Reflection on Resistive loads
 

I am trying to understand forward and reflection power
throught practical circuits , instead of going through books.

I found forward/reverse power meters measure reverse
power by measuring the phase difference between current
and voltage at load terminals. If current and voltage
are in phase, there is no reflection . am i correct ?

if it is correct ,
if transmitter has 50+j0 ohm output impedance , and if i connect
antenna tuned to 100+j0 ohm. (assume no long cable is used ,
directly connected ) , reverse power is zero ?

regards,
palani

Reflection on Resistive loads
 

On 27 Jul 2006 20:30:57 -0700, "palaniappan chellappan"
wrote:


I am trying to understand forward and reflection power
throught practical circuits , instead of going through books.

Hi Palani,

The remainder of your post suggests you should go through the books.

I found forward/reverse power meters measure reverse
power by measuring the phase difference between current
and voltage at load terminals.

WRONG. However, what you say is right by parts. They measure
voltage, they measure current, and they measure differences, but not
in the simple manner you describe. The sum of the parts does not
equal the total of the statement.

If current and voltage
are in phase, there is no reflection . am i correct ?

WRONG. See following:

if it is correct ,
if transmitter has 50+j0 ohm output impedance , and if i connect
antenna tuned to 100+j0 ohm. (assume no long cable is used ,
directly connected ) , reverse power is zero ?

12% of the power is reflected with a 2:1 (100:50) mismatch, not zero.
Current and voltage are in phase (resistive load by your definition of
100+j0 ohm).

73's
Richard Clark, KB7QHC

Reflection on Resistive loads
 

palaniappan chellappan wrote:
If current and voltage
are in phase, there is no reflection . am i correct ?

No. Assuming Z0 is purely resistive, here's what it
takes to make your statement true:

If current and voltage are in phase *at every point*
up and down a transmission line, there is no reflection.

Just to be sure we have cause and effect straight, the
statement should probably be:

Assuming the presence of a signal, if there are no
reflections on a purely resistive Z0 transmission line,
the current and voltage will be in phase at every point
on the transmission line.
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Cecil Moore wrote:
palaniappan chellappan wrote:
If current and voltage
are in phase, there is no reflection . am i correct ?

No. Assuming Z0 is purely resistive, here's what it
takes to make your statement true:

If current and voltage are in phase *at every point*
up and down a transmission line, there is no reflection.


Thanks for replies, but i couln't able to conclude anything from the
two complementary replies i got :-(.
Which book is best for learning about concepts like
antenna,reflection,swr, etc. I am working only at hf band. If there is
any online tutorial ?

regards,
palam

Reflection on Resistive loads
 

palaniappan chellappan wrote:
Thanks for replies, but i couln't able to conclude anything from the
two complementary replies i got :-(.

If the voltage and current are in phase at every point up
and down a transmission line (with a resistive Z0), there
are no reflections. If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Which book is best for learning about concepts like
antenna,reflection,swr, etc. I am working only at hf band. If there is
any online tutorial ?

Unfortunately, only the older (pre 1990's) ARRL Antenna
Books present a decent explanation of reflections. The ARRL
has stopped discussing forward and reflected waves and
started discussing impedance matching. IMO, it's part of
the dumbing down of the ARS along with the rest of the US.

Walter Maxwell literally wrote the book on "Reflections". His
web page is: http://www.w2du.com Worldradio magazine is on
the verge of publishing "Reflections III". Their web page
is: http://www.wr6wr.com

You might get something useful from my Worldradio magazine
article available at: http://www.qsl.net/w5dxp/energy.htm
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Which book is best for learning about concepts like
antenna,reflection,swr, etc. I am working only at hf band. If there is
any online tutorial ?


Try these links:

http://en.wikipedia.org/wiki/Smith_chart

http://en.wikipedia.org/wiki/Reflection_coefficient

Don't feel too bad if this subject is not too clear. It used to be
introduced in about the fourth semester of Electrical Engineering, usually
in Complex Circuit Analysis.

Here's the $2 version:

Impedance is a complex quantity (i.e. it has both Phase and Magnitude.) The
reflection coefficient is a measure of how well a load is matched to its
source impedance; the match determines how much power is reflected by the
load. Whatever is not reflected, is absorbed by the load. In the case of
an antenna, the "load" is free space; therefore all power absorbed by the
load is radiated into free space. We would like the reflection coefficient
to be 0; when that happens we have a perfect match and all power is
radiated.

Like impedance, the reflection coefficient has both a phase and a magnitude.
When most people talk about the reflection coefficient, they only conside
the magnitude and not the phase.

"Cecil Moore" wrote in message
...
palaniappan chellappan wrote:
Thanks for replies, but i couln't able to conclude anything from the
two complementary replies i got :-(.

