Wideband Dual-Polarized Quasi-Omnidirectional Antenna with High Isolation for Mobile Applications in High-Speed Vehicles

A new wideband dual-polarized (DP) quasi-omnidirectional antenna is proposed. The DP antenna consists of a vertical slot for horizontal polarization (HP) and a folded horizontal slot for vertical polarization (VP). A tapered strip is employed to coupling feed the vertical slot while a straight strip is used to excite the horizontal slot. Due to the coupling feed and the orthogonality between the vertical and horizontal slots, the DP antenna features a wide bandwidth and a very high isolation. The simulated isolation is higher than 70 dB, although the measured isolation is about 50 dB. As experimental results indicate, the DP antenna realizes an impedance bandwidth (BW) of 45% (1.7–2.73GHz) with quasi-omnidirectional radiation patterns for both HP and VP. The DP quasi-omnidirectional antenna may be packaged into a radome with a shape of a blade for mobile applications in high-speed vehicles.


Introduction
Recently, a lot of dual-polarized (DP) omnidirectional antennas have been investigated for multiple input multiple output (MIMO) applications [1][2][3][4][5][6][7][8][9][10].Two DP slot antennas with omnidirectional radiation patterns for both VP and HP with small volumes are presented in [1,2].However, they just have a narrow impedance bandwidth of ~5% for return loss RL > 10 dB and an isolation of ~33 dB.The DP planar antenna proposed in [3] consists of four curved branches and an annular-ring patch.The DP planar antenna proposed in [4] is composed of two X-shaped arms etched on a square-shaped substrate.Both antennas achieve a high isolation of ~40 dB but a narrow impedance bandwidth of ~5%.Wideband DP omnidirectional antennas developed in [5][6][7][8][9] have bulky and complicated configurations, not suitable for mobile applications in high-speed vehicles.A sabre-like DP omnidirectional antenna proposed in [10] is composed of a coplanar waveguide-fed monopole for VP and a thin cavity for HP, which exhibits a compact simple structure, and thus is suitable for high-speed vehicle/airborne applications.But the antenna features a narrow bandwidth (~5%) and a low isolation (~16 dB), which cannot sufficiently meet the requirements for wireless communications.In this paper, we present a new wideband DP quasi-omnidirectional antenna with high isolation.The simulated isolation is higher than 70 dB, and the measured result is about 50 dB over an impedance bandwidth (BW) of 45% (1.7-2.73GHz).The DP quasi-omnidirectional antenna may be packaged into a radome with a shape of a blade for mobile applications in high-speed vehicles.A wideband HP quasi-omnidirectional antenna is first introduced in Section 2.Then, the wideband DP quasi-omnidirectional antenna is introduced in Section 3 based on the HP antenna in Section 2.

Wideband HP
Quasi-Omnidirectional Antenna 2.1.Antenna Structure.The structure of a wideband HP quasi-omnidirectional antenna is depicted in Figure 1.The antenna consists of a metal box with a vertical slot which is excited by a tapered patch (Figure 1(a)).The antenna can be packaged into a radome that may have a shape of a blade to alleviate wind resistance (Figure 1(b)), thus being mountable on high-speed vehicles.The metal box is flared by an angle for better impedance matching.The tapered patch is fed by a coaxial probe from one side of the metal box (Figure 1(c)).A nylon bolt is employed to fix the tapered patch.The wideband HP quasi-omnidirectional antenna was designed for 2G/3G/4G mobile applications in the 2 GHz band (1.71-2.69GHz).The wideband HP quasiomnidirectional antenna was simulated and optimized using the software Ansoft HFSS v.15.A list of all the optimized geometric parameters is displayed in Table 1.

Operating Principle.
The wideband operation gets benefit from the tapered-patch excitation and the flared metal box.
When the vertical slot is excited directly by a voltage source, the input impedance (Z in ) is simulated, as displayed in Figure 2. It is easy to see that the resonant resistance is higher than 1000 Ω at the first resonance around 1.75 GHz and 3000 Ω at the second resonance around 3.75 GHz, which means that it is difficult to match the vertical slot to a 50 Ω source.When a tapered patch is used to excite the vertical slot, the simulated return loss (RL) is depicted in Figure 3.
A wideband performance is created by the tapered-patch excitation while a narrow band resonance is observed for the voltage-source excitation.
The impedance matching can be improved by a flared metal box. Figure 4 shows that when a flared metal box is used, a BW of 49% (1.68-2.72GHz) for RL > 10 dB is realized.The flared angle (β) has an effect on the impedance matching, in particular on the lower frequency band.From Figure 5, the optimal value for β is found to be β = 7 °.

