A Design of Dual-Polarized Composite Patch-Monopole Antenna With Reconfigurable Radiation Pattern

This paper describes a design of dual-polarized composite patch-monopole antenna with reconfigurable radiation pattern. The design consists of a double differential-feed patch loaded with four vertical monopoles symmetrically. The monopoles are connected to or disconnected from the ground plane (GND) by changing the ON/OFF states of p-i-n diodes. By adjusting the connections between the monopoles and GND, the pattern reconfigurability (i.e., one widebeam and three narrow-beam modes) can be achieved for each polarization. The proposed antenna can switch the beam in two dimensions separately <inline-formula> <tex-math notation="LaTeX">$(xz-$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$yz-$ </tex-math></inline-formula> plane). The proposed antenna is characterized computationally and validated experimentally. The measurements result in an overlapping 10-dB return-loss bandwidth of <inline-formula> <tex-math notation="LaTeX">$2.40 - 2.51$ </tex-math></inline-formula> GHz and a port-to-port isolation ≥20 dB in all the modes. In the far field, the prototype can deploy switched beam flexibility. Its radiation characteristics are confirmed by the far-field measurements, which shows the reconfigurable patterns for both polarizations.


I. INTRODUCTION
R ECONFIGURABLE techniques have been one of the key drivers for innovation in antenna designs [1], [2].Reconfigurable antennas can offer the flexibility in terms of operational frequency, radiation pattern and/or polarization.Due to the rapid development of the modern wireless communications, such as 5G, Internet of Things, wireless local area network, vehicle to everything, pattern-reconfigurable antennas have been receiving substantial attention.These antennas can direct the main beam to the desired coverage area, consequently, improving the signal-to-noise ratio and the capacity of wireless communication systems.
Two main methods for establishing the switchable broadside and conical patterns are (i) two patches with different operational modes are co-located and incorporated with switching feed networks [6], [7], [9], [11]; (ii) two different modes of a patch are switched by controlling ON/OFF states of integrated p-i-n diodes [4], [5], [8], [10], [12], [13].The patch antennas with two pattern modes, however, may be insufficient for use in some applications that demands the antenna with multi-directional beams.The conventional patch antenna [3] yields a broadside radiation with narrow beamwidth (i.e., half-power beamwidths (HPBWs) of about 60 • and 70 • in the E-and H-planes, respectively).To realize the reconfigurable radiation pattern with multi-directional beam, the patch antenna has been loaded with alterable-shaped elements using p-i-n diodes, such as parasitic patches [14], [15], complementary split-ring resonator embedded on the ground plane [16], metasurfaces [17], and vertical metal walls [18], [19].In [20], a square patch is incorporated with shorting pins and four varactors to generate continuous beam reconfigurability.These patternreconfigurable antennas with multi-directional beam, however, work for a single linear polarization.Under complex scenarios of the modern wireless communication systems, the singlepolarized antenna may not effectively transmit/receive signals from mobile devices.To address this problem and enhance the capacity, antenna with dual-polarized radiation [21] has been considered as an effective solution.Accordingly, several patch antennas with dual-polarized and pattern-reconfigurable characteristics have been proposed.A two-port patch with ±45 • dual-polarization [22] is loaded with two parasitic patches which are connected to/disconnected from the ground plane (GND) for pattern reconfigurability.In [23], a dualport aperture-stacked patch antenna is incorporated with a pair of reconfigurable cross-shaped strips to bestow threedirectional beams.These dual-polarized antennas yield the scanning beam in one dimension only.
In this paper, a design of dual-polarized composite patchmonopole antenna with reconfigurable radiation pattern is presented.In this design, we aim to achieve a large space coverage with higher gain for both polarizations.The antenna consists of a double differential-fed patch symmetrically loaded with four vertical monopoles which are connected to, or disconnected from the ground plane (GND) by changing the ON/OFF states of switches based on p-i-n diodes.By adjusting the p-i-n diode states, reconfigurable patterns can be achieved for both polarizations.For verification, an antenna operating at 2.45 GHz is designed, fabricated, and measured.Both simulation and measurement results illustrate that the proposed antenna achieves an improved coverage for both polarizations.

