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In this letter, a design method of dual-band filtering baluns is presented. The filtering balun operating in 2 bands have the characteristic with flexible frequency ratio in this approach. By properly introducing dual-mode resonators, the filtering balun can work at 2 bands and the frequency ratio of these 2 bands could be flexibly controlled. To prove the effectiveness of the proposed design principle, a dual-band filtering balun working at 3.5 and 4.7 GHz is designed and simulated. Eventually, the results of simulation exhibit favorable performance.

An octagonal prism conformal antenna array for x-band with vertically polarized omnidirectional radiation pattern is presented in this letter. The proposed antenna consists of a feeding network, radiation patches, parasitic patches, and dielectric layer. The 16 patches are placed on the outer surface of the octagonal cylinder and fed by a feeding network with uniform magnitude and phase. The patches are divided into upper and lower layers, each of which has eight units. Thus, the proposed antenna can obtain an omnidirectional radiation. To enhance the isolation between the elements, slots are introduced to decrease electromagnetic coupling between elements through the medium. By utilizing 16 conformal patches, the gain in the horizontal plane can be improved, especially for the range of $\boldsymbol{\theta}=80^{0}$ to $\boldsymbol{\theta}=100^{0}$ . The proposed antenna is designed and simulated. Simulation results reveal that the proposed antenna can provide an impedance bandwidth of 20% (7.16–8.76 GHz). Additionally, the minimum gain of the range which from $\boldsymbol{\theta}=80^{0}$ to $\boldsymbol{\theta}=100^{0}$ is greater than 2.8 dBi over the frequency band.

A wideband omnidirectional vertically polarized antenna is proposed which mounted on an unmanned aerial vehicle (UAV). The antenna adopts the structure of the printed dipole array back to back microstrip patches which fed by coaxial cable. The antenna is loaded by two shorted pin and realized characteristics of low profile and vertical polarization. The patch radiators and feed lines on one side of the ground plane are reversed in positions compared to the other side, and hence with radiators on both sides an azimuthal omnidirectional pattern is formed. The antenna realizes omnidirectional radiation in the horizontal plane. The antenna has a measured $\vert \mathbf{S}_{11}\vert bandwidth of 3.9-5.0 GHz (24.7%). The prototype antenna can be widely used in the field of communication antenna on unmanned aerial vehicle for its advantages of simple structure, small wind resistance, omnidirectional patterns, and low profile.

A circularly polarized (CP) antenna which is a set of 2×2 patch array with parasitic rectangular elements is designed and measured. The parasitic patch can enhanced the gain of the antenna. Each antenna element is fed by two probe that is connected to the feeding network. The final design, with an overall size of 160mm×160mm×28mm $(\sim 1.1\lambda_{0}\times 1.1\lambda_{0}\times 0.2\lambda_{0}$ at 2.09 GHz), was fabricated and measured. The proposed antenna has been fabricated and measured. The antenna array has a measured $\vert \mathrm{S}_{11}\vert dB bandwidth of 1.82-2.4 GHz(27.5%), the 3-dB axial-ratio(AR) bandwidth of 1.9-2.2 GHz is also obtained. Besides, the peak gain of the antenna array is 13.46 dBi at 2.01 GHz.

In this paper, a new design of an dual-band printed quadrifilar helical antenna(QHA) for S-band has been presented. This quadrifilar antenna has four helical arms wrapped around a dielectric cylinder which has low permittivity and low loss. Each arms consists of two parallel elements long and short which opened at the top of the helix. The lower end of helix is simultaneously fed, the long element resonates at 1.995GHz (1.98GHz–2.01GHz with the center frequency 1.995GHz), and the short element resonates at 2.185GHz(2.17GHz–2.2GHz with the center frequency 2.185GHz). The four arms are fed with equal amplitudes with 0°, 90°,180°,270° phase by integrated feeding network. Simulated results of the proposed antenna show that the antenna can provide good wide-beam circular polarization characteristics in both frequency bands. The proposed antenna has a smaller size, light weight and low cost to be suitable for mobile handset applications.