Minimum Noise Levels Obtained on Short-Wave Radio Receiving Systems

The theoretical minimum noise level of receivers in the absence of any interference, the source of which is external to the receiver, is discussed and compared with the limit actually measured on various antennas over a limited frequency range in the short-wave spectrum. It is pointed out that, on the shorter wave lengths and in the absence of man-made interference, the usable signal strength is generally limited by noise of interstellar origin. The powers obtained from this noise with the various antennas and for different times of the day are given. Recently, man-made interference, of which that caused by diathermy machines constitutes the greatest part, has become so extensive that it is now the limiting noise during most of the daylight hours. Data are given on the intensity and extent of this form of interference.