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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.