Exploiting your antenna analyser #3

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.

Screenshot - 9_12_2015 , 8_24_15 AM

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.

Screenshot - 9_12_2015 , 8_27_17 AM

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.

Watch the blog for continuing postings in the series Exploiting your antenna analyser. See also Exploiting your antenna analyser – contents.