Broadband and Dual-Band Coplanar Folded-Slot Antennas (CFSAs) [Antenna Designer's Notebook]

This paper presents a unified design methodology for dual-band and broadband coplanar folded-slot antennas (CFSAs). The design is achieved by using a coplanar waveguide-fed antenna element that is asymmetric with respect to the folded slot, and by adjusting the length of the stub inside the slot. A mathematical derivation, based on a transmission-line model for the asymmetrically-fed coplanar folded-slot antenna, is used on a single-frequency coplanar folded-slot antenna to determine the condition for a second resonance. In this way, when the second resonant frequency is close to the first, broadband uniplanar antennas with ~30%, bandwidth can be designed. In addition, at the dual-band mode, a frequency ratio (f₂/f₁) of the order of 2.5 or more can be obtained. The effect of the ratio of the feed-shift distance to the length of the stub on the bandwidth is an important parameter, and is shown for both the broadband and dual-band designs. The presented method and tables are simple to use, provide very accurate results, and correctly predict the resonant frequencies for the dual-band coplanar folded-slot antennas. The theoretical and simulated results were verified by measurements of fabricated prototypes. The design guidelines cover a broad range of applications in the 2.4 GHz to 5.25 GHz range, with various bands and bandwidths.