This is a follow up to a dialog several months back on impedance
measurements. The question relates to smith charts and an HP 8407a with a
polar display. I was under the impression that the polar display and the
smith chart were pretty closely related. In fact hp made smith chart
overlays for the screens.

The smith chart predicts a variable capacitor connected across a source
will be represented by an impedance semicircle in the capacitive domain,
one of the points on the semicircle passes through the 50 ohm center. I
would expect to be able to create this using the 8407.

The 8407 creates a circle around the outside edge of the display. It is a
circle, however it does not pass through the center.

Why are these different? What above is incorrect?

My first question would be: is the 8407 calibrated correctly?

When you apply a 50ohm load do you get close to a single spot in the centre
of the screen, and when you apply your calibrated Open & Shorts do you get
close to single points on the X axis on the outer ring of the Smith Chart?
If not then you need to re-calibrate your test set up.

It is a long time since I used an HP8407, but I do recall that they were a
pain to calibrate correctly, lacking the auto calibration routines of modern
VNA's.

73
Jeff