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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 multi-band vehicle-mounted garden-themed antenna with isolation improvement for 2G/3G/LTE/5G sub-6 GHz/WiFi/Bluetooth is proposed in this paper. This antenna is mainly comprised of three printed circuit board (PCB) substrates. For good isolation level, two pavilion-shaped 4G-band (including the 2G/3G/LTE frequency bands) patches and two tree-shaped 5G-band (including the 5G sub-6 GHz bands) patches are diagonally placed at the four corners of the top-layer substrate, whereas the other two tree-shaped 5G-band patches and a peach-shaped WiFi-band patch, are arranged in a line on the bottom substrate. To further improve the isolation, two fence posts are arranged between the neighboring antenna elements on the bottom substrate. Finally, a flower-shaped patch array printed on the middle substrate is employed to reduce the mutual coupling factor, broaden the frequency bandwidth, improve the peak gain and the radiation efficiency, especially for the 5G-band antenna element printed on the bottom-layer substrate. Characteristic mode analysis (CMA) method is introduced to the antenna radiation pattern and frequency bandwidth. The measured results showed good impedance bandwidths of 20.69$\%$ (0.78-0.96 GHz) and 106.8$\%$ (1.37-4.51 GHz) for 2G/3G/LTE bands, 71.81$\%$ (2.41-5.11 GHz) for 5G bands, and 84.34$\%$ (2.4-5.9 GHz) for WiFi bands are achieved with a compact size of 0.338 $\times\; 0.130\;\times$ 0.094 $\lambda _{0}^{3}$ (free-space wavelength at the starting frequency). Furthermore, across the above bands of interest, an isolation level of better than 16 dB can be achieved.