---
title: "Measurement parameters and low power device"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:low-power-measurement-device"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/low-power-measurement-device
site: BCT Engineering
---

# Measurement parameters and low power device

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

- Asked in: 2073 Bhadra, 8 marks
- Video: [Introduction To Power Meter - Microwave Measurement - Microwave Engineering](https://www.youtube.com/watch?v=iAO-FJMqo5w) — Ekeeda
- 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/low-power-measurement-device
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Define major microwave measurement parameters and explain the working principle of a low microwave power measurement device.

## Short answer

Major parameters: power, VSWR, reflection coefficient, S-parameters, impedance, Q factor, noise figure. A bolometer bridge measures low power by DC substitution: RF heats a thermistor, then DC is reduced to rebalance; P_RF = (I1² − I2²)R.

## Answer

Microwave measurement parameters are the quantities used to describe signals and components at microwave frequencies, where power and wave patterns are measured instead of voltage and current.

#### Major microwave measurement parameters
- **Power:** average or peak power delivered to a matched load (W, dBm).
- **Frequency / wavelength:** source frequency f and guide wavelength λg.
- **VSWR:** S = V_max / V_min on the line; shows mismatch.
- **Reflection coefficient:** Γ = (Z_L − Z0)/(Z_L + Z0); |Γ| = (S − 1)/(S + 1).
- **Impedance:** load impedance found from VSWR and position of minimum.
- **Attenuation / insertion loss:** power loss through a component in dB.
- **S-parameters:** reflection and transmission ratios of a network.
- **Q factor:** stored energy / energy lost per cycle in a cavity.
- **Noise figure:** degradation of SNR by a device.

#### Low power measurement device: the bolometer bridge
Low power (below about 10 mW) is measured with a **bolometer** – a temperature-sensitive resistor – in a bridge.

Types of bolometer:
- **Barretter:** thin platinum wire; positive temperature coefficient; sensitive but burns out easily.
- **Thermistor:** semiconductor bead; negative temperature coefficient; robust, commonly used.

```text
          +---R1---+---R2---+
          |        |        |
 DC bias -+        G        +-
          |        |        |
          +---R3---+--Rb----+
                       ^
                       | microwave power
                  bolometer mount
```

Working principle (DC substitution):
1. With no RF, DC bias current I1 through the bolometer Rb heats it until Rb equals the value needed for balance (R1/R2 = R3/Rb); the galvanometer G reads zero.
2. The microwave power is applied to the bolometer mount. It heats the bolometer further, its resistance changes, and the bridge unbalances.
3. The DC bias is reduced to I2 until balance returns. Now Rb is again at the same value and temperature, so the drop in DC power equals the RF power:

```text
P_RF = (I1² − I2²) · Rb
```

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

To remove ambient temperature drift, practical meters use a **self-balancing double bridge** with a second compensating thermistor: P = (V_c² − V_rf²)/(4R).

Other low-power sensors: **thermocouple** sensors (about 1 µW–100 mW) and **Schottky diode** sensors (down to about 100 pW, square law below about −20 dBm).

## Check yourself

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

1. Why are power and wave patterns measured instead of voltage and current at microwave frequencies?
   A. Voltage and current are not uniquely defined, e.g. in waveguides
   B. Meters are too cheap
   C. Frequency is too low
   D. Power is easier to generate
   Answer: A. At microwave frequencies V and I cannot be measured directly, so power and standing-wave quantities are used. (Key point: Why power is measured)

2. At balance of the bolometer bridge, what holds?
   A. Rb = 0
   B. R1/R2 = R3/Rb, galvanometer reads zero
   C. R1 = R2 = open
   D. I1 = I2
   Answer: B. A Wheatstone bridge balances when R1/R2 = R3/Rb, giving zero galvanometer current. (Key point: Bridge balance condition)

3. Rb = 200 Ω, I1 = 8 mA and I2 = 5 mA. What is P_RF?
   A. 12.8 mW
   B. 5 mW
   C. 7.8 mW
   D. 3 mW
   Answer: C. P = (I1^2 - I2^2)Rb = (64 - 25)e-6 x 200 = 7.8 mW. (Key point: Power calculation)

4. Which statement is correct?
   A. Thermistor: platinum wire, burns out easily
   B. Barretter: semiconductor bead, negative coefficient
   C. Both are robust
   D. Barretter: platinum wire, positive coefficient, burns out easily; thermistor: robust, negative coefficient
   Answer: D. The barretter is sensitive but delicate; the thermistor is robust and commonly used. (Key point: Barretter vs thermistor)

## Related in RF and Microwave Engineering

- [Device to measure 7.5 mW microwave power](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Ameasure-7-5mw-power) — RF and Microwave Engineering, chapter 8
- [Microwave vs low frequency measurement; static calorimeter](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Ameasurement-differences-static-calorimeter) — RF and Microwave Engineering, chapter 8
- [VSWR meter and low power (bolometer) measurement](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Avswr-meter-power-measurement) — RF and Microwave Engineering, chapter 8, asked 2×
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- [Behavior of systems at microwave vs low frequency](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Amicrowave-vs-low-frequency-behavior) — RF and Microwave Engineering, chapter 1, asked 3×
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