Bolometer Method of Power Measurement

In this method of R.F. power measurement, the unknown power will be absorbed in a specially constructed bolometer element of resistance material. The resultant temperature raise is then detected by measuring the change in bolometer resistance using a bridge circuit.

(a) Bolometer:

A bolometer consists of a coaxial cable for the input through which the R.F. power will be fed . The cable has a short tapered section to transform the line impedance to that of the bolometer. A stub provides a ground return path as provided by a capacitor for the R.F. currents.

The bolometer consists of an element, which may be a thermistor or a barretter. A barretter is basically a wire mounted in a catridge similar to a fuse. Its resistance increases, with temperature. A thermistor is a small semiconductor bead with connecting wires. It has a negative temperature coefficient. The bead material is a mixture of manganese and nickel oxides combined with finely divide copper particles to control the resistivity. The bead diameter is about 0.5 cm. There are several shapes of thermistors. They are available as rods disks, washers or flakes.

The bolometer element is connected to the bridge. The bridge will be excited simultaneously by direct current from the voltage source and by the alternating voltage at audio frequency. Thus the bolometer element is simultaneously heated by the D.C. power, audio frequency power and radio frequency power being measured. The resistance of the bolometer is dependent on the total power and is dependent on where this power comes from.

(b) Arrangement for Measurement:

The arrangement for Bolometer method of R.F. Power Measurement is given in Figure.

Arrangement for Bolometer Method

The output from the bolometer element is connected to the arm CD. Exitation of the bridge is by two sources namely the A.F. or low R.F. source through a capacitor C and through the battery supplying the voltage E. The galvanometer G is used to act as the null detector.

(c) Measurement Method: 

The radio frequency power reaches the bolometer element through the coaxial cable. A nominal voltage from the audio frequency source is applied to superimpose upon the radio frequency power being measured. The direct current from the voltage source E will be next adjusted by the use of the rheostat R till the heating of the bolometer causes its resistance to have the value required to balance the bridge. The galvanometer acts as null detector and indicates the balancing of the bridge.

When once the balance is obtained the radio frequency power is switched off. This leads to unbalancing of the bridge as the bolometer element loosing the heat in the absence of R.F. power.

The audio frequency power will now be increased to re-balance the bridge. The resistance of bolometer depends on the total power dissipated irrespective of the frequency. Hence the radio frequency power measured will be directly proportional to the audio frequency power that causes the balance condition.

Taking the voltage from auxiliary supply as V1, before the radio frequency power is switched off, and further taking the voltage from the auxiliary supply after the second balance as V2, the R.F. power is

V22-V12/ 4R1

To obtain the maximum accuracy the audio power initially supplied to the bolometer element should not be large compared to the radio frequency power being measured.

(d) Precautions:

The bolometer and the associated bridge circuit must be properly shielded.

The initial audio power supplied to the bolometer element should not be large compared to the R.F. power being measured.

(e) Applications of RF Power Meter:

They are useful in the measurement of RF power at

The output of transmitters
Cable terminations
Antenna feeders

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