THE TREND TOWARD BUNDLING multiple components into a single module for wireless communications has researchers eying a dual-function DRA that can simultaneously act as the antenna and packaging cover. The problem is that the DRA's quality (Q) factor tends to be low to enhance both radiation and bandwidth. In contrast, the Q-factor of the dielectric-resonator filter (DRF) is usually high to reduce insertion loss. It is therefore contradictory to implement both the DRA and DRF using a single dielectric resonator. At the City University of Hong Kong's Wireless Communications Research Centre and Department of Electronic Engineering, Eng Hock Lim and Kwok Wa Leung have been working on a dual-function DRA filter that combines the DRA and DRF.

The researchers used a cylindrical dielectric resonator, which was top-loaded with a metallic disk, as the resonator for both the antenna and the bandpass filter. The metallic disk was used to improve the DRF's insertion loss and tune the filter with negligible effect on the DRA's radiation efficiency. The researchers found that the filter part's operating frequency can be made equal to or different from that of the antenna part.

The researchers concluded that the antenna and filter parts of the DRA filter could be designed and tuned separately and almost independently. In addition, the antenna and filter parts could be designed at the same or different frequencies. They also designed a second-order DRA filter that achieved better out-of-band rejection than the first version. Due to the orthogonality of their fields, the resonance modes of the second-order antenna and filter are almost independent of each other. See "Use of the Dielectric Resonator Antenna as a Filter Element," IEEE Transactions on Antennas and Propagation, January 2008, p. 5.