The rapid growth of terrestrial and satellite communications in the last two decades has driven the development of relevant equipment. In satellite communication and long-range terrestrial systems, the high gain antenna is a key component to transmit/receive modulated radio frequency signals. The solution to feed this gain requirement is the aperture antenna and antenna arrays. Of all these high gain antenna models, the reflectarray antenna is an attractive candidate for satellite communication due to its low cost and high reliability. However, the reflectarray antenna suffers the disadvantages of narrow operational bandwidth and lack of beam steering ability. These shortcomings of reflectarray are unwelcome in many applications.
In this thesis, theoretical and experimental investigations are carried out to study the bandwidth improvement and reconfigurable capability design of the microstrip reflectarray antenna. In the bandwidth investigations, two types of phasing element structure are proposed at X band: a double ring configuration and a circular ring attached with a circular open-ended stub structure. In the reconfigurable reflectarray design, PIN diodes/transistors are accommodated into the circular phase delay line of the phasing element for phase switching at C band.
In the first research stage, the design of a microstrip reflectarray formed by 81 variable-size double elliptical and circular rings illuminated by a conical feed is presented. Due to the double resonance of the radiating structure, the double circular and elliptical rings provide an increased phasing range of unit cells that exceeds the required minimum of 360°. Printed on a single substrate and supported by a layer of foam and a conducting ground they also offer a slow phase slope as a function of their size and thus provide an increased operational bandwidth and a good manufacturing error tolerance. The performance of the designed double circular and elliptical rings reflectarrays is investigated via full-wave electromagnetic simulations. Measurements are also performed to verify the performances of the manufactured double elliptical reflectarray. The presented results show a wideband performance of the designed reflectarrays. However, this series of phasing element structures suffers from a serious manufacturing difficulty, which is that the gap between the adjacent rings in a unit cell may become too narrow to fabricate. Having noted this issue, a new phasing element investigation is carried out with a different method for bandwidth enhancement.
In the second stage of research, the design of a single-layer reflectarray, which employs a new phasing element in the form of a fixed-size circular ring and a variable length open-circuited stub, is presented. The array is developed on a thin substrate supported by a thick foam material. Investigations are performed to obtain a linear reflection phase as a function of the stub’s length when the element operates in a unit cell. This goal is achieved by a suitable choice of the ring’s radius and width and the stub’s width. In order to validate the simulated element’s reflection phase behaviour, a waveguide simulator is fabricated and used to perform the experimental tests. The phasing element offering the best linear phase characteristics is used to design an X-band offset fed 13×13 elements reflectarray pointing at both 0° and 20° from the broadside direction. Full wave simulations performed using CST Microwave Studio show that the desired radiation characteristics of the designed array antenna are achieved in both main focus and offset feed design. The simulated performance is confirmed by experimental tests performed on the fabricated reflectarray prototype showing a 17.8% 3-dB gain drop bandwidth.
In the final research stage, an electronically controlled phasing element for a beam-steered single-layer microstrip reflectarray operating at 4 GHz is presented. The phasing element is formed by a printed circular ring equipped with a variable length arc stub. The reconfigurable design is accomplished by including an open gate PIN transistor in the variable length stub to offer a 1 bit beam steering ability. Full-wave electromagnetic simulations and measurements are performed to prove the linear phase performance of the proposed cell including the effect of the utilised active chip and its biasing circuit. Those results indicate the high isolation between the phase performance of the utilised cell between the ON and OFF states of the PIN switches. An array of 8×8 elements is designed in both the gathered element method and traditional individual element approach to verify the performances of the reconfigurable phasing element by steering the radiation patterns between 20° and 30° from the broadside direction. The obtained results show that the two phasing methods produce similar radiation patterns around the desired directions. However, the reflectarray using gathered elements suffers from an increased level of grating lobe.