Power measurement tool for airport surveillance radar

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

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:

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:

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

3. Peak power of rectangular pulses:

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.