Microwave Measurements Lecture Notes

1. Power Measurements

• Power is defined as quantity of energy dissipated.

• The microwave power meter has a power sensor which converts the microwave power into heat energy.

• The rise in temperature provides a change in electrical parameters, resulting in output current.

• The three commonly used power measurement methods are,

a) Schottky Barrier Diode Sensor:

It is used as a square law detector whose output is proportional to input.

b) Bolometer Sensor:

It is a power sensor whose resistance changes with temperature. The two common types of bolometer sensors are barretter and thermistor. The barretter has positive temperature coefficient of resistance. Thermistor has negative temperature coefficient of resistance.

c) Thermocouple Sensor: 

A thermocouple is a junction of two dissimilar metals or semiconductors. The EMF generated by the thermocouple, when it is heated is proportional to microwave power to be measured.

2. Impedance Measurement:

a) Slotted Line Method: 

In this method, the impedance of the load can be measured by measuring the phase angle and reflection coefficient. The load impedance is given by,

Z_L = Z₀(1 + Γ_L)/(1 – Γ_L)

Where, Γ_L = ρ_L e^jΦL

b) Reflectometer Method:

The unknown impedance can be measured by the equation,

Z_L = (AΓ_L+B)/(CΓ_L+D)

Where A, B, C and D are the signal amplitudes.

3. Frequency Measurement:

a) Down Conversion Method:

This method uses a heterodyne converter. It converts unknown frequency by mixing with a known frequency, such that the difference between frequencies is amplified and measured.

b) Slotted Line Method:

In this method, the frequency is determined using the equation,

λ = 2d

where, d = minimum distance

c) Wave meter Method:

Wave meter is a cylindrical cavity which changes the resonant frequency by changing the length. The cavity resonates at different frequencies for different positions or lengths.