De-embedding transmission line to allow remote measurement

Recent articles An interesting case study of measurement of a balun's Insertion VSWR and Another measurement of a balun's Insertion VSWR made measurements through a transmission line of the load on a balun.

In the first article, the measurements at the input of around 7m of 50Ω line were adjusted to move the reference plane to the load end of the coax using the add/subtract cable feature of Antscope to de-embed the transmission line.

The second article used a FA-VA5 analyser and VNWA software to make the measurements and to some extent, de-embed the transmission line. In this case the transmission line was quite short at 370mm, and whilst the facility adjusted for propagation time, it did not adjust for attenuation though that was very small in this case and of little consequence. The FA-VA5 analyser and VNWA software combination would not suit the scenario in the first article as will be demonstrated.

This article examines the response to a 6m length of RG58 with O/C load at 30MHz.

We can see that although the phase of Gamma (phase of 0.85+j0.01) is close to zero the magnitude is 0.85 when the magnitude should be 1.00 for an O/C load.

The reason for this is that the port extension facility does not de-embed the attenuation of the line.

I can use TLLC to calculate the ratio of Vout/Vin for that scenario as 1.182e+0∠2.3°, so |Vin/Vout|=0.868 which reconciles well with the measured |Gamma|of 0.85.

In this case the attenuation is significant, and the port extension facility falls short in modelling the effect.

By contrast, Rigexpert's Antscope use for the first article does include attenuation in the cable add/subtract facility and gives acceptable results.

Antscope's cable add/subtract facility which includes a database of common cable parameters provides a convenient means of backing out the transmission line when measurements are made remotely, and whilst dependent on the accuracy of the line characteristics, it is a very useful facility not found universally.