The estimation of aerial radiation patterns from limited near-field measurements

A series expression for the near-field radiation pattern of an aerial obtained with a directive measuring aerial in a two-dimensional system is derived from a cylindrical-mode expansion of the radiated field. Patterns calculated from this series show better agreement with experimental results at shorter ranges than is provided by previous series evaluations. An inversion of the first few terms of this series suggests a convenient technique for the determination of the radiation pattern from near-field measurements. Calculations based on this method of inversion indicate that the approximate shape of the pattern may be obtained with relatively little effort from the near-field pattern yielded by a small measuring aerial at a range of a2/¿, where a is the aperture length of the transmitting aerial. At ranges considerably shorter than a2/¿, a more directive measuring aerial, which provides a closer initial approximation to the far-field pattern, has the advantage of reducing the amount of calculation required.