Roy,

Thank you. Yes, I understood both magnitude and phase are required for both.

The question arose after looking into the block diagram of an HP 4805a Impedance
Meter. It measures voltage and current as the basis for determining impedance.

My work with a vector voltmeter has been through using directional couplers and
measuring forward and reflected power.

My question is: Is there an inherent accuracy benefit to one measurement basis
over the other?

Or: Are current and voltage probes inherently easier to build and more accurate
then directional couplers?

- Dan


Roy Lewallen wrote:
dansawyeror wrote:

All,

What is the best way to base impedance measurements, by voltage and
current phase or by forward and reflected power?


Measurement or calculation of forward and reverse power don't give you
enough information to determine impedance. There are an infinite number
of impedances which result in any given set of forward and reverse powers.

Likewise, voltage and current phase don't provide enough information.
Again, there are an infinite number of impedances possible for any set
of voltage and current phases.

First I assume both methods provide the data to measure impedance.


No, neither one does. Among adequate sets of measurements which will
allow you to determine impedance are both magnitude and phase of both
voltage and current; or the ratio of voltage and current and the
difference between their phase angles; or the magnitude and angle of the
reflection coefficient.

It would seem that both suffer from the real effects of the taking the
measurement. Measuring voltage suffers from the effects of the
reflected signal, I would assume current does as well. Couplers have a
built in cross talk parameter.


It's much more difficult than most people realize to make accurate RF
measurements. Any measurements have numerous potential sources of error.
If you need accurate results, you should always measure a number of
known impedances approximately equal to the measurement being questioned
to verify that the system is sound. Measuring antennas brings the
additional problems of coupling among the antenna, test equipment, and
you; disturbances from other signals being received by the antenna;
common mode currents; and properties of connecting feedlines.

Roy Lewallen, W7EL