II. ANTENNA DESIGN AND CHARACTERISTICS A. MECHANISM OF RECONFIGURABLE PATTERN
Fig. 1 shows the schematic model of a differential feed patch antenna symmetrically loaded with two vertical ∼ λ/4 monopoles in the E-plane.The patch is designed to operate at TM 10 mode.The monopoles can be connected to/disconnected from the GND by controlling ON/OFF states of two switches (S 1 and S 2 ).Accordingly, the antenna can operate in four modes (i.e., one widebeam and three narrowbeam modes), as given in Fig. 2.
In Mode 1 with S1 and S2 OFF, the two monopoles are not induced by the patch, and consequently, the antenna operates as a conventional patch with the radiated field in E-plane [3] as follows: where β is the propagation constant in free space, L is the patch length, and E 1 is the complex amplitude.Equation (1) indicates that the antenna working in Mode 1 yields a broadside unidirectional radiation.In Modes 2 or 3 (S 1 -ON and S 2 -OFF/S 1 -OFF and S 2 -ON), one of the two monopoles is connected to the GND and inducted by the patch.The monopole with switch ON acts as a reflector, whereas the other one with switch OFF acts as a director.Accordingly, the main radiation beam of the antenna is tilted.In Mode 4 with two switches ON, there are two vertical inducted currents (J v1 and J v2 ) appear on the monopoles that have the same amplitude and opposite phase.In this state, the design works as the composite patch-monopole antenna whose total radiated field (E total ) can be synthesized as follows [24]: where E monopole is the radiation pattern of a monopole and AF monopole is the array factor of these two monopoles.
According to [3], the E-plane radiated field of a λ/4 monopole and AF monopole of two monopoles with the same amplitude and opposite phase are given by: where E 2 is the complex amplitude and d is the spacing between two monopoles.By adjusting the ratio of E 1 /E 2 , the composite patch-monopole antenna achieves a very wide beam in the E-plane (φ = 0 • ) radiation pattern [24].Since AF monopole = 0 with φ = 90 • , the monopoles do not affect the H-plane pattern of the antenna.

B. GEOMETRY OF THE PROPOSED ANTENNA
Based on the conceptual design in Fig. 1, we propose a dual-polarized composite patch-monopole antenna with multiple-directional beam, as shown in Fig. 3.The antenna is composed of a double differential-feed square patch, four vertical monopoles, four switches (S1 − S4), two dielectric substrates (Sub. 1 and 2), and the GND.The patch is printed on the top side of Sub. 1, while the GND is placed on the top side of Sub. 2. The two substrates are Roger 4003 sheets (ε r = 3.38, tanδ = 0.0027, and thickness of 0.8128 mm), which are separated by an air-gap of h a .The vertical monopoles are passed through Sub. 1 and connected to the GND via switches.A zoom-in view of a switch is illustrated in Fig. 3(c).Each switch consists of two p-i-n diodes, two resistors (R d = 100 ), two inductors (L d = 56 nH), a capacitor (C d = 100 pF), and two shorting vias.The bias circuits are designed for ensuring forwarded-DC currents and RF-choke; i.e., the resistors are adopted to limit DC current follows to the diodes.The capacitor is to isolate the DC current, but allow flowing of the RF current (the impedance of capacitor is 0.64 at 2.5 GHz).The inductors are used to isolate the RF currents (their impedances are 880 at 2.5 GHz), but allow the forwarded DC current.Surface-mountable SMP1340-079 models from Skyworks Solutions [25] are selected for the pin diodes.In the full-wave simulations, the equivalent circuit of the diode is applied as in Fig. 4. To balance the inducted current on the monopoles, each switch needs two pin diodes.The proposed antenna is characterized using ANSYS Electronics Desktop for the center frequency of 2.45 GHz and its optimized design parameters are given in Table 1.

