Microwave antenna system for passive discrimination

A novel passive antenna system, capable of discriminating specific electromagnetic signals is addressed. This antenna system will be able to detect signals of certain bandwidths, amplitudes and propagation directions. The philosophy behind this design was to maximise the signal discrimination at a stage prior to reception. The development of such systems could relieve the work involved in post detection discrimination, which may be time consuming and expensive. A major motivation of these studies lies in the difficulties inherent in signal detection for mobile radio communication systems operating at microwave frequencies. Such an antenna system consists of two components. They are the filter section and the detector array. The filter is designed in such a way that only the near normal signal to the locally flat area will be admitted and the rest reflected. The detector array will be at an appropriate position below the filter. Two types of filter structures have been studied for this angular filtering property. They are the Dielectric Multilayers (DML) and periodic arrays of slots as Frequency Selective Surfaces (FSS). DML are constructed by stacking layers of dielectric material whose permittivities vary in a near sinusoidal manner. Such a structure is known to have the ability to admit certain frequency bands of signals. The conventional transmission/reflection matrix method is used for its analysis. Also an optimisation procedure is carried out to minimise the loss of the signal in the DML. The characteristics of the DML as a beam-director and Beam-shaper have also been investigated. FSS exhibit the characteristics of band pass and band stop filters, depending upon the nature of the surface (periodic arrays of elements or slots) . Here the band pass nature is utilised by using arrays of slotted elements. These surfaces are tuned to admit narrow band signals. The well-known modal analysis method has been employed to study the FSS characteristics. The FSS have been studied in the context of frequency scanning, beam shaping, beam directing as well as angular scanning. A prototype has been constructed to simulate a multi signal environment in which the above structures have been experimentally assessed.