| This dissertation presents a new approach to designing spatial filters (frequency-selective surfaces) having extensive applications in communications and radar systems. Unlike conventional surfaces composed of resonance-length elements, the new structures use sub-wavelength elements, and therefore, operate in TEM mode. Consequently, their frequency response is harmonic-free up to a frequency where their elements' dimensions become comparable with the wavelength, lambda. Hence, their behavior is well described through quasi-static circuit models. These surfaces, which will be referred to as miniaturized-element surfaces, are easily synthesized since filter theory and circuit simulators can be utilized in their design process. The small dimensions of the elements of the surface and its TEM mode of operation decrease the surface sensitivity to the incidence angle of the excitation (plane-wave). This allows the application of such surfaces in conjunction with phased-arrays and their placement in close proximity (< lambda/10) to an antenna. These surfaces can also operate properly with smaller panel dimensions.;The theory of the new surfaces is introduced in Chapter 3 where a surface consisting of an array of lambda/12-long elements is presented. The transmission response of this surface includes a passband and a transmission zero. For this design, the first harmonic is located at a frequency six times higher than the operation frequency. Using varactors, frequency tuning of nearly an octave is shown. Chapter 4 presents multipole spatial filters. Through an accurate circuit model, dual-bandpass and maximally fiat filters that are lambda/240 thick are demonstrated. Chapter 5 introduces a reconfigurable surface that produces a frequency response with two operation modes: bandstop and bandpass. In addition, using varactors, the center frequency and the bandwidth are tuned independently. The discussion on tunability is continued in Chapter 6 which introduces another varactor-tuned structure that operates, similar to the previous designs, without additional biasing circuitry for the varactors. However, this structure is immune to single point failure as it uses a parallel biasing method. Finally, Chapter 7 demonstrates a lambda/10-thick, coupled filter-antenna array to achieve a high-order filtering for beamforming systems. This design eliminates the need for integrating bulky RF filters required in the receive chain of each element of the system. |
