Design of a Crossed Dielectric Resonator-Loaded, Dual-Band Dual-Polarized Differential Patch Antenna with Improved Port Isolation and Gain

Abstract

To meet the urgent requirement for more channel capacity in modern wireless communication systems, antennas with more operation bands are demanded. However, large amounts of antennas suffer from low radiation gains and low port isolation levels. In view of this, a differentially fed, dual-wideband, dual-polarized patch antenna is proposed in this paper. Compared with conventional crossed-feeding structures, the proposed crossed dielectric resonator (CDR) can provide extra resonances with improved isolation levels and radiation gain. Further, four shorting pins are introduced to the radiating patch to help improve the impedance-matching performance. In addition, the proposed antenna also has a very compact size of 0.46λ × 0.46λ × 0.12λ. Finally, a prototype of the proposed antenna is fabricated to validate the design concept. The measured results show that the proposed antenna generates dual wide bands of 1.86–2.52 GHz and 3.26–3.72 GHz for |S₁₁| < −10 dB. High radiation gains of 8.9 ± 0.9 dBi and 10.8 ± 1.2 dBi are also obtained, as well as high port isolation levels of better than 38.4 dB and 36.2 dB at the two bands. The excellent performance of the proposed antenna makes it a promising candidate for 4G/5G wireless communication systems.

1. Introduction

As modern wireless communication devices such as base stations get concentrated in crowded spaces, electromagnetic environments get worse, which leads to unexpected interference between different wireless communication systems. One of the solutions is integrating different functions into one wireless communication device. As a result, antennas are demanded to have wider operation bands or multiple operation bands [1,2,3].

In multi-band antenna designs, parasitic structures [4,5,6,7] and slots [8,9,10,11] are commonly used to generate multiple operation bands. For example, fan-shaped parasitic patches are introduced to circularly polarized patches [4], and additional resonant mode can be excited at a lower band with linearly polarized characteristics. In [5,6,7], stacked patches are utilized to generate extra resonant modes for dual-band operation. In [8] and [9], symmetric slots etching at the edges of radiating patches contribute to a higher resonant mode at the upper band. Except for the above designs, multi-band characteristics can also be obtained by loading shorting pins or stubs [11,12,13]. For example, a dual-band microstrip patch antenna is designed in [12] using four shorting pins and a slot, and the radiation gain reaches 10.2 and 10.0 dBi at the two bands. In recent years, dielectric resonators have been investigated in antenna designs because of their stable, resonant characteristics [14,15,16,17,18]. For example, the higher-order resonance in [14] is obtained by loading copper tape on the surface of a defected DRA. In [15], a coupling slot and a metal strip are used to excite HEM₁₁₁ and HEM₁₁₃ modes with orthogonal polarizations. In [16], higher-order resonant modes are excited using crossed DRA differentially fed by probes, and two operation bands are obtained. In [17], dual-operation bands can also be excited by etching a cylindrical hole in the middle of a cylindrical DRA.

However, very few studies concentrate simultaneously on gain and isolation improvement for dual-band antennas. In view of this, a differentially fed, dual-band, dual-polarized differential patch antenna is developed in this paper. By introducing a crossed dielectric resonator as the feeding structure, extra resonant modes are excited with better impedance matching performance, as well as improved port isolation levels. In addition, the shorted patch antenna can not only excite extra resonances but also work as a 1 × 2 array at the two bands. Thus, the realized gain in the main radiating direction is relatively higher than in other designs. Measured results show that the proposed antenna has two wide operation bands of 1.86–2.52 GHz and 3.26–3.72 GHz for |S₁₁| < −10 dB with high port isolation levels of higher than 38.4 dB and 36.2 dB at the two operation bands. The antenna configuration and detailed design process and analysis are presented in Section 2. The experiment results are listed in Section 3.

2. Antenna Design Process and Analysis

Figure 1 shows the configuration of the dual-band dual-polarized differential antenna. The proposed antenna consists of a CDR with relative permittivity of 9.2, an F4B substrate (the relative permittivity is 2.65, the loss tangent is 0.002), four printed patches, four shorting pins, as well as a metal ground. The shorted patches are printed on the top of the F4B substrate. The CDR is differentially fed by two pairs of coaxial cables with inner conductors of the coaxial cables connected to each edge of the CDR. When port1+ and port1− are differentially fed, vertical polarization is obtained. On the other hand, horizontal polarization can be obtained when port2+ and port2− are differentially excited. Due to the symmetry of the antenna structure, the two differential ports have similar frequency responses. Thus, only the simulated and measured results when port1+ and port1− are differentially fed are displayed in the following analysis.