Experimental Results
. The wideband HP quasiomnidirectional antenna was made and measured.An archetype of the fabricated wideband HP quasi-omnidirectional antenna is pictured in Figure 6.
The measured and simulated results for RL are compared in Figure 7; good concordance is achieved.It is obvious that the measured BW for RL > 10 dB is approximately 51% (1.67-2.79GHz).The measured and simulated radiation patterns of the wideband HP quasi-omnidirectional antenna at 1.7 GHz, 2.2 GHz, and 2.7 GHz are presented in Figure 8; good concordance is again achieved.HP quasi-omnidirectional patterns are observed in the horizontal plane (i.e., the x-y plane) over the wide bandwidth with an    International Journal of Antennas and Propagation unroundness of about 5 dB.The copolarization level is 20 dB higher than the cross-polarization.The measured antenna gain is plotted in Figure 9, which was compared with the simulated results; good agreement is observed.It is found that the peak gain of the wideband HP quasi-omnidirectional antenna is ~5 dBi.

Wideband DP Quasi-Omnidirectional Antenna
The structure of a wideband DP quasi-omnidirectional antenna is depicted in Figure 10.The DP antenna is     4 International Journal of Antennas and Propagation slot and the horizontal slot, a very high isolation can be achieved.For mobile applications in high-speed vehicles, the wideband DP quasi-omnidirectional antenna may be packaged into a radome which has a shape of a blade (see Figure 10(c)) to alleviate wind resistance.
The wideband DP quasi-omnidirectional antenna has been fabricated and measured.An archetype and a measurement setup of the fabricated antenna are pictured in Figure 11.A semirigid coaxial cable for HP is connected to feeding port 1 while another one for VP is connected to feeding port 2. Ten ferrite rings are employed for each coaxial cable to forbid the current from flowing on the surface of the cable.
The measured and simulated S-parameters are depicted in Figure 12.The impedance BW overlapped for S 11 and S 22 <−10 dB is about 45% (1.7-2.73GHz), covering the frequency bands for DCS1800, PCS1900, UMTS, LTE2300/2500, and Wi-Fi 2.4 GHz systems.The simulated isolation (− S 12 in dB) is higher than 70 dB, and the measured result is about 50 dB.The disagreement between the measurement and simulation is owing to the effect of the feeding coaxial cables even though the effect has been mitigated by the ferrite rings.The measured radiation patterns for HP and VP at 1.7 GHz, 2.2 GHz, and 2.7 GHz are depicted in Figure 13.Quasi-omnidirectional patterns are observed for both polarizations.The measured and simulated gains of the DP quasi-omnidirectional antenna are depicted in Figure 14.The averaged antenna gains are about 4 dBi.The antenna efficiencies for both VP and HP are displayed in Figure 15.The measured averaged antenna efficiencies are ~85%.The discrepancies between the measured and simulated results of the antenna gains and efficiencies are due to the losses caused by the metal box, the feeding line, the coaxial cables, and SMA connectors which were not taken into consideration in the simulation.
To verify the advantage of the proposed wideband DP omnidirectional antenna, a comparison with other reported   2. It is evident that [1][2][3][4] and [10] all have narrow bandwidth, even though the antennas presented in the rest of references listed in Table 2 realize wide bandwidth; the isolation they achieved is still much lower than that of the DP antenna realized in this work, which features a wide bandwidth (45%) and a much higher isolation (>50 dB).In addition, the proposed DP antenna is also mountable on high-speed vehicles for mobile applications.

Conclusion
A wideband DP quasi-omnidirectional antenna with high isolation is proposed.Experimental results show that the impedance bandwidth of the DP quasi-omnidirectional antenna is about 45%.The simulated isolation is higher than 70 dB, although the measured result is about 50 dB.The wideband DP quasi-omnidirectional antenna can be applied in high-speed vehicles for 2G/3G/4G/Wi-Fi communications.

Figure 1 :
Figure 1: The structure of a wideband HP quasi-omnidirectional antenna: (a) perspective view; (b) package with a radome of blade shape; (c) top view.

Figure 2 :
Figure 2: Input impedance as the vertical slot is excited directly by a voltage source.

Figure 3 :
Figure 3: Return loss as the vertical slot is excited by a tapered patch compared with a voltage-source excitation.

Figure 4 :훽Figure 5 :
Figure 4: Improvement of impedance matching when a flared metal box is used.

Figure 6 :
Figure 6: An archetype of the wideband HP quasi-omnidirectional antenna: (a) front view and (b) right-side view.

Figure 7 :Figure 8 :
Figure 7: Measured and simulated return loss of the wideband HP quasi-omnidirectional antenna.

5
International Journal of Antennas and Propagation DP omnidirectional antennas in terms of overlapped bandwidth and isolation is demonstrated in Table

Figure 11 :
Figure 11: An archetype of the wideband DP quasi-omnidirectional antenna: (a) an archetype and (b) measurement setup.

Figure 12 :
Figure 12: S-parameters of the wideband DP quasi-omnidirectional antenna: (a) S 11 and S 22 and (b) S 12 .

Figure 13 :
Figure 13: Measured radiation patterns of the wideband DP quasi-omnidirectional antenna: (a) port 1 for HP and (b) port 2 for VP.

Table 1 :
Geometric parameters of the wideband HP antenna.