The HP 478A Thermistor Mount is a coaxial power measurement device engineered for use with HP 431, 432, and N432A RF power meters. Operating across 10 MHz to 10 GHz, it measures microwave power from 1 µW to 10 mW with 50 ohm impedance and 200 ohm ±1% operating resistance. The mount employs two series pairs of matched thermistors to eliminate drift caused by ambient temperature fluctuations. Thermal stability is achieved by mounting thermistor leads on a common thermal conductor, thermally isolated from the mount body, which shields the sensing elements from external thermal disturbances and enables accurate microwatt-level measurements.
Technical Specifications
• Frequency Range: 10 MHz to 10 GHz
• Power Range: 1 µW to 10 mW (minimum measurable power: 1 µW)
• Maximum Average Power: 30 mW
• Maximum Peak Power: 200 W
• Maximum Energy per Pulse: 10 µs (PRF ≥ 1 kHz); 5 µs (PRF 95% (98% actual) from 5 MHz to 1 GHz; marked on instrument label with NIST traceability
• RF Connector: Type-N male (MIL-C-71 or MIL-C-39012 compatible)
• Weight: 5 oz (140 g)
– Key Features
• Dual-pair matched thermistor design for drift compensation across temperature variations
• Thermally isolated lead configuration minimizes environmental thermal noise
• Field-replaceable thermistor assembly
• Low reflection coefficient across operating band (max return loss 11.3 dB at 10 MHz–25 MHz)
– Compatibility & Integration
The 478A connects directly to HP Model 431 and 432 power meters via dedicated bridge connectors. Option H75 variant is designed for 432 Power Meter operation only and is incompatible with the 431.
