HP / Agilent 8478B Coaxial Thermistor Mount 10 MHz to 18 GHz

The HP/Agilent 8478B is a precision coaxial thermistor mount for microwave power measurement from 10 MHz to 18 GHz. It terminates a 50-ohm transmission line with a thermally balanced thermistor assembly - two matched series pairs designed to cancel ambient temperature drift and minimize thermoelectric error. This architecture enables measurement of power levels as low as 1 µW with exceptional stability, even under thermal shock.

The mount delivers calibration factor and effective efficiency data at selected frequencies, traceable to NIST, and integrates with HP/Agilent 432A, 431C, 431A/B, 430 Series, and N432A power meters.

Technical Specifications

• Frequency Range: 10 MHz to 18 GHz


• Power Measurement Range: 1 µW to 10 mW (with Model 432 Power Meter)


• Input Impedance: 50 ohms


• Operating Resistance: 200 ohms, unbalanced


• Maximum Peak Power: 200 W


• Maximum Average Power: 30 mW


• Maximum Energy per Pulse: 10 W-µs (PRF ≥ 1 kHz); 5 W-µs (PRF < 1 kHz)


• Return Loss/SWR: - 10–30 MHz: ≤ 1.75 SWR (11.3 dB) - 30–100 MHz: ≤ 1.35 SWR (16.5 dB) - 100 MHz–1 GHz: ≤ 1.10 SWR (26.4 dB) - 1–12.4 GHz: ≤ 1.35 SWR (16.5 dB) - 12.4–18 GHz: ≤ 1.60 SWR (12.8 dB)


• RF Connector: Type N male (APC-7 optional)


• Weight: 140 g

Key Features

• Thermally balanced dual-pair thermistor assembly with matched leads on common thermal conductor


• Temperature-compensated design for low drift and minimal thermoelectric error


• Field-adjustable zero-set restoration capability post-overload


• NIST-traceable calibration data provided at selected frequencies

Typical Applications Sensitive RF power measurement in test, calibration, and standards laboratories where low-level microwave power detection and thermal stability are critical.

Compatibility & Integration HP/Agilent 432A, 431C, 431A/B, 430 Series, and Agilent N432A power meters.