C. ANTENNA CHARACTERISTICS
As mentioned in Section II-A, the monopoles do not affect the H−plane pattern, and consequently, each pair of Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.In the simulations, differential signals are applied to the two pairs of P 1 + /P 1 − and P 2 + /P 2 − (as shown in Fig. 3) for modeling the double differential feed (i.e., P d1 and P d2 ports).The differential S-parameters of the antenna can be calculated as follows [26]: Fig. 5 shows the simulated |S d1d1 | and |S d2d2 | of the proposed antenna in different operating modes.In Modes 1 and 5, the antenna works as a conventional patch which yields a resonance at 2.46 GHz and 10-dB return loss bandwidth of 2.42 − 2.50 GHz.When one or two monopoles are connected to the GND which generate extra-resonance for the antenna system, and consequently, broaden the impedance matching bandwidth [24].In Modes 2, 3, 6, and 7, the antenna yields two resonances at 2.4 GHz and 2.58 GHz and an impedance matching bandwidth of 2.33 − 2.59 GHz.With the two switches ON, it yields two resonances at 2.33 GHz and 2.60 GHz and a bandwidth of 2.27 − 2.64 GHz for 10-dB return loss.The coupling coefficients among P d1 and P d2 ports are calculated using ( 6) and ( 7) and given in Fig. 6.As analyzed in [27], for double differential feed scheme, the antenna needs to exhibit a structural symmetry across two planes xz and yz for zero coupling.As shown in Fig. 6, the cases with structural symmetry achieve very small coupling coefficients of ≤ −43 dB which is due to the symmetry of switches.When one switch of the two pairs is ON (such as S 1 ON -S 2 OFF and S 3 ON -S 4 OFF), the antenna structure is asymmetrical and yields a less perfect coupling coefficient of ≤ −20 dB.
For better illustrating the reconfigurability, the contour 2D plot of the 2.45-GHz radiation patterns of the proposed antenna are simulated in the different operating modes and shown in Fig. 7.In these plots, the patterns are presented in 3 rd definition of Ludwig [28], i.e., Co-polar (L3X) for Mode 1−4 and Co-polar (L3Y) for Mode 5−8.It is observed that the antenna achieves multi-directional beam with extending coverage for both polarizations, i.e., x-and y-polarization.When the antenna is in Mode 1 (S 1 and S 2 OFF), the TM 10 mode resonance is excited, and therefore, a broadside radiation with narrow beam is achieved for the x−polarization.For Mode 2, the left monopole is connected the GND via S 1 ON, the narrow-beam radiation pattern is deflected at θ = 25 • in the xz-plane.For Mode 3, the right monopole is connected the GND via S 2 ON, the narrowbeam radiation pattern is deflected at θ = −25 • in xz-plane.Based on the operation mechanism of a Yagi antenna, the deflecting angle is mainly determined by lengths (H m ) of reflector and director, and their spacing (d) over to the driven element.In the proposed antenna, these parameters of H m and d, however, considerably affect the impedance matching in the other modes (e.g., Modes 4 and 8).Accordingly, the H m = 24 mm and d = 56 mm are chosen for the final design based on a tradeoff between the maximum deflecting angle and good impedance matching in all operation modes.When the antenna is in Mode 4, the two monopoles on the x axis are connected to the GND via S1 and S2 ON, which causes a widebeam radiation pattern in the xz plane.Similarly, the reconfigurability for the y−polarization is similar to the beam steerable along the yz-plane when the antenna is in Mode 5 − 8, respectively.These results demonstrate that the proposed antenna achieves reconfigurable patterns with wide space coverage in both polarizations.

III. FABRICATION AND MEASUREMENTS
For verification, the dual-polarized composite patchmonopole antenna with reconfigurable radiation pattern is realized and measured.Fig. 8 shows a fabricated prototype of the antenna which has an overall size of 120 mm × 120 mm × 24 mm (0.98λ 0 ×0.98λ 0 ×0.2λ 0 at 2.45 GHz).The double differential feeding network consists of two Wilkison power divider and phase-delay lines, which is designed for the center frequency of 2.45 GHz.Plastic posts and screws are utilized to construct the antenna prototype.A 3-V battery is used to charge DC voltage to control the ON/OFF states of each switch.All DC wires are placed under the GND to avoid undesired effects on the antenna radiation.with structural symmetry, the measured coupling coefficients are less than −30 dB.For the cases with asymmetric structure (such as S 1 ON -S 2 OFF and S 3 ON -S 4 OFF), within the operational bandwidth, both measured and simulated |S 21 | are still better than −20 dB.There is a slight discrepancy between the measured and simulated results in Figs. 9 and 10, which are attributed to small fabrication tolerances, imperfect feeding network, and tolerance of the pin diodes.
Fig. 11 shows the radiation patterns of the antenna prototype with different modes.There is a good agreement between the simulation and measurement which indicate that the antenna achieves the pattern reconfigurability for both polarizations.For each polarization, the antenna achieves beams, including three narrowbeam one widebeam modes.When all switches are OFF (modes 1 and 5), the measurements result in broadside narrow-beam pattern with half-power beamwidth (HPBW) of 68 • and cross-polarization level of ≤ −18 dB at the broadside direction.In Modes 2, 3, 6, 7, the narrow-beam radiation patterns are tilted to θ = ±25 • in both xz and yz planes.These modes yield HPBWs of about 100 • and cross-polarization level of about −10 dB.In these modes, the cross-polarization level is larger than other modes due to the asymmetric structure.When all switches are ON (mode 4 and 8), the antenna achieves a widebeam radiation pattern with HPBW of ≥ 160 • and cross-polarization level of ≤ −19 dB at the broadside direction.
The peak realized gains of the fabricated prototype are given in Fig. 12. Again, the measurement results agreed rather closely with the simulations.Since the peak gain  and HPBW are inversely proportional, there is discrepancies in the peak gain between each operational mode.With the broadside narrow-beam pattern in Modes 1 and 5, the prototype achieves a measured peak gain of 7.0 dBi.For the tilted in modes 2, 3, 6, and 7, the measurements result in the peak gains of about 5.3 dBi.In modes 4 and 8, the widebeam pattern is accompanied by a lower gain which is 3.4 -3.9 dBi across the operational bandwidth.Due to the function limitation of the chamber, the radiation of the antenna have not been measured.Nevertheless, since the measured gains are very close to the simulated values, the simulations should give a reasonable prediction of the of the prototype.In all operational modes of the proposed antenna, the result in radiation efficiency of greater than 90% across its operational bandwidth.
Table 3 shows a performance comparison between the proposed design and the reconfigurable-pattern patch antennas with multi-directional Most of the priors, such as [14], [15], [17], [18], work for a single linear polarization.The dual-polarized antennas [22], [23] yield scanning beam in one dimension.Different [22], [23], our design targets to improve the coverage in the upper hemisphere for both polarizations, thus provides scanning in two a result, the features of the proposed antenna include simple configurable, dualpolarization, reconfigurable with multi-directional beam, and scanning beam in two dimensions.

