Radio field-strength measurement

Abstract

The subject of radio field-strength measurement is of direct interest in connection with all applications of radio wave technique. Furthermore, since radiocommunication, broadcasting and, to a lesser extent, navigational aids concern many countries other than that in which the service originates, it is important to ensure that there is international agreement on the methods of measurement used and on the results obtained with the equipment available. The British National Committee for Scientific Radio has been active in this field in connection with the General Assemblies of the Union Radio Scientifique Internationale since 1934, when a comprehensive review of the then existing methods of measuring radio field strengths was presented. Two years later, the National Physical Laboratory conducted a comparison between the various field-strength measuring sets then available in Great Britain for wavelengths between 7 and 1 500 m (40 Mc/s and 200 kc/s). The results of this comparison showed that while in the wavelength band 6 to 10 m (50 to 30 Mc/s) the measurements on different sets agreed to within ±20% of the mean value, greater divergences were obtained at longer wavelengths; and in the range 100 to 500 m (3 to 0.6 Mc/s) the departures from the mean value of field strength ranged from -30% to +60%. An examination by the U.S. Bureau of Standards of a number of commercial equipments for field-strength measurement at wavelengths within the range 200 to 500 m (frequencies 1 500 to 600 kc/s), showed that special precautions were necessary to ensure the attainment of an absolute accuracy within 20%. Apart from the introduction of the use of a shot-noise diode in place of thermal-agitation noise in the first tuned circuit, for adjusting the amplifier in a measuring set to a standard gain, there is no evidence from the published literature that any appreciable advance on the position described above has been made in the past ten years. What has been done, however, is to im