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Measurement parameters and high VSWR measurement

Short answer

Major parameters: power, frequency, VSWR/Γ, impedance, attenuation, S‑parameters, Q, noise figure. For VSWR > 10, the double‑minimum method: measure guide wavelength λg from successive minima; locate the voltage minimum, find positions d1 and d2 where power doubles (3 dB up), then S ≈ λg/[π(d2−d1)].

List out the major RF/MW measurement parameters. How the VSWR of any microwave transmitter (In case of VSWR > 10) can be measured? Explain.

RF/MW measurement is mainly based on power and wave patterns because voltage and current cannot be measured directly at these frequencies.

Major RF/MW measurement parameters

  1. Power (low, medium, high; average and peak)
  2. Frequency and guide wavelength (λg)
  3. VSWR and reflection coefficient (Γ)
  4. Impedance (load impedance, using Smith chart)
  5. Attenuation and insertion loss
  6. Scattering parameters (S11, S21, …)
  7. Q factor of cavities
  8. Noise figure, dielectric constant, phase shift

Measurement of high VSWR (S > 10): double minimum method

For high VSWR the minimum is very sharp and E_min is very small, so the direct method (E_max/E_min) is inaccurate: the crystal leaves its square-law region at E_max and the reading near the minimum is noisy. Instead the width of the minimum is measured.

Set-up:

Source -> Isolator -> Atten. -> Freq. -> Slotted -> DUT
(1 kHz AM)                      meter    line
                                          |
                                     probe+crystal
                                          |
                                      VSWR meter

Procedure:

  1. Find λg: λg = 2 × distance between two successive minima.
  2. Move the probe to a voltage minimum; adjust gain so the meter reads a convenient value (e.g. 3 dB on the dB scale).
  3. Move the probe to one side until the reading is twice the minimum power (3 dB higher, i.e. E = √2 E_min). Note position d1.
  4. Move to the other side of the minimum and find the same level; note d2.
  5. Calculate:
S = √[ 1 + 1 / sin²(π(d2 − d1)/λg) ]
For large S:  S ≈ λg / [π (d2 − d1)]

 |E|
  |\                 /
  | \               /
  |  \   √2Emin    /
  |---\-----------/---
  |    \_________/
  |   d1   Emin   d2
  +--------------------> position

Example: λg = 4 cm, d2 − d1 = 0.1 cm S ≈ 4 / (π × 0.1) = 12.73; exact formula gives 12.78.

Other method for high VSWR: the calibrated attenuator method – read the attenuator change (in dB) needed to give the same meter reading at E_max and E_min; S = 10^(dB/20).

✓ Answer checked by Claude Opus 5.5 (high effort)

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