The sign of reactance
At Exploiting your antenna analyser #2, the matter of determining sign of reactance was mentioned.
If you have an analyser that does not measure the sign of reactance, this information should be important to you.
Above is a Smith chart plot of measurements from 15MHz to 25MHz.
One can see that the locus of Zin on the Smith chart forms an arc, and the points on the arc rotate clockwise about the arc centre with increasing frequency.
(In the general case, the locus may be a quite complicated spiral from end to end, but in any small neighbourhood, the locus tends to an arc, clockwise about the arc centre as frequency increases.)
Were the three points taken to be and plotted as +ve X values, we would get the following chart.
The points would not rotate clockwise about the arc centre with increasing frequency, and that suggests that they are not the response of a real system… that the sign of one or more points is probably wrong.
There is deeper discussion of this issue at The sign of reactance.