ohm feedline and we have reflected
energy incident upon the source, just as you described above.
--
73, Cecil http://www.w5dxp.com