This thesis presents the design, fabrication, and measurement of tunable frequency microstrip antennas using RF MEMS (Microelectromechanical Systems) technology. The integration of RF MEMS components with radiators enable to implement tunable systems due to the adjustable characteristics of RF MEMS components. In the frame of this thesis, different types of structures have been investigated and designed. The first structure consists of a microstrip patch antenna which is loaded with a microstrip stub whose length is controlled by RF MEMS switches. In the other structure, the length of a microstrip patch antenna is changed by connecting a metal plate using RF MEMS switches. The third structure is composed of a microstrip patch antenna and a microstrip stub on which RF MEMS variable capacitors are placed periodically to control the resonant frequency. In order to maintain an easier integration with RF MEMS capacitors, another structure consisting of a microstrip patch antenna and a coplanar waveguide (CPW) stub which is loaded with variable RF MEMS capacitors is designed. The final structure is a dual frequency CPW-fed rectangular slot antenna whose resonant frequencies are shifted by RF MEMS variable capacitors placed on a short circuited stub inserted inwards the antenna. The fabrication of CPW-fed rectangular slot antenna is completed in the MEMS fabrication facilities of METU using RF MEMS process based on electroforming on glass substrate. The measurement results show that RF MEMS components might be a proper solution to obtain tunable frequency antenna structures.

In this research we attempted several modifications to microstrip ring/loop antennas to design multi-frequency antennas through systematic approaches. Such multi-frequency antennas can be useful while building compact terminals to operate at multiple wireless standards. One of the primary contributions was the use of a capacitive feed arrangement that enables simultaneous excitation of multiple concentric rings from an underlying transmission line. The combined antenna operates in the same resonant bands as the individual rings and avoids some of the bands at harmonic frequencies. A similar feeding arrangement is used to obtain dual band characteristics from just one ring, with improved bandwidth. This is made possible by widening two adjacent sides of a square ring antenna symmetrically, and attaching an open stub to the inner edge of the side opposite to the feed line. Use of fractal segments replacing the side with the stub also results in a similar performance. Use of fractal geometries has been widely associated with multi-functional antennas. It has been observed from the parametric studies that, the ratio of the resonant frequencies can range from 1.5 to 2.0. This shows some flexibility in systematically designing dual-band antennas with a desired pair of resonant frequencies. An analysis technique based on multi-port network modeling (MNM) has been proposed to accurately predict the input characteristics of these antennas. This approach can make use of the ordered nature of fractal geometries to simplify computations. Several prototype antennas have been fabricated and tested successfully to validate simulation and analytical results.