The design and construction of a short-wave field-strength measuring set

The paper contains a discussion of the principal features of the design of apparatus for the measurement of field strength at very short wavelengths by means of a loop aerial. The apparatus consists of a high-gain intermediate-frequency amplifier (about 1 Mc./sec.) associated with a signal-frequency and frequency-change unit of constant conversion efficiency. The e.m.f. of thermal agitation in the input tuned circuit of the intermediate-frequency amplifier is used as a standard signal for setting the amplifier to a known gain. The combination of intermediate-frequency amplifier and signalfrequency unit is calibrated over its signal-frequency range by means of a radiator which gives a horizontally-polarized radiation of calculable intensity. It is shown that a single-phase, i.e. unsymmetrical, frequency-change system has certain practical advantages over biphase systems, and that this can be used without loss of the necessary balance of the aerial system, if the latter is coupled inductively through a Faraday screen to the frequency-change circuit. The detailed design of the various couplings and of the diode frequency-conversion circuit are analysed in Appendices. The intermediate-frequency amplifier embodies continuously-variable gain-controls, calibrated in decibels, these components being a particular type of commercial carbontrack ¿volume control¿ which has been found satisfactory for this quantitative use. The set can be used for the measurement of field strengths down to the order of microvolts per metre at wavelengths from 7¿11 metres (27¿43 Mc./sec.) with the frequency-change and aerial units described.