How to determine the measurement plane?

[kdavis]

I have access to a lab grade VNA at work. After I calibrate the instrument with an attached coaxial test lead using the "Open, Short, Load" process, there is a "Port Extension" feature which moves the measurement plane to the end of the test lead.  This "Port Extension" procedure is required to position an "Open" and a "Short" at their proper positions on the Smith Chart display.
It seems to me that the "OSL" calibration SHOULD have already positioned the measurement plane at the end of the test lead cable.

Questions:

1. Why is the "Port Extension" needed when the OSL calibration was performed at the end of the test lead?

2. Does the AIM 4170 have some sort of "Port Extension" feature or is it not necessary?

3. If it is not necessary for the 4170, what is the difference in the method between the two instruments?

[Bob]

The OSL loads are normally made such that their effective measurement planes are at the essentially same distance. In the UHF region an error of even a fraction of a millimeter may be noticeable. It's the phase shift that matters. In the gigahertz range, a few degrees of phase is a very short distance.

Ideal loads can only be approximated by real world loads, so high quality cal loads for the multi-GHZ range include a set of tables that have corrections as a function of frequency.

Sometimes an adapter may be added to the test setup. Then the port extension can be used to fine-tune the calibration without having to go back through the whole calibration procedure.

The AIM program does have a port extension option. This can be a positive or a negative number.
The impedance and velocity factor of the port extension can be specified. The loss of the port extension is assumed to be zero.
The use of the port extension is normally limited to a few centimeters.

The AIM also has a function called "Custom Calibration" that makes it easy to calibrate at the end of an arbitrarily long transmission line. This function takes into account: cable length, impedance, velocity factor and loss automatically. This can be used to calibrate long leads that are used to connect to components for measurements. It's very handy for calibrating long transmission lines so the impedance at the driving point of an antenna can be measured directly.   Custom Cal also compensates for filters that may be inserted in the signal path.

--73/Bob