Working principle of bolometry

Bolometry measures microwave power by the change in resistance of a small temperature-sensitive resistor (bolometer) when it absorbs the microwave power. It is a substitution method: the RF heating is compared with an equal amount of DC (or low-frequency) heating.

Bolometer elements

Barretters are fast but fragile and burn out easily; thermistors are more rugged and are preferred.

Working principle (balanced bridge)

          +----R1----+----R2----+
          |          |          |
   DC  ---+        [ G ]        +---
  bias    |          |          |
          +----R3----+---[Rb]---+
                          |
                   bolometer in
                   waveguide mount
                   (RF input)

1. The bolometer is one arm of a Wheatstone bridge and sits in a matched waveguide or coaxial mount that absorbs the RF power.
2. With no RF, DC bias current is adjusted until the bolometer reaches the balance resistance (e.g. 200 Ω) and the galvanometer reads zero. DC power P1 = I1²Rb.
3. RF power is applied. The bolometer heats and its resistance changes, unbalancing the bridge.
4. The DC bias is reduced until the bridge balances again (same Rb, same temperature). DC power P2 = I2²Rb.
5. The RF power equals the reduction in DC power:

P_RF = P1 − P2 = (I1² − I2²)·Rb

Example: Rb = 200 Ω, I1 = 10 mA, I2 = 8 mA → P_RF = (1×10⁻⁴ − 0.64×10⁻⁴)×200 = 7.2 mW.

Self-balancing bridge

In practical power meters, an amplifier automatically adjusts the bias to keep the bridge balanced. A second, identical thermistor (not exposed to RF) in another bridge compensates for ambient temperature changes.

Features

• Range: about 1 µW to 10 mW (low power); higher power is measured with a calibrated attenuator or directional coupler in front.
• Accuracy: about ±1–5 %; errors come from mount mismatch, mount losses (mount efficiency) and DC–RF substitution error.
• Slow response (thermal), so it measures average power.