In this paper, a class of bandpass frequency selective structures (FSSs) with improved out-of-band rejection is proposed by using stacked slotlines. The proposed FSS consists of a periodic array of slotlines etched in pieces of single-layer printed circuit boards, which are vertically placed along the periodic plane and then stacked together with a certain distance. Due to this special arrangement, slotline resonators are constructed based on the desired propagation modes, thus leading to a stable bandpass filtering response in a wide frequency range. To better understand the operating principle, design procedure of this class of FSSs with uniform slotline unit cells, inductor-loaded uniform slotline unit cells, and stepped-impedance slotline unit cells is presented step by step. In particular, the equivalent transmission line models are established and the even–odd mode analysis method is introduced to intensively analyze the resonant and filtering mechanisms. As two examples, a single-polarized FSS and a dual-polarized one, both based on stepped-impedance slotline unit cells, are designed, fabricated, and measured. Measured results indicate that the proposed FSSs feature miniaturized unit-cell size, wide upper rejection band, and stable frequency filtering response under a large variation of incident angles.

A design approach to a novel wideband circular sector patch antenna is proposed. Design guidelines are laid down based on an approximate 1.5-wavelength, multimode magnetic dipole, and the cavity model. Then, the flared angle of the circular sector patch and the corresponding usable resonant modes for wideband radiation are determined. It is demonstrated that the resonant TM _4/3,1 and the TM _8/3,1 modes within a 270° circular sector patch radiator can be simultaneously excited, perturbed, and employed to form a wideband unidirectional radiation characteristic with two resonances. Prototype antennas are designed and fabricated to experimentally validate the dual-resonant wideband property on a single-layered substrate. It is further demonstrated that the antenna designed on a 5-mm-thick air substrate exhibits an available radiation bandwidth (ARB) of 14.5%, while the printed one designed on a 2-mm-thick modified Teflon substrate exhibits an ARB of 6.5%. It is evidently validated that the proposed approach can be employed to effectively enhance the operational bandwidth of microstrip patch antennas without increasing antenna profile, inquiring multiple radiators or employing reactance compensation techniques.

This paper proposes a novel circularly polarized antenna operating at 2.4–2.48 GHz industrial, scientific, and medical (ISM) band for biomedical applications. This proposed structure is developed from a simple pin-loaded patch antenna, whose circular polarization (CP) is mainly attributed to the loading of shorting pins and L-shaped open-end slot. Based on this, pin-loaded annular ring (PLAR) with dual-mode operation is adopted to introduce another resonance. Additionally, two closely coupled rectangular patches are added to bring these two resonant frequencies close to each other in muscle. Furthermore, by introducing arc-shaped slots, CP property and improved impedance matching are achieved with size miniaturization. This proposed structure shows a simulated impedance bandwidth of 8% and a wide axial-ratio (AR) bandwidth of 19.1%. Finally, measurement is performed for the proposed antenna, and the results coincide well with the simulation ones, indicating that it is a good candidate for biomedical applications.

A novel wideband planar endfire circularly polarized (CP) antenna with wide 3 dB axial ratio (AR) beamwidth is presented. The operation principle of the proposed CP antenna is described at first through the combination of a planar magnetic dipole and a V-shape open loop. Then, it is demonstrated how its AR beamwidth can be precisely controlled with resorting to the shape and thickness of the open loop element. Finally, the theoretical design approach is numerically verified, and the results are then experimentally validated. It is observed that a 3 dB AR beamwidth is achieved in an extremely wide angular region up to 250° within the principal elevation plane over a frequency range of 2.41-2.51 GHz. The proposed antenna has exhibited a wide impedance bandwidth of 22.23% in fraction, a 3 dB AR bandwidth of no less than 8.00% in fraction, and a simple planar profile in geometry. In addition, its endfire beam is in parallel with its plane.

The radiation behavior of the fractional-order, resonant mode within a circular sector cavity radiator is revealed at first, and then, it is employed to design a novel, planar quasi-isotropic magnetic dipole antenna. A set of closed-form formulas is derived and employed to determine the key parameters of the proposed antenna. The resultant circular sector magnetic dipole antenna operates at its dominant TM(2/3), 1 mode. It is numerically verified and experimentally validated at the 2.45-GHz band. It is seen that the antenna exhibits a good non-uniformity of less than 5.7 dB within the three principal planes, and an average radiation efficiency up to 82% within its impedance bandwidth from 2.4 to 2.5 GHz (for reflection coefficient smaller than -10 dB). Good agreement between the theoretical, simulated, and measured results has evidently verified the proposed antenna design approach.