Modelling antennas on structures

ALDAS is a versatile program which has been in use for a number of years and which uses diffraction theory for computing the effects of a conducting structure on the RF performance of an antenna mounted on or off the structure. Geometrical Theory of Diffraction consists of a number of solutions to canonical problems such as diffraction at an edge, at cone tips, creeping waves et cetera. Since these are solutions to canonical problems, compromises have to be made between runtime and complexity on one hand and accuracy on the other. The major decisions in writing the program were to use a structure model which included canonical shapes rather than the computer intensive solution of fitting NURBS to a surface and to write in FORTRAN using a reliable compiler (Salford FTN77). These decisions reflect an engineering approach to the software and mean that the program can be used on a desktop machine rather than a more expensive workstation. ALDAS can carry out the computation of radiation patterns (nearfield and farfield) and isolation between antennas. At each stage in the model build, decisions have had to be taken which trade-off complexity, runtime and accuracy. The purpose of this paper is to review the aspects of accuracy and comment on the effects as far as the user is concerned.