A Pulse Field-Strength Measuring Set for Very High Frequencies

The paper describes a portable equipment for the measurement of the field strength of pulse and continuous-wave signals in the frequency bands 20¿30 and 40¿650 Mc/s (wavelengths 10¿15 and 0.46¿7.5 m). The equipment consists essentially of a receiver, in which are incorporated calibrated signal-frequency and intermediate-frequency attenuators and an output meter, and a cathode-ray output-indicator unit. The field strength is measured by adjustment of the attenuators for a standard output, which for pulse signals is read on the cathode-ray tube and for continuous-wave signals on the meter. A half-wave dipole aerial is used, and the initial calibration of the standard output in terms of the field strength at the aerial is carried out by a radiation method. The noise voltage of the first circuit of the intermediate-frequency amplifier is used as the reference voltage for setting up the gain of that amplifier, and an internal oscillator supplies the reference voltage for setting the gain of the indicator unit. The minimum measurable field strength varies with frequency between 3 and 500 ¿V/m on short pulse (< 2 microsec) signals, and between 1.5 and 250 ¿V/m on long-pulse and continuous-wave signals. The maximum field strengths which can be measured are 140 db above these minimum values. The accuracy of relative measurements on any one frequency is within ± 0.5 db, and that of absolute measurements is within ± 2 db. Within these limits of accuracy the measurements are independent of pulse width for values greater than 0.5 microsec. The recurrence frequency range is 25¿100 000 c/s. Various types of field-strength measurement which have been made with the equipment are described, and they illustrate its wide range of application.