Design and Investigation of Differential-Fed Ultra-Wideband Patch Antenna with Polarization Diversity

A novel singleor dual-polarized ultra-wideband (UWB) patch antenna fed by coupled feedingmechanism is proposed.The singlepolarized antenna consists of a square ring patch and two Γ-shaped patches which are coupled to the radiating patch. The vertical portions of the Γ-shaped patches are connected to the microstrip lines which are printed on the bottom layer of the grounded FR4 substrate. To realize the differential feeding mechanism for enhancing the polarization purity, a tapered balun is employed to excite the antenna. Further to provide dual linear orthogonal polarizations, another pair of Γ-shaped patches is added in the singlepolarized UWB antenna.The dual-polarized UWB antenna prototype can achieve two orthogonal polarizations with an impedance bandwidth (|S 11 | ≤ −10 dB) of 113% and isolation of over 25 dB across the entire frequency band.


Introduction
The development of UWB radars has received significant interests, particularly after the Federal Communications Commission (FCC) designated the frequency band from 3.1 to 10.6 GHz for unlicensed use, as high-resolution sensing and imaging can be realized due to the large bandwidth.UWB radars can be applied for indoor and outdoor environments, including medical imaging for cancerous tissue detection, detection of human beings behind the wall and ground penetrating radar for mine detection [1,2].Moreover, dualpolarized UWB antennas have substantial advantages in comparison with linearly polarized antennas, as the performance of the UWB radar can be improved by using an appropriate polarimetric method [3].For radar applications, the proper antennas are required to achieve a wide impedance bandwidth with dual orthogonal polarization, high port-toport isolation, stable directional patterns, and compact size.
It is well known that patch antennas have been widely utilized in wireless communication systems and radar systems mainly due to their characteristics of low profile, easy fabrication, and low-cost.However, patch antennas can only operate in a limited frequency bandwidth which can not satisfy the FCC allocated UWB bandwidth.Consequently, various methods have been proposed for enhancing the bandwidth of dual-polarized patch antennas.The most used methods include optimizing the shapes of feeding probes [4][5][6], multiple radiating patches [7,8], and aperture-coupled feeding mechanism [9][10][11].The dual-polarized magnetoelectric dipole antenna in [6] is fed by four Γ-shaped probes and can achieve stable radiation characteristics across a wide frequency range.However, the relative operating bandwidths of the listed dual-polarized patch antennas [4][5][6][7][8][9][10][11] are less than 40%.Recently, a popular method for designing a dualpolarized antenna is developed from the uniplanar monopole antenna which can work from 3 to 10 GHz.The planar dualpolarized antenna is obtained by employing two identical monopoles with one rotated by 90 ∘ [12].However, the antenna achieves doughnut-shaped patterns with omnidirectional radiation in the H-plane and bidirectional radiation in the E-plane, thus leading to low antenna gain.In addition, the pattern is split at higher frequencies and unstable across the entire frequency band though the impedance bandwidth can meet UWB regulations.Another promising design is to employ two Vivaldi antennas with cross-shaped configuration to achieve a wide bandwidth [13], while it requires a large space for accommodating the structure.A differential-fed slot-loaded patch antenna proposed in [14] can operate across the FCC UWB band with dual orthogonal polarizations and the impedance matching can be tuned by adjusting the tilt angle between the triangular feeders and the ground plane.Good impedance matching is obtained when the tilt angle is very small, thus leading to a large ground plane for supporting unidirectional radiation.
In this paper, a novel coupled-fed single-or dualpolarized patch antenna is proposed.The coupled feeds are applied for ensuring good impedance matching across the UWB frequency range.The proposed dual-polarized antenna achieves an impedance bandwidth of over 110% (| 11 | ≤ −10 dB) and port-to-port isolation of above 25 dB by adopting the differential feeding technique.The rest of the paper is organized as follows: the design of the single-polarized UWB patch antenna is demonstrated in Section 2. The parametric study of three key parameters has also been carried out in this section to understand the operating principles and choose the proper values of the dimensions.Section 3 presents the design of the dual-polarized UWB patch antenna, and both simulation and measurement results are given.A conclusion is drawn in Section 4.

