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.

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.


If the reflection is exactly in phase with the next wave arriving
through the feed line, then it just raises the impedance the line
sees. In other words, the reflection acts as a large part of the feed
energy for the next cycle. It doesn't bounce into and out of the feed
line, it bounces back and forth from end to end of the dipole.
Actually there are two reflected waves going in opposite directions,
end to end, simultaneously.

On Mon, 4 Sep 2006 20:44:17 +0100, "David" nospam@nospam 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.

Accepting the figures as an example and not necessarily a reality...

You have just described a point which is a junction between:
- a load where the ratio of voltage to current is 50+j0;
- a feedline whe
* the ratio of the voltage to current due to the forward travelling
wave must each be in the ratio 50+j0;
* the ratio of the voltage to current due to the reflected
travelling waves must each be in the ratio 50+j0.

Having regard for the sign of the traveling waves, the only solution
to those constraints / conditions is that the reflected travelling
wave must have zero amplitude.


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.

Lets be clear that we are now talking about a single frequency.

At any point, the forward and reflected waves resolve to a single
voltage at that point, and a single current flowing at that point, and
the ratio of voltage to current is the impedance (and these are all
complex quantities, ie they have real and imaginery parts).

If the point you consider is the feedpoint, and the ratio of voltage
to current is 50+j0, then that is the impedance, it fully describes
the load at that frequency.

You talk of the "reflected wave" as if it has inertia, that it must
keep travelling when it reaches the junction of the feedline and the
antenna? You are not alone in speaking that way, but thinking that way
will get in the way of understanding what is happening. Next thing,
you will be thinking that the reflected wave must travel back to the
PA anode and will be absorbed there causing overheating.

Owen
--

"Owen Duffy" wrote:
You talk of the "reflected wave" as if it has inertia, that it must
keep travelling when it reaches the junction of the feedline and the
antenna? You are not alone in speaking that way, but thinking that way
will get in the way of understanding what is happening. Next thing,
you will be thinking that the reflected wave must travel back to the
PA anode and will be absorbed there causing overheating.
_______

If you write of reflected power existing on the transmission line between
the tx output connector and the antenna input connector, then yes -- with
sufficient tx output power and a poor enough match at the antenna feedpoint,
that reflected power can cause transmission line and/or tx PA component
failure.

I've analyzed and repaired many such failures of these installed systems in
my pre-retirement career, and know this first hand. Conventional theory
also supports this result.

RF

Visit http://rfry.org for FM transmission system papers.

John Popelish wrote:
You might. What is the feed point impedance of a dipole that has its
ends terminated in the complex conjugate of their local impedance, so
that no energy reflects at the ends?

It's roughly approximately the same as a terminated
Rhombic antenna, i.e. hundreds of ohms.

Let's make some rough assumptions. The SWR on the
1/2WL dipole is 20:1 which makes rho roughly
(20-1)/(20+1)= 0.9 That makes the power reflection
coefficient (rho^2) roughly equal to 0.8

If we are supplying 100 watts to the antenna then
Pfor - Pref = 100W and we know that Pref/Pfor = 0.8
So we can solve for Pfor = 500W and Pref = 400W.

If we assume the Z0 of the dipole is 600 ohms, that
makes Vfor = 548 volts and Ifor = 0.91 amps. Also
Vref = 490 volts and Iref = 0.81.

So (Vfor-Vref)/(Ifor/Iref) =
(548-490)/(0.91+0.81) = 58V/1.72A = 34 ohms.

But that is just half of the dipole's impedance so we
have to double it to get a feedpoint impedance in the
ballpark of 68 ohms. However, please note that 34
ohms is roughly the feedpoint impedance of a 1/4
wavelength monopole, i.e. half a dipole.

These are obviously rough ballpark assumptions but
you can observe the concepts involved. For the dipole
feedpoint impedance to be low, the voltages have to
subtract and the currents have to add. This agrees
with the extra 180 degree phase shift that happens
when the current is reflected at the ends of the dipole.
--
73, Cecil http://www.w5dxp.com

On Mon, 4 Sep 2006 17:03:39 -0500, "Richard Fry"
wrote:

"Owen Duffy" wrote:
You talk of the "reflected wave" as if it has inertia, that it must
keep travelling when it reaches the junction of the feedline and the
antenna? You are not alone in speaking that way, but thinking that way
will get in the way of understanding what is happening. Next thing,
you will be thinking that the reflected wave must travel back to the
PA anode and will be absorbed there causing overheating.
_______

If you write of reflected power existing on the transmission line between
the tx output connector and the antenna input connector, then yes -- with
sufficient tx output power and a poor enough match at the antenna feedpoint,
that reflected power can cause transmission line and/or tx PA component
failure.

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. At the tx end of the line, the forward and reflected wave
components resolve to a 50+j0 load, and the transmitter sees the same
50 ohm load as it would were 50 ohm line used. Increasing power or
increasing line Zo for a higher VSWR will not change the outcome of
this example.

Owen
--

Owen Duffy wrote:
Having regard for the sign of the traveling waves, the only solution
to those constraints / conditions is that the reflected travelling
wave must have zero amplitude.

Sorry Owen, that's not true. Assuming a 1/2WL dipole,
the reflected voltage has traveled 180 degrees. The
reflected current has traveled the same 180 degrees
plus experienced a 180 degree phase shift at the tip
of the dipole. Assuming 100 watts is being applied to
the antenna, the following conditions satisfy the
observed conditions on the dipole at the feedpoint.

Pfor = 500W, Vfor = 548V, Ifor = 0.91A

Pref = 400W, Vref = 490V, Iref = 0.81A

Pnet = 100W, Vnet = 58V, Inet = 1.72A

You talk of the "reflected wave" as if it has inertia, that it must
keep travelling when it reaches the junction of the feedline and the
antenna? You are not alone in speaking that way, but thinking that way
will get in the way of understanding what is happening.

It is well known that all EM waves contain energy and
momentum (inertia) so David is correct. The thing that
reverses the momentum of the reflected wave is destructive
interference at the Z0-match point. Anyone interested in
understanding how/why it happens is invited to peruse
my energy analysis article at: http://www.w5dxp.com/energy.htm
--
73, Cecil http://www.w5dxp.com

Owen Duffy wrote:
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.

Of course not. Destructive interference redirects the energy
back toward the load at the impedance discontinuity. The same
thing happens with a 1/4WL thin-film on non-reflective glass.
--
73, Cecil http://www.w5dxp.com

On Mon, 04 Sep 2006 22:53:34 GMT, Cecil Moore
wrote:

Owen Duffy wrote:
Having regard for the sign of the traveling waves, the only solution
to those constraints / conditions is that the reflected travelling
wave must have zero amplitude.

Sorry Owen, that's not true. Assuming a 1/2WL dipole,
....

Cecil, the statement was in the context of the previous paragraph. The
travelling waves I was referring to where the ones I had just
discussed which were on the transmission line (and NOT the dipole).

Owen
--

Owen Duffy wrote:
Cecil Moore wrote:
Sorry Owen, that's not true. Assuming a 1/2WL dipole,

Cecil, the statement was in the context of the previous paragraph. The
travelling waves I was referring to where the ones I had just
discussed which were on the transmission line (and NOT the dipole).

I apologize. I thought you were talking about the antenna.
I just reread it and it's not clear to me what was being
discussed.
--
73, Cecil http://www.w5dxp.com