Exploiting your antenna analyser #11

Backing out transmission line

Often we make measurements through a section of transmission line, and the measurements are wrt the reference plane, which for many analysers is the connector on the instrument.

Some analysers, or their associated software allow the effects of the transmission line to be backed out.

Screenshot - 17_01_16 , 09_16_18

The actual length could be some integral number of halfwaves greater.

Screenshot - 17_01_16 , 09_15_56

Above is the result after tweaking the cable distance for a plot that has very slowly increasing R about the point where X passes through zero, the cable length was in fact 7.7m, which reconciles with the measured length of cable.

Screenshot - 17_01_16 , 09_28_44

Above, the R,X plot adjusted to the feed point shows the classic behaviour of this type of antenna (forget the |Z| plot, it is meaningless and I cannot disable it).

It was convenient that Antscope showed the angle of Γ on the chart. If your system does not do that, you could use Calculate VSWR and Return Loss from Zload and Zo to calculate the value from R,X.

Screenshot - 17_01_16 , 09_34_26

Above is the angle -15.1° calculated from R,X shown on the first Smith chart.

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