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

Above is a Smith chart view of measurement of a test antenna through some length of RG58. The antenna will have R<50Ω at minimum VSWR, so the angle of the complex reflection coefficient Γ will be close to 180° at the feed point. Antscope uses a different notation, but shows here the angle at the point of measurement to be -15.1°, so we need to increase it by 180–15.1=195.1°, which will take about half that electrical length of line, 97.6°. From TLLC, I calculate the length involved is 7.6m of RG58, which is an estimate that gives a starting point for backing out the cable.

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.