IV. CONCLUSION
A dual-polarized composite patch-monopole antenna with reconfigurable radiation pattern has been described.It consists of a double differential-feed patch loaded with four vertical monopoles which are connected to the GND vias p-i-n based switches.By adjusting the ON/OFF states of switches, the multi-directional beam reconfigurability, including three narrow-beam and one widebeam modes, can be achieved for both x-and y-polarization.The final prototype with overall-size of 0.98λ 0 × 0.98λ 0 × 0.2λ 0 at 2.45 GHz achieves a bandwidth of 2.40 − 2.51 GHz in eight modes.For each polarization, the radiation pattern can be reconfigured to be either: (i) broadside narrow beam with HPBW of 68 • and peak gain of 7.0 dB; (ii) titled narrow-beam at θ = 25 • and peak gain of 5.3 dB; (ii) titled narrow-beam at θ = −25 • peak gain of 5.3 dB; (iv) widebeam with HPBW of ≥ 160 • and peak gain of 3.7 dB.These features make the proposed antenna a promising candidate for modern wireless communication systems that require large coverage with polarization diversity.

FIGURE 1 .
FIGURE 1. Schematic model of a differential feed patch antenna loaded with switchable monopoles.

FIGURE 2 .
FIGURE 2. Mechanism of the pattern reconfigurability.

FIGURE 7 .
FIGURE 7. Contour 2D plots of 2.45-GHz radiation patterns of the proposed antenna in different operating modes.The range of θ is from 0 • to 180 • along the radius axis, while φ ranging from 0 • to 360 • .

FIGURE 8 .
FIGURE 8. Fabricated prototype of the proposed antenna: (a) perspective view, (b) top view, (c) ground plane with switches, and (d) double differential feeding network and DC-bias.

Fig. 9 shows
|S 11 | and |S 22 | values of the antenna prototypes in different operating modes.Both simulation and measurement indicate that the antenna yields a good impedance matching at the desired frequency band for all operating modes.Its whole pattern reconfigurable performances are yielded within the impedance matching bandwidth of 2.40 − 2.51 GHz for 10-dB return loss.Simulated and measured coupling coefficients of the prototype in different modes are illustrated in Fig. 10.In all cases

FIGURE 9 .
FIGURE 9. Simulated and measured reflection coefficients (|S11| and |S22|) of the antenna prototype in different operating modes.

FIGURE 10 .
FIGURE 10.Simulated and measured coupling coefficients of the antenna prototype.

FIGURE 11 .
FIGURE 11.Normalized 2.45-GHz radiation pattern (in dB scale) of the antenna prototype in different modes.

FIGURE 12 .
FIGURE 12. Peak realized gains of the antenna prototype in different modes: (a) P1 is excited, (b) P2 is excited.

TABLE 2 . Radiation pattern of the antenna in eight modes.
switches (i.e., S 1 -S 2 or S 3 -S 4 ) can be used to control the radiation pattern in the xz-and yz-planes separately.The proposed antenna can operate in eight reconfigurable modes by switching ON and OFF states of the two pairs of switches.Its possible radiation patterns and the corresponding switch states are given in Table2.It is noted that for Mode 1 − 4, S 3 and S 4 do not affect the antenna performance and are set to be OFF (similarly for S 1 and S 2 for Mode 5 − 8).