Abstract :
The paper consists of a review of recent work in the field of aerial noise at microwave frequencies, together with the author´s detailed estimates of the resulting noise temperature of an aerial caused by direct radiation from atmospheric oxygen and water-vapour, direct radiation from the ground and indirect radiation from the atmosphere via the ground, for frequencies in the range 1?10Gc/s. Three differerent water-vapour distributions and several types of ground surface have been considered, and the finite beamwidth and side-lobe level of the aerial have been allowed for. Sufficient data are given to allow an approximate value of the minimum background noise temperature that can be expected with any given site and aerial system to be estimated for any angle of elevation of the aerial. For an aerial pointing vertically, the contribution to the noise temperature from oxygen over the whole frequency range, from the main beam, under fine-weather conditions (no clouds or rain), is of the order of 1?2?K, whilst for a horizontally pointing aerial it is of the order of 100?K. Water vapour in the atmosphere produces only a small effect at frequencies below about 4Gc/s (less than 0.05?K when the inclination ? to the vertical is zero, and less than 6?K when ? = 90?, for a sea-level water-vapour content of 14g/m3). At 10Gc/s, the corresponding figures for this very moist atmosphere are 5?K and 50?K, respectively. The ground, too, can be important, and even with a side- and back-lobe level of ?20dB relative to an isotropic aerial, the noise temperature from the ground can be over 1?K for vertical polarisation and just under 1?K for horizontal polarisation. Clouds and rain can produce noise temperatures considerably in excess of the fine-weather temperatures, particularly at the higher frequencies (a few degrees at 3Gc/s to over 100?K at 10Gc/s under exceptional conditions). As a part of the study, simplified formulas for the calculation of the absorption coefficients of oxygen- and water vapour as functions of height and frequency in the range 1?10Gc/s have been developed.
