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Abstract-A miniaturized crescent-shape microstrip antenna is proposed for ultrawideband (3-10 GHz) applications. The crescent antenna is evolved from an elliptical patch antenna by carving a circular hole inside symmetrically. The circular aperture introduces an additional antenna in-band resonance and provides wider bandwidth with more design flexibility. The radiation characteristics of this crescent antenna are investigated with full-wave electromagnetic simulations and compared with an elliptical antenna. A crescent antenna prototype that occupies only 60% area of the elliptical patch is fabricated and tested. Antenna pattern and impedance measurements show good performance over the 3-10 GHz band with consistent radiation patterns, low cross polarization, and a substantial gain. The design method is also described.

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It is found through a vector integral-equation analysis and the reciprocity theorem that the gain of a microstrip antenna can be greatly enhanced with a photonic band-gap (PBG) material layer either as the substrate or the superstrate. The beam angle is found to coincide with that of a leaky-wave mode of a planar-grating structure. This observation suggests that high gain is due to the excitation of strong leaky-wave fields.

Abstract-Slow-wave structures using distributed periodic inductive and capacitive loadings have found many microwave circuit applications as left-handed (bandpass) or right-handed (lowpass) transmission lines. A large slow-wave factor (SWF) could result in a much smaller passive component, but also a much lower bandgap (cutoff) frequency and a larger dispersion. This paper addresses the issues and the design tradeoff between the SWF, group delay (dispersion), and the cutoff frequency of a right-handed (lowpass) quasi-lumped transmission line. A new two-layer transmission line structure using 3-D substrate metallization with an SWF of 5.8 is designed. A prototype of a 3-GHz branch-line coupler with a 70% size reduction using such a transmission line structure is fabricated and tested.Index Terms-Branch-line coupler, dispersion, electromagnetic bandgap (EBG), periodic structures, slow wave.