David wrote:
RF transmitter output has impedance of 50 ohms and is connected to
dipole with a feedpoint impedance of 50 ohms via feeder with
characteristic impedance of 50 ohms. System is perfectly matched. I
expect SWR meter to show perfect match of 1:1.

Yes.

Dipole has a standing wave on it. Ends of dipole are at high voltage.
Dipole is centre-fed with centre being high current point. Standing
wave means that a reflected wave exists. Wave is reflected from open
ends of dipole. What happens to the reflected wave? How does it
vanish at centre of dipole? Why does reflected wave not go along
feeder into transmitter output? There cannot be a reflected wave on
feeder because SWR is 1:1.

There is no standing wave on the antenna. The distribution patterns of
voltage and current on a half-wave dipole shown in antenna books, is not
a standing wave. They are the RMS values of voltage and current along
the dipole. At the centre, the ratio of Vrms to Irms is 50 ohms.
Alan

David wrote:
RF transmitter output has impedance of 50 ohms and is connected to dipole
with a feedpoint impedance of 50 ohms via feeder with characteristic
impedance of 50 ohms. System is perfectly matched. I expect SWR meter to
show perfect match of 1:1.

Dipole has a standing wave on it. Ends of dipole are at high voltage. Dipole
is centre-fed with centre being high current point. Standing wave means that
a reflected wave exists. Wave is reflected from open ends of dipole. What
happens to the reflected wave? How does it vanish at centre of dipole? Why
does reflected wave not go along feeder into transmitter output? There
cannot be a reflected wave on feeder because SWR is 1:1.


Sorry about my last post - complete drivel - please ignore.
Can't think what was going through my mind at the time.
Alan

"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 Clark wrote:

On Mon, 4 Sep 2006 20:44:17 +0100, "David" nospam@nospam wrote:
What happens to the reflected wave?

Hi David,
It is radiated.

How does it vanish at centre of dipole?

It does not "vanish."

Why does reflected wave not go along feeder into transmitter output?

Because you established there was a match at that port.

73's
Richard Clark, KB7QHC

Awesome. I get it. Thanks.

Alan Peake wrote:
There is no standing wave on the antenna. The distribution patterns of
voltage and current on a half-wave dipole shown in antenna books, is not
a standing wave. They are the RMS values of voltage and current along
the dipole. At the centre, the ratio of Vrms to Irms is 50 ohms.

From "Antennas" by Kraus & Marhefka: "A sinusoidal current
distribution may be regarded as the standing wave produced
by two uniform traveling waves of equal amplitude moving in
opposite directions along the antenna."

From "Antenna Theory" by Balanis: "Because of the standing
wave pattern, it is also classified as a standing wave antenna"
"The sinusoidal current distribution of long open-ended linear
antennas is a standing wave constructed by two waves of equal
amplitude and 180 deg phase difference at the open-end traveling
in opposite directions along its length."

Of course, a 1/2WL dipole is a standing wave antenna.
--
73, Cecil http://www.w5dxp.com

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