A Novel Filter in Antenna (FiA) With Compact Size and High Performance

In this paper, a novel and effective integrated design method for filtering antennas is proposed for the first time, with the name of Filter in Antenna (FiA). This method takes a holistic perspective, utilizing the antenna structure to build a shielding cavity, and implanting the smaller-sized filter into the larger-sized antenna. To demonstrate the superiority of this method, an FiA prototype is designed by integrating a magneto-electric (ME) dipole and a compact filter. Initially, a metal shielding cavity is built with the M-dipole structure of the low-profile ME dipole. Secondly, a miniaturized filter is built using multi-layer coupling structures. Subsequently, the filter is inserted in the feeding line inside the shielding cavity of the ME dipole. Accordingly, the ME dipole and the multi-layer filter are integrated into an FiA. The design preserves the antenna's size while achieving full filter and antenna functionality integration. As a validation, a fully functional prototype is manufactured and tested. The FiA prototype has compact dimensions of 0.47λ ₀ ×0.48λ ₀ ×0.076λ ₀ (λ ₀ is the free-space wavelength at the center frequency), and measured results coincide well with the simulation. Excellent filtering performance (return loss of 21 dB, insertion loss of 0.6 dB, 6 transmission zeros to achieve 15-dB out-of-band suppression bandwidth of 1.6 f ₀ , etc.) and nice antenna performance (stable gain of 7.8 dBi, low cross polarization, low back radiation, etc.) are obtained, verifying the feasibility of the proposed method.