---
title: "Working principle of bolometry"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:bolometry-working-principle"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/bolometry-working-principle
site: BCT Engineering
---

# Working principle of bolometry

RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements

- Asked in: 2080 Chaitra, 5 marks; 2079 Chaitra, 10 marks (asked 2 times)
- Video: [Bolometer | Power measurement |  Microwave Engineering | Lec-118](https://www.youtube.com/watch?v=kCT40ZZ3jd0) — Education 4u
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/bolometry-working-principle
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Explain the working principle of bolometry used in microwave measurements.

## Short answer

Bolometry measures microwave power by resistance change of a barretter or thermistor absorbing RF in a Wheatstone bridge. DC bias is reduced after RF is applied until the bridge rebalances; P_RF = (I1^2 - I2^2) R. It covers about 1 uW to 10 mW with substitution accuracy.

## Answer

**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

| Type | Material | Temperature coefficient | Typical R |
|---|---|---|---|
| **Barretter** | Thin Pt/W wire | Positive (R rises) | ~200 Ω |
| **Thermistor** | Semiconductor bead | Negative (R falls) | ~100–200 Ω |

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

#### Working principle (balanced bridge)

```text
          +----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:

```text
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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a bolometer measure microwave power?
   A. By the change in resistance of a temperature-sensitive element heated by the absorbed power, compared with an equal DC heating (substitution)
   B. By counting photons
   C. By measuring the voltage standing wave ratio only
   D. By the magnetic field of the wave
   Answer: A. RF heating changes the element's resistance; DC bias is adjusted to restore it so the RF power equals the DC power change. (Key point: Principle of bolometry)

2. How do a barretter and a thermistor differ?
   A. Barretter: Pt/W wire with positive coefficient and fragile; thermistor: semiconductor bead with negative coefficient and more rugged
   B. Barretter: negative coefficient bead; thermistor: positive wire
   C. Both are negative coefficient wires
   D. Barretter is more rugged and preferred
   Answer: A. A barretter's R rises with temperature but it burns out easily; a thermistor's R falls and it is preferred for ruggedness. (Key point: Barretter vs thermistor)

3. In the balanced bridge, how is RF power calculated?
   A. P_RF = (I1^2 - I2^2) Rb, the reduction in DC power needed to rebalance
   B. P_RF = (I1 + I2) Rb
   C. P_RF = I1^2 Rb only
   D. P_RF = (I2^2 - I1^2) Rb
   Answer: A. The RF heating replaces part of the DC heating, so the DC power drop from P1 to P2 equals the RF power. (Key point: Power from bridge balance)

4. With Rb = 200 ohm, I1 = 10 mA and I2 = 8 mA, what is the RF power?
   A. 7.2 mW
   B. 0.72 mW
   C. 4 mW
   D. 72 mW
   Answer: A. (1x10^-4 - 0.64x10^-4) x 200 = 7.2 mW. (Key point: Numerical example)

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