Both the filter and antenna are indispensable components of modern wireless communication systems. A filtering antenna that integrates the filter and antenna can reduce insertion loss, overall volume, and the mutual coupling between adjacent antennas operating at different frequencies, thereby preventing the distortion of their radiation patterns. This thesis investigates the application of filtering antennas in wireless communications in three parts.

The first part focuses on linearly polarized (LP) and circularly polarized (CP) filtering dielectric resonator antennas (DRAs) with conducting loops. A cylindrical DRA is used for the LP design, whereas an elliptical DRA with two notches is used for the CP design. The DRA is excited in the HEM_11δ mode in each case. Equivalent horizontal magnetic dipoles can be obtained from the HEM_11δ mode of the DRA and the loop structure. When they are equal in magnitude but opposite in phase, the radiated fields can be canceled out and a good filtering response can be obtained. LP and CP filtering DRAs operating at 2.4 GHz were designed, fabricated, and tested to demonstrate such idea.

The second part extends the concept of radiation cancellation to obtain a millimeter-wave (mmWave) filtering response. A substrate-integrated filtering DRA and its 2 × 2 array for 28 GHz applications are also presented. The DRA is fabricated out of a substrate with a high dielectric constant by using two concentric arrays of air holes and metalized vias. The top side of the antenna substrate has four separate strips that are connected to the ground plane through the metalized vias. The metalized vias, ground plane and four strips form a substrate-integrated cavity that prevents energy leakage and enhances antenna gain. Similar to the designs presented in the first part, a dual-loop structure is introduced into the DRA to provide two radiation nulls and a filtering response. For impedance matching, a multi-section microstrip feedline printed on a second substrate is used to excite the DRA through the four slots in the common ground plane. Three modes can be excited simultaneously, namely, the cavity, DRA HEM_11δ, and perturbed HEM_11δ modes. A 2 × 2 filtering DRA array operating in the 28GHz licensed band (27.5 GHz–28.35 GHz) was designed to demonstrate this idea.

The third part examines a compact absorptive filtering patch antenna that comprises a filtering patch antenna and a bandstop filter (BSF) whose transfer functions are complementary to each other. A slot is fabricated in each patch and ground to provide two radiation nulls for the lower bandedge. Two radiation nulls are also obtained for the upper bandedge by using a dual-stub
feed. A λ_g/2 defected ground structure and λ_g/4 defected microstrip structure are used in the BSF design, which is terminated by a chip resistor. Given the complementary transfer functions of the filtering patch antenna and BSF, incident energy can be effectively radiated in the passband yet largely absorbed by the resistor in the stopbands. As a result, only a little amount of energy is reflected over a wide frequency range, thereby generating a reflectionless characteristic. An absorptive filtering antenna operating at 5.8 GHz was designed, fabricated, and tested to demonstrate this idea.

All filtering antennas presented in this thesis achieve a good filtering response without requiring filtering circuits in the feed network. Therefore, these antennas are promising candidates for the development of compact, efficient wireless communication systems.