If the voltage and current are in phase at every point up
and down a transmission line (with a resistive Z0), there
are no reflections. If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Which book is best for learning about concepts like
antenna,reflection,swr, etc. I am working only at hf band. If there is
any online tutorial ?

Unfortunately, only the older (pre 1990's) ARRL Antenna
Books present a decent explanation of reflections. The ARRL
has stopped discussing forward and reflected waves and
started discussing impedance matching. IMO, it's part of
the dumbing down of the ARS along with the rest of the US.

Walter Maxwell literally wrote the book on "Reflections". His
web page is: http://www.w2du.com Worldradio magazine is on
the verge of publishing "Reflections III". Their web page
is: http://www.wr6wr.com

You might get something useful from my Worldradio magazine
article available at: http://www.qsl.net/w5dxp/energy.htm
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Just a nit: You said:

If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Actually, if there are reflections, the voltage and current ar NEVER in
phase.

"Cecil Moore" wrote in message
...
palaniappan chellappan wrote:
Thanks for replies, but i couln't able to conclude anything from the
two complementary replies i got :-(.

If the voltage and current are in phase at every point up
and down a transmission line (with a resistive Z0), there
are no reflections. If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Which book is best for learning about concepts like
antenna,reflection,swr, etc. I am working only at hf band. If there is
any online tutorial ?

Unfortunately, only the older (pre 1990's) ARRL Antenna
Books present a decent explanation of reflections. The ARRL
has stopped discussing forward and reflected waves and
started discussing impedance matching. IMO, it's part of
the dumbing down of the ARS along with the rest of the US.

Walter Maxwell literally wrote the book on "Reflections". His
web page is: http://www.w2du.com Worldradio magazine is on
the verge of publishing "Reflections III". Their web page
is: http://www.wr6wr.com

You might get something useful from my Worldradio magazine
article available at: http://www.qsl.net/w5dxp/energy.htm
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

On Sun, 30 Jul 2006 16:04:10 -0700, "Bob Agnew" wrote:

Just a nit: You said:

If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Actually, if there are reflections, the voltage and current ar NEVER in
phase.

In fact, voltage and current in the reflected wave are ALWAYS 180° out of phase,
while in the forward wave they are always in phase. Thus, along the line they
alternately add and subtract, first reinforcing and then cancelling each other
at every quarter wave, to form the standing wave.

Walt, W2DU

Reflection on Resistive loads
 

Bob Agnew wrote:
Just a nit: You said:

If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Actually, if there are reflections, the voltage and current ar NEVER in
phase.

So how does a 1/2WL piece of transmission line driving a
50 ohm load wind up with the voltage and current in phase
no matter what the SWR?
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Walter Maxwell wrote:
On Sun, 30 Jul 2006 16:04:10 -0700, "Bob Agnew" wrote:
Actually, if there are reflections, the voltage and current ar NEVER in
phase.

In fact, voltage and current in the reflected wave are ALWAYS 180° out of phase,
while in the forward wave they are always in phase. Thus, along the line they
alternately add and subtract, first reinforcing and then cancelling each other
at every quarter wave, to form the standing wave.

All true, Walt, but I think we are discussing the net voltage
and net current which are in phase only every 1/4 wavelength
where the SWR circle crosses the horizontal purely resistive
line on the Smith Chart.
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Walter Maxwell wrote:
On Sun, 30 Jul 2006 16:04:10 -0700, "Bob Agnew" wrote:

Just a nit: You said:

If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.
Actually, if there are reflections, the voltage and current ar NEVER in
phase.

In fact, voltage and current in the reflected wave are ALWAYS 180° out of phase,
while in the forward wave they are always in phase. Thus, along the line they
alternately add and subtract, first reinforcing and then cancelling each other
at every quarter wave, to form the standing wave.

Walt, W2DU

And, if you assume the line is lossless, the voltage and current are in
phase every 90 degrees along the line regardless of the amount of mismatch.

This is easily illustrated with a Smith chart -- choose any point you'd
like, representing an arbitrary load impedance. Then draw a circle
through that point, with the center of the circle at the chart's origin.
Moving clockwise along this circle represents moving along the
transmission line from the load toward the source. You'll cross the
chart's axis, where the impedance is purely real, in a half revolution
(90 degrees of movement along the line) or less, and cross it each half
revolution (90 degrees) from then on.

Roy Lewallen, W7EL

Reflection on Resistive loads
 

Roy Lewallen wrote:
And, if you assume the line is lossless, the voltage and current are in
phase every 90 degrees along the line regardless of the amount of mismatch.

If the line has losses, the SWR circle becomes an SWR spiral
but the spiral still crosses the purely resistive axis like
the circle does, just not at the same points.

Does your answer imply that the number of degrees between
purely resistive crossings is not equal to 90 degrees when
the line is lossy? (Not a trick question)
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Cecil Moore wrote:
Roy Lewallen wrote:
And, if you assume the line is lossless, the voltage and current are
in phase every 90 degrees along the line regardless of the amount of
mismatch.

