Is the characteristic impedance of the sampler line section important?
The sampler line section behaves like any transmission line, and may transform impedance. Quality instruments will usually have a sampler line section of exactly the same Zo as the VSWR calibration base so that impedance transformation caused by the sampler line is minimised. However, the detectors may be calibrated (nulled) at some Z that is different to the sampler section line Zo. If the sampler line section is very short, the impedance transformation will be very small. For example, consider a HF sampler line section of length 20mm and Zo=75+j0 calibrated for reflected null at 50+j0Ω, the insertion VSWR in a 50Ω system is around 1.02 which is comparable to the error using a true 50+j0Ω sampler section in RG58C/U nominal 50Ω cable at low HF (ignoring cable tolerance errors).
