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VSWR meter and low power (bolometer) measurement

Short answer

Standing waves: a slotted-line probe with crystal detector feeds a VSWR meter. For low VSWR, it directly reads E_max/E_min; for high VSWR, the double-minimum width gives S≈λg/[π(d2−d1)]. Low power: a bolometer (barretter/thermistor) in a bridge; RF heating unbalances it, and DC is reduced to rebalance, yielding P_RF=(I1²−I2²)R.

Describe how standing waves and microwave powers are measured with VSWR meter and bolometry (low power measurement) respectively.

The VSWR meter with a slotted line measures the standing-wave ratio on a line, and the bolometer bridge measures low microwave power (below about 10 mW).

Measuring standing waves with a VSWR meter

Set-up:

Klystron -> Isolator -> Var. -> Freq. -> Slotted -> Load
(1 kHz AM)              atten.   meter    line
                                           |
                                    probe + crystal
                                           |
                                       VSWR meter
  • The source is square-wave modulated at 1 kHz.
  • A probe moves along the slotted line and samples the field. A crystal detector (square law, V ∝ E²) recovers the 1 kHz envelope.
  • The VSWR meter is a high-gain amplifier tuned to 1 kHz, with a meter calibrated in VSWR and dB for a square-law detector.

Steps (low VSWR, S < 10, direct method):

  1. Move the probe to a voltage maximum; adjust the gain so the meter reads full scale (VSWR = 1.0).
  2. Move the probe to the next minimum; read the VSWR directly from the scale.
  3. S = E_max / E_min. Also |Γ| = (S − 1)/(S + 1).
  4. Twice the distance between two successive minima gives λg.

For high VSWR (S > 10), the double minimum method is used: find the two positions d1, d2 on either side of the minimum where the power is twice the minimum (3 dB up). Then S ≈ λg / [π(d2 − d1)].

Low power measurement by bolometry

A bolometer is a temperature-sensitive resistor: a barretter (platinum wire, R rises with power) or a thermistor (semiconductor, R falls with power). It is placed in a mount at the end of the line and forms one arm of a Wheatstone bridge.

        +----R1----+----R2----+
        |          |          |
  DC  --+          G          +-- 
 bias   |          |          |
        +----R3----+--Bolom.--+
                       ^ RF power

Working (DC substitution):

  1. With no RF, DC bias current I1 heats the bolometer to the balance resistance R; the galvanometer G reads zero.
  2. RF is applied. It also heats the bolometer, so its resistance changes and the bridge becomes unbalanced.
  3. The DC current is reduced to I2 until balance returns. The bolometer is again at the same R and temperature, so the RF power has replaced the DC power removed:
P_RF = (I1² − I2²) · R

Example: R = 200 Ω, I1 = 8 mA, I2 = 5 mA P_RF = (64 − 25)×10⁻⁶ × 200 = 7.8 mW.

Limitations: ambient temperature drift (solved by a compensating bridge), low power only, and mismatch errors.

✓ Answer checked by Claude Opus 5.5 (high effort)

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