Write short note on Microwave Power Measurement

Microwave power measurement devices can be divided into two categories: devices such as detector , bolometer and thermocouples, whose resistance changes with the applied power and calorimeters in which the temperature rise of a known quantity of liquid being used as a load measured. Devices if the former type is used to measure power in microwaves and are very sensitive, while calorimeters are not so sensitive but are capable of measuring powers as high as hundreds of kilowatts. In microwave, power lower than 1 watt is defined as low power. Thus the power measurement can be classified as

1. Low microwave power measurement
2. Medium and high power measurement

As microwave power sources are very sensitive to load impedance variation and thus during the measurement of power, the power source should be isolated from the load with the help of isolator.

Bolometer:

Bolometer measurements are based on the dissipation of the RF- power in a small temperature sensitive resistive element called bolometer. This bolometer may be a short ultra thin wire having a positive temperature coefficient of resistance, called Baretter, or a bead of semiconductor having a –ve temperature coefficient called thermistor. The microwave power to be measured heats the bolometer and causes change in its electrical resistance, which serves as an indication of magnitude of power. The bolometer is generally incorporated in to a bridge network, so that small changes in resistance can be readily detected. Here the bridge is initially balanced with low frequency power and finally the low frequency power is withdrawn until the bridge is balanced again. Symmetric view of bolometer mount and bolometer element is shown in the Fig.8.2.

Fig. 8.2 Schematic view of bolometer mount

Fig. 8.3 Bolometer bridge

To measure unknown RF power, a small know AF power, indicated by the volt meter V₁, is super imposed on the RF test power. The direct current from voltage E is next adjusted by varying R, which heats the bolometer element until its resistance equal R₁, which is the value required to balance the bridge. The test RF power is then turn off, which unbalances the bridge. Balance is now restore by increasing the AF voltage to V₂ hence RF powers equals

\(RF\ power=\frac{{V_2}^2-{V_1}^2}{4R_1}\)

In a coaxial and waveguide transmission system, the bolometer mount must provide the necessary impedance transformation (matching). This is usually done by means of a tapered section as shown in the Fig. 8.3

Thermocouple:

This is more simple and does not required any biasing. Two dissimilar metals make thermocouple and when temperature difference exists at the junctions, current flows through it. This temperature difference is achieved with the help of a carbon film resistor connected between two dissimilar metal wire junctions of one end. Incident microwave power produces heat in the resistor connected at one end of thermocouple.

Thermocouples are sensitive and easy to use, since only dc volt meter is required with them, but they are rarely used, because of resistance changes with temperature, making matching difficult. Another difficulty faced is the need for a small diameter of heater resistance to remove skin effect and frequency dependence, due to which the thermocouple operates closed to its limit, with a resultant lack of dynamic range and sensitivity to overloads. Thermocouple has a use where constant power level monitoring has to be done with little change in the power level. It can be very easily used to measure the change in the klystron power output due to frequency or repeller voltage variation.

Calorimeter- Wattmeter:

High power microwave energy is usually measure by a calorimeter-wattmeter. The power is directed into a well matched liquid load and is completely absorbed by the liquid. The fluid may be water, oil, or in fact any liquid which a good absorber of microwaves. The advantage of this method is that there is no leakage of microwave energy either by radiation or through lossy line. The arrangement is shown in the Fig.8.4

Fig. 8.4 Calorimeter- Wattmeter

The fluid is allowed to flow towards the load at a constant rate with the help of a pump and is heated up by the microwave energy as it passes through. The rise in temperature is measured from the difference of reading of out let and inlet temperature monitor.

Now from the theory of calorimetry the amount of heat absorbed by the liquid load

\(Q=m\times s\times∆t\)

Where m=mass of the liqued flow in m/s

          s=sp.heat of liqued in cal/gm ℃

          ∆t=rise in temperature

Now ; if the same rise in temperature can be produced in the liquid load by passing a known amount of dc power or low frequency ac power (p), then we can write

\(P=4.18\times m\times s\times∆t\)

A calorimetric power meter calibrated directly in terms of power can be used for this purpose.

The power determined by the calorimeter – wattmeter is average power but it can be converted to peak power by the relationship

\(P_p=\frac{P_A}{duty\ cycle}\)