Antenne analysis and matching.
Updated: May 30, 2020
How to measure antenna impedance?
Different types of VNA can be used. When using a professional VNA (Agilent, R/S,…) the measuring cable between the VNA and the antenna can be calibrated out.
With Ham equipment (MFJ,RigExpert,miniVNA, nanovna,…) this is not always the case.
Therefor, you can use after calibrating your VNA a coax line which has an electrical length of the wavelength / 2 or a multiple of this length.
Mechanical length is electrical length multiplied by the Vf (0.66 for RG213, 0.86 for Hyperflex13,….)
When using a half wavelength (or multiple) you will see exact the same impedance at the end of the cable or at the beginning.
In other words, you will see the antenna impedance at the end of your measuring cable.
Just one warning: Be sure to avoid common mode current. This can simply been done by rolling up de coax on a diameter of 30 cm. 5 turns is enough for 14 MHz use.
Example: Antenna to be measured at 14.250 MHZ:
Wavelength is 300/14 = 21,05 meter. Wavelength / 2 = 10,52 meter.
When using RG213 (Vf = 0.66)
Mechanical length is 10,52 x 0.66 = 6.95 meter or a multiple of 6.95 meter.
Currently the Nanovna is becoming popular.