On Sun, 04 Dec 2005 07:57:55 -0800, dansawyeror
wrote:

Wes,

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected signals
changed by the same about?

Thanks - Dan kb0qil

I'm not sure I understand the question(s) but in the case of a vector
reflectometer using a dual directional coupler maybe this will help.

Here is a dual directional coupler.


Reverse Forward
| |
| |
|----------R R ---------|
X X
Input --A-----------------------B--C Load


Let's say that at frequency, F, the coupling factor (X) is -10 dB with
no phase shift between point B and the forward port and between point
A and the reverse port to keep it simple.

So a wave propagating in the forward direction (Input -- Load)
induces a signal at the forward port that is 10 dB below the input at
0 degrees phase with respect to point B. A wave propagating in the
opposite direction has the same relationship at the reverse port; 10
dB down and 0 degrees phase with respect to point A.

A to Reverse and B to Forward -might- track reasonably well in both
magnitude and phase, but in this case, it's immaterial.

Because B-A and C-B 0 there will be a frequency dependent phase
difference between A, B and C.

When we calibrate using a short on the load port here's what happens.

The signal at the forward port becomes the reference, i.e., unity
amplitude and 0 degrees phase.

The short creates a 100% reflection and -180 degree phase shift. This
signal propagates back down the main line to the source, which is
assumed to be a perfect match, so there is no re-reflection. A -10 dB
sample (by definition: unity) is coupled to the reverse port, with a
phase shift, theta(F), determined by the electrical length of the line
C - B - A.

Unless we are lucky enough to be Lotto winners, the signal at the
reflected port -will not- be 1 @ ang-180 deg. So our calibration
routine must do whatever math is necessary to make the ratio B/A = 1 @
ang-180. This fudge factor is then applied to all subsequent
measurements to "correct" the data.

Now to address (I think) your question. If we change frequencies,
theta(F) changes and the fudge factor no longer corrects for it.
While the coupling factors might track, it is of little consolation
because the calibration is good only at the frequency where it was
performed. Automatic network analyzers perform calibration at each
test frequency, or at least enough points to interpolate between.

Wes,

Thanks. If I read the gist of your reply the physical dimensions are the root
cause of the phase difference between the forward and reflected signals. Is this
true?

Thanks again - this is very helpful. Dan - kb0qil

Wes Stewart wrote:
On Sun, 04 Dec 2005 07:57:55 -0800, dansawyeror
wrote:


Wes,

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected signals
changed by the same about?

Thanks - Dan kb0qil


I'm not sure I understand the question(s) but in the case of a vector
reflectometer using a dual directional coupler maybe this will help.

Here is a dual directional coupler.


Reverse Forward
| |
| |
|----------R R ---------|
X X
Input --A-----------------------B--C Load


Let's say that at frequency, F, the coupling factor (X) is -10 dB with
no phase shift between point B and the forward port and between point
A and the reverse port to keep it simple.

So a wave propagating in the forward direction (Input -- Load)
induces a signal at the forward port that is 10 dB below the input at
0 degrees phase with respect to point B. A wave propagating in the
opposite direction has the same relationship at the reverse port; 10
dB down and 0 degrees phase with respect to point A.

A to Reverse and B to Forward -might- track reasonably well in both
magnitude and phase, but in this case, it's immaterial.

Because B-A and C-B 0 there will be a frequency dependent phase
difference between A, B and C.

When we calibrate using a short on the load port here's what happens.

The signal at the forward port becomes the reference, i.e., unity
amplitude and 0 degrees phase.

The short creates a 100% reflection and -180 degree phase shift. This
signal propagates back down the main line to the source, which is
assumed to be a perfect match, so there is no re-reflection. A -10 dB
sample (by definition: unity) is coupled to the reverse port, with a
phase shift, theta(F), determined by the electrical length of the line
C - B - A.

Unless we are lucky enough to be Lotto winners, the signal at the
reflected port -will not- be 1 @ ang-180 deg. So our calibration
routine must do whatever math is necessary to make the ratio B/A = 1 @
ang-180. This fudge factor is then applied to all subsequent
measurements to "correct" the data.

Now to address (I think) your question. If we change frequencies,
theta(F) changes and the fudge factor no longer corrects for it.
While the coupling factors might track, it is of little consolation
because the calibration is good only at the frequency where it was
performed. Automatic network analyzers perform calibration at each
test frequency, or at least enough points to interpolate between.

