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