---
title: "Power measurement tool for airport surveillance radar"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:airport-radar-power-measurement"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/airport-radar-power-measurement
site: BCT Engineering
---

# Power measurement tool for airport surveillance radar

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

- Asked in: 2074 Magh, 8 marks
- 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/airport-radar-power-measurement
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Choose a proper power measurement tool to measure power of an Airport Surveillance Radar.

## Short answer

For off-air average power, use a flow calorimeter; for online monitoring, use a calibrated directional coupler, attenuator, and thermistor power meter, then compute peak power from average and duty cycle.

## Answer

An **Airport Surveillance Radar (ASR)** is a high-power **pulsed** transmitter (S-band, about 2.7–2.9 GHz, peak power of the order of 1 MW, pulse width about 1 µs). A bolometer alone would burn out, so the proper choice is:

- **Flow (circulating) calorimeter wattmeter** with a water-cooled dummy load to measure the full **average power** when the radar is off-air, and
- in normal operation, a **calibrated directional coupler + attenuator + thermistor (bolometer) power meter** to sample a small known fraction of the power. **Peak power** is then found from average power and duty cycle (or with a peak-power diode sensor).

#### Set-up

```text
Magnetron/  -> Directional coupler -> Antenna or
klystron Tx    (e.g. 50 dB)           water load
                     |                (flow calori-
                 attenuator (10 dB)    meter)
                     |
               thermistor mount -> power meter
```

#### Working principle
1. **Flow calorimeter:** water flows through a matched dummy load at a known rate v. All RF power turns into heat:

```text
P_avg = ρ · v · c · ΔT  ≈ 4.187 · v(cm³/s) · ΔT  W
```

2. **Coupler method:** the coupler sends a fixed fraction of power (−50 dB) to the side arm; an attenuator brings it into the bolometer range (below 10 mW). The thermistor bridge reads P_meter, and:

```text
P_avg(dBm) = P_meter(dBm) + coupling(dB)
             + attenuation(dB)
```

3. **Peak power** of rectangular pulses:

```text
Duty cycle D = τ × PRF
P_peak = P_avg / D
```

#### Example
Assume P_peak = 1 MW, τ = 1 µs, PRF = 1000 Hz.
- D = 1×10⁻⁶ × 1000 = 0.001
- P_avg = 1 MW × 0.001 = 1 kW
- With 50 dB coupler + 10 dB attenuator (60 dB total): power at the meter = 1000 W × 10⁻⁶ = 1 mW, which suits a thermistor mount.
- In the flow calorimeter, 1 kW with ΔT = 10 °C needs a flow of 1000/(4.186 × 10) ≈ 23.9 cm³/s of water.

#### Reasons for this choice
- Calorimeter handles kW average power and is absolute and broadband.
- The coupler method allows monitoring while the radar operates.
- Bolometer and diode sensors are only for mW levels, so they must be used behind a coupler and attenuator.

## Check yourself

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

1. Why can a bolometer not be connected directly to a radar transmitter?
   A. Its frequency range is too low
   B. It would burn out due to the very high power
   C. It measures only peak power
   D. It needs no matching
   Answer: B. A bolometer handles only about 10 mW, whereas the radar has about 1 MW peak power. (Key point: Why not bolometer alone)

2. What is the role of the directional coupler plus attenuator?
   A. To increase the power
   B. To measure frequency
   C. To sample a small known fraction into the bolometer's range
   D. To pulse the transmitter
   Answer: C. Coupling and attenuation (60 dB total in the example) bring kW down to mW, allowing monitoring during operation. (Key point: Coupler method)

3. Radar has tau = 1 µs, PRF = 1000 Hz and P_avg = 1 kW. What is P_peak?
   A. 1 kW
   B. 1 MW
   C. 1 GW
   D. 1 W
   Answer: B. D = 0.001, so P_peak = P_avg / D = 1 kW / 0.001 = 1 MW. (Key point: Peak power from average)

4. With a 50 dB coupler and 10 dB attenuator, what power reaches the meter from 1000 W average?
   A. 1 W
   B. 10 mW
   C. 100 µW
   D. 1 mW
   Answer: D. 60 dB is a factor of 10^6, so 1000 W becomes 1 mW. (Key point: Coupler budget)

## Related in RF and Microwave Engineering

- [Duplexer for half-power airport surveillance radar](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aradar-duplexer-smatrix) — RF and Microwave Engineering, chapter 4
- [Measurement tool to test an antenna as DUT](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aantenna-dut-measurement-tool) — RF and Microwave Engineering, chapter 8
- [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