Bolometric Microwave Power Measurement In a microwave power meter using a bolometer, which physical quantity is intentionally varied with absorbed RF power to indicate power level?

Introduction / Context:
Bolometric power meters are classic instruments for measuring microwave power with high accuracy. They convert RF power into heat, then sense a resulting change in an electrical property to infer the incident power.

Given Data / Assumptions:

• Bolometer element (thermistor or barretter) is used as the sensing element.
• Absorbed RF power raises the temperature of the element.
• A bridge or bias circuit monitors the change to produce a calibrated reading.

Concept / Approach:

The defining characteristic of a bolometer is that its electrical resistance varies predictably with temperature. When RF power is absorbed, temperature increases and the resistance changes. By keeping the bridge in balance (or reading the resistance shift), one can determine the absorbed power. Therefore, the measured quantity that varies with power is resistance, not inductance or capacitance.

Step-by-Step Solution:

Verification / Alternative check:

Thermistor bolometers have negative temperature coefficient; barretter types have positive temperature coefficient. Both rely on resistance-temperature dependence.

Why Other Options Are Wrong:

Inductance/capacitance (A,C) are not the primary sensing mechanisms in bolometers; cavity dimension (D) belongs to resonant-cavity power methods; Q-factor (E) also refers to resonator-based measurements, not bolometric sensing.

Common Pitfalls:

Confusing bolometric methods with thermocouple or diode detectors; assuming frequency-selective cavities when the device is broadband absorptive.

Final Answer:

resistance with absorption of power