Device to measure 7.5 mW microwave power

7.5 mW is a low microwave power:

P (dBm) = 10 log10(7.5 mW / 1 mW) = 8.75 dBm

This is inside the range of a thermistor (bolometer) power meter, about 1 µW–10 mW. A diode sensor is not suitable, because it is square law only below about −20 dBm, and a calorimeter is too slow and insensitive at mW level. So the proper device is a thermistor mount with a self-balancing (double) bolometer bridge – the DC substitution bolometer method.

Bolometer element

• Thermistor: semiconductor bead, negative temperature coefficient, robust – chosen here.
• Barretter: fine platinum wire, positive temperature coefficient, more delicate.

The thermistor sits in a matched waveguide/coax mount that absorbs all the incident power.

Basic bridge (DC substitution)

          +---R1---+---R2---+
          |        |        |
 DC bias -+        G        +-
          |        |        |
          +---R3---+--Rth---+
                       ^ 7.5 mW RF

1. With no RF, DC bias current I1 heats the thermistor until it reaches the balance resistance R (e.g. 200 Ω); G reads zero.
2. RF power heats the thermistor more; R changes and the bridge unbalances.
3. DC is reduced to I2 to restore balance. The thermistor is at the same temperature, so

P_RF = (I1² − I2²) · R

Practical self-balancing double bridge

To avoid drift with room temperature and manual balancing:

1. Two matched thermistors share one thermal block. One gets RF (RF bridge), the other only sees ambient temperature (compensating bridge).
2. Feedback amplifiers keep both bridges balanced automatically, with bridge voltages V_rf and V_c.
3. The meter shows

P_RF = (V_c² − V_rf²) / (4R)

Temperature drift changes both channels equally and cancels.

Practical precautions for 7.5 mW

• Zero the meter with RF off.
• Use a well-matched mount (low VSWR) to avoid mismatch error, and apply the mount calibration factor.
• 7.5 mW is close to the 10 mW upper limit; if the level may rise, add a 10 dB attenuator (reading becomes 0.75 mW) and add 10 dB back.