Wes,

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected signals
changed by the same about?

Thanks - Dan kb0qil


Wes Stewart wrote:
On Sat, 03 Dec 2005 17:33:18 -0800, dansawyeror
wrote:


All,

I am trying to measure antenna impedance. For this I intend to us a directional
coupler to isolate reflected signal. After using the coupler for a while I
believe that it introduces a phase shift, that shift seems to be related to
frequency. This creates a bit of a catch 22. Antenna resonance is defined as the
frequency where there is no reflected complex component. If the tool to measure
this is also frequency dependent how can this be accomplished? Is this even the
best method?


This depends a lot on what instrument you are connecting to this
coupler. If it's nothing more than a power sensor, then you are
making scalar measurements and phase is meaningless.

You would calibrate by placing a short on the measurement (antenna)
port and getting a 100% reflection reference (rho=1). You would
determine the magnitude of the reflection coefficient by ratioing this
to the measured value.

If you have a magnitude and phase sensitive instrument (vector
analyzer) then, as others have answered, you calibrate with additional
reference standards. In any event, the phase shift through the
coupler is compensated for by the calibration process.


Do bi-directional couplers automatically compensate for frequency shift?



No. The provide for a simultaneous sample of the forward and
reflected signals.

Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil

The phases seen at each coupled port should be identical to the phase of the
forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler directivity,
and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the standards
do not require quantification by the use of "Standard definitions" -- see
www.vnahelp.com.


Regards,

Frank

Frank,

The bi-directional coupler is a machined block about 1 x 3 x 5. The inside is a
straight through line, the pickups are simply terminated one loop lines. It is a
UHF coupler that works reasonably down to 2 meters. When I configure this to
look at the forward and reflected 'open' circuit case they are not in phase.
Reflected lags forward by about 40 degrees. (I checked the connection delay and
this is not a cable issue.) This is frequency independent. Shorting the output
reverses this relationship. The outputs are terminated in 50 Ohms so I conclude
it is a 50 Ohm device. When I terminate the device in 50 Ohms the forward and
reflected outputs are out of phase by about 140 degrees.

What is the significance a non frequency dependent phase shift between forward
and reflected? This shift is frequency independent.

Thanks - Dan kb0qil


Frank wrote:
Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil


The phases seen at each coupled port should be identical to the phase of the
forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler directivity,
and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the standards
do not require quantification by the use of "Standard definitions" -- see
www.vnahelp.com.


Regards,

Frank

Dan,

your original posting says the shift you are getting is frequency dependent.
Your last posting says it is not. Which one I read wrong?

Thks
Ivan

"dansawyeror" wrote in message
...
Frank,

The bi-directional coupler is a machined block about 1 x 3 x 5. The inside
is a
straight through line, the pickups are simply terminated one loop lines.
It is a
UHF coupler that works reasonably down to 2 meters. When I configure this
to
look at the forward and reflected 'open' circuit case they are not in
phase.
Reflected lags forward by about 40 degrees. (I checked the connection
delay and
this is not a cable issue.) This is frequency independent. Shorting the
output
reverses this relationship. The outputs are terminated in 50 Ohms so I
conclude
it is a 50 Ohm device. When I terminate the device in 50 Ohms the forward
and
reflected outputs are out of phase by about 140 degrees.

What is the significance a non frequency dependent phase shift between
forward
and reflected? This shift is frequency independent.

Thanks - Dan kb0qil


Frank wrote:
Your answer to the question about bidirectional couplers was they do not
compensate for phase shift. Let me ask it again:

Do the measuring ports of a bi-directional coupler accurately represent
or
preserve the relative phases of the signal?

To put it another way is the phase shift of the driving and reflected
signals changed by the same about?

Thanks - Dan kb0qil


The phases seen at each coupled port should be identical to the phase of
the
forward and reflected signals. This is easily verifiable, and frequency
independant, as follows:

No load -- forward and reverse amplitudes equal, and in phase;
Short circuit at output -- forward and reverse amplitudes equal, and 180
degrees phase difference;
50 ohm load -- reverse than forward by = specified coupler
directivity,
and phase difference can 0 theta +/ 180.

This is only true if the frequencies are low enough such that the
standards
do not require quantification by the use of "Standard definitions" --
see
www.vnahelp.com.


Regards,

Frank