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A vehicular antenna with diverse radiation patterns is proposed for both the dedicated short range communications (DSRC) and the fifth-generation (5G) millimeter-wave (MMW) communications. The proposed antenna mainly comprised a 2 × 2 MMW subarray, an 8-element DSRC subarray and a corresponding 5-port feeding network. The radiating patches of the MMW subarray are excited by two pairs of coaxial-to-substrate integrated waveguide (SIW) transition (CST) feeding networks that are comprised of two 3-dB couplers, two 90$^{\circ }$-phase shifters, and four power dividers (PDs), which provide in-phase and differential-phase output signals to achieve single-beam and conical-beam radiation patterns. On the other hand, the 8 monopoles of the DSRC subarray arranged around the aforementioned MMW radiator are excited by a coaxial-to-SIW-to-coaxial transition (CSCT) feeding network that includes a 1-to-4 SIW PD and four 1-to-2 microstrip PDs to form an omni-directional radiation and achieve a large frequency ratio (LFR). Measured results indicate that wide bandwidths of 15.44% (24.14-28.18 GHz), 16.09% (24.29-28.54 GHz), 15.01% (23.85-27.72 GHz), and 14.29% (24.05-27.75 GHz) with corresponding high peak gains of 10.97$\pm$1.03 dBi, 11.71$\pm$1.32 dBi, 10.84$\pm$1.00 dBi, and 11.29$\pm$1.28 dBi are obtained by the four MMW subarray ports, while a bandwidth of 11.40% (5.46-6.12 GHz) with an azimuth-plane gain of 0.64 dBi and a peak gain of 4.29$\pm$0.19 dBi for the DSRC subarray port are also obtained. Besides, low cross-polarization (X-pol) levels of −24 dB and −27 dB are achieved for the two subarrays, respectively.

A wideband, low-profile endfire antenna with polarization reconfiguration is proposed. The endfire antenna can be electronically switched among the left-hand circular polarization (LHCP), right-hand circular polarization (RHCP), and linear polarization (LP) using six PIN diodes in conjunction with biasing lines. The proposed antenna consists of a wideband half-mode substrate integrated waveguide (HMSIW) cavity (vertically polarized magnetic dipole) and tapered switchable electric dipole (horizontally polarized) with a feed line. The impedance bandwidth of the HMSIW cavity is increased by combining the resonance frequencies of quasi-TE1,0.5,0 mode with quasi-TE3,0.5,0 mode by placing the two slotted pairs at the null of quasi-TE3,0.5,0 mode. At LHCP/RHCP states, the proposed endfire antenna exhibited a stacked 10-dB impedance bandwidth (IBW) of 11.66% (6.54-7.35 GHz) and a 3-dB axial ratio bandwidth (ARBW) of 11.23% (6.515-7.29 GHz) with a peak gain of approximately 4.20 dBic. When switched to the LP state, it shows a 10-dB impedance bandwidth of 11.58% (6.87-7.715 GHz) with a peak gain of 5.35 dBi. Therefore, it can exhibit good effective bandwidths (stacked 10-dB IBW and 3-dB ARBW) of 5.93% (6.87-7.29 GHz) and 10.84% (6.54-7.29 GHz) across the three polarization states (LP/RHCP/LHCP) and two CP states (RHCP/LHCP), respectively.

In the present era, the key challenge for a low-frequency planar antenna applied to a wireless underground sensor network (WUSN) and other underground wireless communication is to achieve positive gain with good efficiency. Besides that, a relatively wider bandwidth with more than one operating frequency is desirable for operational versatility. In this work, a simple artificial magnetic conductor (AMC)-integrated rectangular microstrip antenna (RMA) of size $0.28\lambda _{0} \times 0.28\lambda _{0} \times 0.07\lambda _{0}$ is employed. By meticulously exploiting a dual-periodic AMC along with dielectric strips (AMC radiating patch and AMC ground plane) and proper utilization of an L-probe feed, two resonances at 572 MHz (low band) and 768 MHz (high band) can be yielded with a wide bandwidth of around 12%. Unlike the classical RMA design, a transverse electric (TE) mode excited RMA is proposed. The proposed antenna can also achieve good positive gains of 4.3 dBi (572 MHz) and 6.1 dBi (768 MHz) across both bands with an efficiency of 71% and 75%, respectively.