Elimination of errors from crossed-dipole direction-finding systems

Radio direction-finding systems have mainly been used in conjunction with vertically-polarized radiation. For the detection of aircraft by radar on metre wavelengths, horizontal polarization has certain advantages and is normally used. The early technique, involving the use of a crossed-dipole system and goniometer suitable for use with horizontally-polarized radiation only, is briefly described. The main sources of error in such systems are discussed. It is shown that special care must be taken to ensure that, when long feeders are involved, aerials are carefully matched to the feeders. In order to achieve this, accurate impedance measurements are required. Measurements on a dipole fed by twin concentric transmission lines are described, and it is shown how the impedance of stacked arrays may be calculated from these measurements. The need for careful bonding of transmission lines is discussed, and a design using heavy brass clamps to bond all feeders to a central lightning conductor is described. Errors due to reflector systems used for sense determination are discussed. A multi-element reflector system for use with stacked d.f. arrays is described. The receiving aerials for the C.H. radar systems, as designed on the above principles and used for the determination of range, bearing and elevation of aircraft, are described.