If the line has losses, the SWR circle becomes an SWR spiral
but the spiral still crosses the purely resistive axis like
the circle does, just not at the same points.

Does your answer imply that the number of degrees between
purely resistive crossings is not equal to 90 degrees when
the line is lossy? (Not a trick question)

After further thought, I think Roy's point is that a lossless
transmission line has a purely resistive Z0 so the voltage
and current are in phase every 90 degrees. Z0 is not purely
resistive for ordinary transmission lines. But real-world
distortionless lines are indeed lossy while possessing a
purely resistive Z0.
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

So how does a 1/2WL piece of transmission line driving a
50 ohm load wind up with the voltage and current in phase
no matter what the SWR?
--

If the characteristic impedance of the transmission line is 50 ohms, then
there are no reflections; furthermore the current and voltage are in phase
at every point along the line, There are no standing waves in this case.

In fact it doesnt matter how long the line is as long as it is terminated in
its charcteristic impedance. This case corresponds to the circle in the
middle of the Smith Chart on which the impedance is constant.


"Cecil Moore" wrote in message
. com...
Bob Agnew wrote:
Just a nit: You said:

If there are reflections, the voltage
and current are in phase only every 1/4 wavelength.

Actually, if there are reflections, the voltage and current ar NEVER in
phase.
c 73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Bob Agnew wrote:

So how does a 1/2WL piece of transmission line driving a
50 ohm load wind up with the voltage and current in phase
no matter what the SWR?
--

If the characteristic impedance of the transmission line is 50 ohms, then
there are no reflections; furthermore the current and voltage are in phase
at every point along the line, There are no standing waves in this case.

In fact it doesnt matter how long the line is as long as it is terminated in
its charcteristic impedance. This case corresponds to the circle in the
middle of the Smith Chart on which the impedance is constant.

It doesn't matter what the characteristic impedance of the
transmission line is as long as it is an electrical 1/2WL. As such,
you can have reflections, feeding a 50 ohm resistive load, and the
voltage and current will be in phase 1/2WL back fron the load. Antenna
matching with transmission line transformers use different impedance
transmission lines to wind up with a perfect match.
73 Gary N4AST

Reflection on Resistive loads
 

I haven't taught this subject at the University level for a few years now,
so my terminology is a little rusty. VSWR = (1 + abs(rho))/(1 - abs(rho))

When the reflection coefficient rho is +/- 1 the VSWR is infinite. When rho
= 0 the VSWR is 1:1. A VSWR of 1:1 corresponds to the unit circle at the
center of the Smith Chart.

wrote in message
oups.com...

Bob Agnew wrote:

So how does a 1/2WL piece of transmission line driving a
50 ohm load wind up with the voltage and current in phase
no matter what the SWR?
--

If the characteristic impedance of the transmission line is 50 ohms, then
there are no reflections; furthermore the current and voltage are in
phase
at every point along the line, There are no standing waves in this case.

In fact it doesnt matter how long the line is as long as it is terminated
in
its charcteristic impedance. This case corresponds to the circle in the
middle of the Smith Chart on which the impedance is constant.

It doesn't matter what the characteristic impedance of the
transmission line is as long as it is an electrical 1/2WL. As such,
you can have reflections, feeding a 50 ohm resistive load, and the
voltage and current will be in phase 1/2WL back fron the load. Antenna
matching with transmission line transformers use different impedance
transmission lines to wind up with a perfect match.
73 Gary N4AST

Reflection on Resistive loads
 

Bob Agnew wrote:
I haven't taught this subject at the University level for a few years now,
so my terminology is a little rusty. VSWR = (1 + abs(rho))/(1 - abs(rho))

When the reflection coefficient rho is +/- 1 the VSWR is infinite. When rho
= 0 the VSWR is 1:1. A VSWR of 1:1 corresponds to the unit circle at the
center of the Smith Chart.

Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects
back the identical load impedance, no matter the transmission line
characteristic impedance (neglecting losses). If you have a 1/2WL 600
ohm line driving a 50 ohm load, then you have reflections (SWR), but at
the source end, you see 50 ohms (V and I in phase).
73 Gary N4AST

Reflection on Resistive loads
 

Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects

OK __ I won't post here anymore. I once was an Amateur when I was 14.
I thought that I wanted to get back into the hobby now that I am retired.

wrote in message
ups.com...

Bob Agnew wrote:
I haven't taught this subject at the University level for a few years
now,
so my terminology is a little rusty. VSWR = (1 + abs(rho))/(1 -
abs(rho))

When the reflection coefficient rho is +/- 1 the VSWR is infinite. When
rho
= 0 the VSWR is 1:1. A VSWR of 1:1 corresponds to the unit circle at the
center of the Smith Chart.

Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects
back the identical load impedance, no matter the transmission line
characteristic impedance (neglecting losses). If you have a 1/2WL 600
ohm line driving a 50 ohm load, then you have reflections (SWR), but at
the source end, you see 50 ohms (V and I in phase).
73 Gary N4AST

Reflection on Resistive loads
 

On Mon, 31 Jul 2006 16:30:21 -0700, "Bob Agnew"
wrote:

Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects

OK __ I won't post here anymore. I once was an Amateur when I was 14.
I thought that I wanted to get back into the hobby now that I am retired.

Hi Bob,

You as a prof. are not alone here. We have several, some still in the
saddle, others retired, and all Hams. You and Gary are not wrong
either, simply posting at cross purposes when the topic was sidelined
into half wave lines, SWR, and a demand for an explanation for a
problem that was never offered in the first place. This was a troll
that bit you, not Gary.

73's
Richard Clark, KB7QHC

Reflection on Resistive loads
 

"Richard Clark" wrote in message
...
On Mon, 31 Jul 2006 16:30:21 -0700, "Bob Agnew"
wrote:

Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects

OK __ I won't post here anymore. I once was an Amateur when I was 14.
I thought that I wanted to get back into the hobby now that I am retired.

Hi Bob,

You as a prof. are not alone here. We have several, some still in the
saddle, others retired, and all Hams. You and Gary are not wrong
either, simply posting at cross purposes when the topic was sidelined
into half wave lines, SWR, and a demand for an explanation for a
problem that was never offered in the first place. This was a troll
that bit you, not Gary.

73's
Richard Clark, KB7QHC

I have been an amateur continuously since I was 13: 1963.

Transmission line: 1/2 half wave repeats the load, 1/4 wave inverts the
load.
That I knew in Jr High.

Finally understood it when I studied E&M in college.

73
H.
NQ5H

www.hep.utexas.edu/mayamuon

Reflection on Resistive loads
 

Bob Agnew wrote:
So how does a 1/2WL piece of transmission line driving a
50 ohm load wind up with the voltage and current in phase
no matter what the SWR?

If the characteristic impedance of the transmission line is 50 ohms, then
there are no reflections; furthermore the current and voltage are in phase
at every point along the line, There are no standing waves in this case.

But I didn't say Z0 equals 50 ohms. What if Z0 equals 75 ohms?
300 ohms? 450 ohms? 600 ohms? Assuming lossless lines, those
Z0's will all result in 50 ohms looking into them when they
are 1/2WL long. Thus the voltage and current will be in phase
for 1/2WL no matter what the characteristic impedance and no
matter what the SWR.
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

Bob Agnew wrote:
When the reflection coefficient rho is +/- 1 the VSWR is infinite. When rho
= 0 the VSWR is 1:1. A VSWR of 1:1 corresponds to the unit circle at the
center of the Smith Chart.

And when SWR = 1:1, that circle has a radius of zero, i.e.
it is a point at the center of the Smith Chart.

But it seems to me that no matter what the Z0, whether purely
resistive or containing some reactance as does ordinary transmission
line, on a line with reflections, there will be a point where the
ratio of net voltage to net current has the current leading the
voltage and a short distance away, the current is lagging the voltage.
In between those two points, it seems to me that there must exist a
point where the voltage and the current are in phase. How could the
net (virtual) reactance go from capacitive to inductive without having
the voltage and current in phase at a point in between?
--
73, Cecil http://www.qsl.net/w5dxp

Reflection on Resistive loads
 

On Mon, 31 Jul 2006 19:35:45 -0500, "H. Adam Stevens, NQ5H"
wrote:

That I knew in Jr High.

Finally understood it when I studied E&M in college.

You're miles ahead of me then. I'm tackling phonon dispersion right
now. I know it, I should understand it (especially after weeks in the
library), but there's always something in the corner of my mind on all
sorts of these subjects that asks:
"What the #%!@ ?"
or as we said in tech school to conform to comm systems lingo:
"What the #%!@ ? - Over."

73's
Richard Clark, KB7QHC

Reflection on Resistive loads
 

Bob Agnew wrote:
Hi Bob, I have never taught at the University level myself, this is an
"Amateur" newsgroup. If you look at the Smith Chart, 1/2WL reflects

OK __ I won't post here anymore. I once was an Amateur when I was 14.
I thought that I wanted to get back into the hobby now that I am retired.

Hi Bob, I'm sorry if you read something into my "amateur" statement
that I did not mean. I for one would welcome you to post on anything,
anytime you wish. I would also like to welcome you back into the
hobby, if that what you would like to do. I became a Ham when I was
about 14. Am an EE, but never worked in the RF end of things. When I
post on the antenna group, I am an Amateur.
73 Gary N4AST