Microwave vs low frequency measurement; static calorimeter

Microwave measurements differ from low-frequency ones because at microwave frequencies the wavelength is comparable to the circuit size, so circuits behave as distributed systems and voltage and current lose their simple meaning.

Differences from low-frequency measurements

Reasons: lead lengths are a large fraction of a wavelength, the meters' own capacitance and inductance load the circuit, and electron transit time limits ordinary devices.

Static calorimeter for power measurement

A calorimeter turns all the microwave power into heat in a matched load and measures the temperature rise. In the static type, the absorbing medium does not flow.

   RF in
 =====+=====================
      |  insulated vessel   |
      |   +-------------+   |
      |   | water/oil   |   |-- thermometer /
      |   | (mass m)    |   |   thermocouple
      |   +-------------+   |
       =====================

Working:

1. A fixed mass m of liquid (or a solid absorber) forms a matched load at the end of the waveguide, inside a thermally insulated vessel.
2. RF power is applied for a time t; the load absorbs it and heats up.
3. The temperature rise ΔT is measured with a thermometer, thermocouple or thermopile.
4. Since heat gained = P × t = m c ΔT,

P = m · c · ΔT / t

Example: m = 0.2 kg water, c = 4186 J/kg·°C, ΔT = 5 °C in t = 120 s P = 0.2 × 4186 × 5 / 120 ≈ 34.9 W.

5. For better accuracy, a known DC heater is placed in the same load and its power is adjusted to give the same temperature rise (DC substitution); then P_RF = P_DC. This cancels heat-loss and heat-capacity errors.

Features: an absolute method used for medium to high power; simple, but slow, and it needs good thermal insulation.