A standard Microwave Bench setup is used for most measurements. It includes a signal generator (Klystron or Gunn oscillator), attenuators, an isolator, a frequency meter, a slotted line with a probe, a detector, and a VSWR meter or CRO.

5.1. Key Measurement Procedures

• Power Measurement:
  • Low Power (0.01-10mW): A Bolometer (using a thermistor or barretter) in a bridge circuit measures power by detecting resistance changes due to heating.
  • Medium Power (10mW-1W): Calorimeter techniques measure the temperature rise of a special load with known specific heat.
  • High Power (>10W): Calorimeter watt meters (dry or flow type) measure the temperature change in a fluid or dielectric that absorbs the microwave power.
• Attenuation Measurement:
  • Power Ratio Method: The ratio of output power with and without the device under test is measured.
  • RF Substitution Method: The device is replaced with a calibrated precision attenuator, which is adjusted to produce the same output power. The attenuator’s reading directly gives the device’s attenuation.
• Phase Shift Measurement: A comparison technique where the signal passing through the device under test is compared on a CRO with a signal passing through an adjustable precision phase shifter. The phase shifter is adjusted until the two signals match, providing a direct reading of the phase shift.
• VSWR Measurement:
  • Low VSWR (<10): Measured directly using a slotted line and VSWR meter by finding the ratio of maximum to minimum voltage readings.
  • High VSWR (>10): The double minimum method is used. The distance between two points on either side of a voltage minimum where the power is double the minimum value (d_2 – d_1) is measured. VSWR is then calculated as: VSWR = \frac{\lambda_{g}}{\pi(d_2-d_1)}.
• Impedance Measurement:
  • Using Slotted Line: The position of the voltage minimum is measured first with the unknown load and then with a short circuit. The shift in the minimum’s position and the VSWR are used to determine the load impedance.
  • Using Reflectometer: Two directional couplers sample the incident (P_i) and reflected (P_r) power. The reflection coefficient magnitude is calculated as \rho = \sqrt{P_r/P_i}, from which VSWR and impedance can be derived.
• Q of a Cavity Resonator Measurement: The Transmission Method involves plotting the resonator’s output power versus frequency to obtain a resonance curve. The quality factor (Q_L) is calculated from the resonant frequency (w_0) and the half-power bandwidth (2\Delta): Q_L = \frac{w}{2(w-w_0)}.