What a VNA measures
A vector network analyser sends a signal whose amplitude and phase it knows, and compares what comes back (S11, the reflection, on port 1) or what goes through (S21, the transmission, from port 1 to port 2). “Vector”: it measures the phase too, so the real and imaginary parts of the impedance, not just the SWR.
The game's formulas
- Reflection coefficient: Γ = (Z − Z0) / (Z + Z0), with Z0 = 50 Ω. |Γ| goes from 0 (matched) to 1 (everything comes back).
- SWR = (1 + |Γ|) / (1 − |Γ|); return loss = −20 log |Γ| dB; reflected power = |Γ|². SWR 2: |Γ| = 1/3, 9.5 dB, 11%.
- A line of length l turns Γ: Γ(l) = Γ e−2γl. Half a turn of the chart per quarter wave; the losses draw the point towards the centre.
- Coil and capacitor: X = 2πfL, so L = X / (2πf); X = −1 / (2πfC), so C = 1 / (2πf|X|). The Q factor = |X| / R.
- Antenna: around its resonance, a series RLC circuit, Z = R + jRQ (f/f0 − f0/f). A half-wave dipole is about 142.5 / f(MHz) metres in all, a quarter-wave vertical 71.25 / f.
- Series S21: a part Z between the two ports gives S21 = 2Z0 / (2Z0 + Z).
Calibration
Three known standards (OPEN, Γ = +1; SHORT, Γ = −1; LOAD, Γ = 0) are enough to work out a port's three errors: directivity, source match and frequency response, cable included. THRU and ISOLN correct the transmission, more simply than the reflection: CH1 only receives, so its response and the isolation are corrected. Calibrate at the cable's end, over the frequency range you will measure.
Take care Before connecting an outdoor antenna, discharge it to ground: static built up by the wind or a nearby storm can destroy a VNA's input. And never connect a transmitter to a VNA!