Dipole with standing wave - what happens to reflected wave?
 

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.

Dipole with standing wave - what happens to reflected wave?
 

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.

Dipole with standing wave - what happens to reflected wave?
 

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
--

Dipole with standing wave - what happens to reflected wave?
 

"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.

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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
--

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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
--

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

"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

Dipole with standing wave - what happens to reflected wave?
 

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.

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

"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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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
--

Dipole with standing wave - what happens to reflected wave?
 

"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

Dipole with standing wave - what happens to reflected wave?
 

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
--

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

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

Dipole with standing wave - what happens to reflected wave?
 

"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

Dipole with standing wave - what happens to reflected wave?
 

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