Single-Polarized UWB Antenna Design
The configuration and geometry of the proposed singlepolarized UWB patch antenna are presented in Figure 1.As shown in Figure 1(a), the radiating square ring patch is coupled by two identical Γ-shaped patches which are symmetrically located with respect to the center of the structure.Each Γ-shaped patch is formed by a horizontal rectangular patch and a vertical isosceles trapezoidal patch.The square ring patch has an outer side with length of  1 and an inner side with length of  2 .It is worth mentioning that the lowest operating frequency of the antenna is mainly determined by the dimensions of the radiating patch.Moreover, the impedance matching is tuned by adjusting the dimensions of the Γ-shaped patches and the gap between the inner sides of the radiating patch and top sides of the Γ-shaped patches.A pair of microstrip lines are fabricated on the backside of the grounded FR4 substrate with thickness of ℎ 2 and connected to the bottom sides of the Γ-shaped patches by a conducting via.In addition, a piece of Rohacell foam with thickness of ℎ 1 is attached on the ground to isolate and support the vertical parts of the Γ-shaped patches.Furthermore, a tapered balun [14] is applied to feed the antenna and play a role as differential feed to suppress cross-polarization and enhance port-to-port isolation.As illustrated in Figure 1(b), the balun is fabricated on a 1.6 mm thick FR4 substrate and perpendicular to the ground plane.The top sides of the balun are attached to the microstrip lines while the bottom sides are soldered to a 50 Ω SMA connector.Figure 1(c) can also show that the feeding balun is mounted perpendicularly to the ground plane and parallel to the vertical parts of the Γshaped patches.
Since the dimensions of the square ring patch and Γshaped patches are the main factors in affecting the operating frequency range of the proposed antenna, the three parameters  1 , , and  1 are selected to implement the sensitivity study and understand the operating principles.The Ansoft HFSS 15.0 using Finite Element Method has been applied to implement the simulation and optimization of the proposed design.The effects of the three parameters on the reflection coefficient of the design are simulated and shown in Figures 2-4. Figure 2 exhibits the influence of the outer side of the square ring patch  1 on the reflection coefficient versus frequency.As expected, the proposed UWB antenna can work across the frequency range from around 3 to 11 GHz and the lowest operating frequency is shifted from 3.25 to 3.08 GHz when the length of the outer side increases.In addition, the impedance matching in the lower band is improved while kept unchanged in the upper band.In this work, the dimension of  1 is chosen to cover the required lowest operating frequency.The effect of the gap between the inner side of the square ring patch and the top part of the Γ-shaped patch is shown in Figure 3.It is noted that the impedance matching in the lower band can be improved and the reflection coefficient is reduced as the gap decreases from 2.1 to 0.1 mm.Also it is observed that when the gap is removed ( = 0 mm), the reflection coefficient from 4 to 7 GHz is further lowered while it becomes large abruptly in the band from 3 to 4 GHz.For achieving good impedance matching across the whole frequency band, the dimension of  is selected as 0.1 mm when considering the fabrication tolerance.Figure 4 presents the effect of the bottom side of the Γ-shaped patch  1 on the reflection coefficient.It is observed that it can change the reflection coefficient in both lower and upper bands.The reflection coefficient decreases in the lower band and increases in the upper band when  1 changes from 10 to 16 mm.

Dual-Polarized UWB Antenna Design
Based on the design and analysis of the single-polarized UWB patch antenna in previous section, a compact UWB antenna with dual orthogonal polarizations is obtained by adding another pair of Γ-shaped patches.As illustrated in Figure 5, the four Γ-shaped patches are symmetrical with respect to the center of the dual-polarized UWB antenna and International Journal of Antennas and Propagation   the baluns are mounted to center of the feeding lines and perpendicularly to the ground plane.
A prototype has been fabricated and measured to validate the proposed design principle.The simulated and measured -parameters of the proposed antenna are presented in Figure 7.The measured results confirm that Port 1 has over 115% impedance bandwidth (from 2.7 to 10.1 GHz) for | 11 | ≤ −10 dB while Port 2 achieves an impedance bandwidth of 113% (from 3 to 10.8 GHz).It is also noted that the measured mutual coupling between the two ports is less than −25 dB across the entire frequency range and the simulation is less than −30 dB.Ideally, the top sides of the feeding baluns should be precisely attached to ends of the feeding lines on the ground plane.However, the fabrication may lead to some tolerances which will deteriorate the performance of the baluns.This is coherent with the phenomenon that the measured isolation is larger than the simulation, as shown in

Figure 2 :
Figure 2: Effect of the outer side of the radiating patch  1 on the reflection coefficient.

Figure 3 :Figure 4 :
Figure 3: Effect of the gap  on the reflection coefficient.

Figure 7 :Figure 8 :
Figure 7: Simulated and measured results of -parameters for the dual-polarized UWB antenna.

Figure 7 .
The measured copolarized patterns at three different frequencies in both principal planes are illustrated in Figure 8.As noticed, both polarizations exhibit similar radiation