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