The evolution process of the proposed antenna is given in Figure 2 to show the detailed design concept. Ant.1 is a conventional patch antenna differentially excited by a bent metal strip. In Ant.2, the metal strip is replaced by a CDR. The proposed antenna has four shorting pins loading on the radiating patches of Ant.2.

The simulated S-parameters and radiation gains for Ant.1 and Ant.2 are illustrated in Figure 3. When Ant.1 is differentially fed, two resonances are obtained at about 1.92 GHz and 3.96 GHz. The isolation level is better than 37.5 dB and 25.2 dB at the two bands, while the realized gains at the two bands are 8.8 and 9.0 dBi, respectively. When the crossed strip is replaced by the CDR in Ant.2, the original two resonant modes move to lower bands, and another resonant mode is generated at about 2.48 GHz. In addition, the isolation level of Ant.2 is 17.5 dB and 16.9 dB higher than that of Ant.1 at the two bands.

Current distributions on Ant.1 and Ant.2 at the two frequencies are shown in Figure 4 for better demonstration. The dominant current of Ant.1 at 1.92 GHz flows along the edges of radiation patches that are close to the feeding structure. The currents flow along the whole feeding structure at 3.96 GHz, which indicates a poor isolation level between the two differential ports. Additionally, the dominant current path of Ant.1 at 1.92 GHz is about twice the current path at the upper band, which corresponds to two different resonant modes. As the dominant currents flow along X-axis, unidirectional radiation patterns can be obtained for Ant.1 at the upper band. When the crossed metal strip is replaced by the CDR in Ant.2, another resonant mode occurs at 2.48 GHz. The current distributions are like Ant.1 at the lower band, as Figure 4 shows. When differentially fed, the resonating mode for the CDR at 3.64 GHz is fixed along the two ports, and the orthogonal mode can hardly be excited, which leads to isolation improvement between the two pairs of differential ports. Additionally, the effective currents flow along the outer side of the radiating patches, which means that the antenna can work as a 1 × 2 array, and radiation gains are relatively higher than that of other patch antennas. Meanwhile, as the impedance matching gets better, the peak realized gains at the two bands increase by 1 dB and 1.2 dB, respectively.

Figure 5 illustrates the input impedance for Ant.1 and Ant.2. It can be clearly seen from the real part that Ant.1 has two resonances. The four resonances of Ant.2 indicate that the CDR offers one more resonance at the upper band. In view of this, a parameter study is developed to validate the influence of the CDR on the impedance-matching performance of Ant.2, as shown in Figure 6 and Figure 7. It can be inferred that both the radiating patch and the CDR provide one resonance at the each operation band. As the additional resonance at the upper band is introduced by the CDR, the resonant frequency is more sensitive to the length and width of the CDR. When L1 or W1 increases, the resonant mode of the CDR at the upper band moves to low-frequency band and fuses with the patch mode. Thus, the impedance matching performance also gets better as L1 or W1 increase.

However, the impedance-matching performance of Ant.2 is still not satisfied. In view of this, four shorting pins are loaded on the radiating patches, which can effectively tune the impedance to a matching status, as shown in Figure 8. As a result, wideband performance with dual resonances is obtained at the lower and upper bands. Additionally, radiation gains of the proposed antenna increase by 0.3 dB and 1.3 dB at the two bands owing to the improvement of impedance matching performance.

To further clarify the influence of the shorting pins, the input impedance of Ant.2 and the final design are investigated, as shown in Figure 9. The four shorting pins have the function of inductors at the lower band, mainly because the length of the shorting pin is 0.09 wavelength. Thus, the capacitive character of Ant.2 can be neutralized by the shorting pins at the lower band. As a result, the impedance matching performance is improved at the lower band. Additionally, as the dominant current path is increased by the shorting pin, resonance for the patch at the lower band moves to a higher frequency band and fuses with the CDR mode. On the other hand, the shorting pin at the upper band is about 0.2 wavelength, which means that it can be used as an open-ended structure approximately. Thus, the shoring pin presents a capacitive character with a low real part. As Ant.2 shows the inductive imaginary part at the upper band with high impedance, the introduction of four shorting pins can help to tune the impedance to a matching status. At last, the proposed antenna exhibits dual operation bands of 1.89 GHz–2.55 GHz and 3.30 GHz–3.76 GHz for |Sdd₁₁| < −10 dB. The realized gains at the two bands are 9.38 ± 0.74 dBi and 10.63 ± 1.44 dBi, respectively. To better understand the influence of shorting pins in the proposed antenna, a parameter study on the position of the shorting pins is made, as Figure 10 shows. As the shorting pin can be regarded as an open-ended structure at the upper band, the dominant current path gets longer at the upper band when d increases, which can be inferred from Figure 4. At the same time, the resonant frequency at the upper band moves towards the CDR mode at about 3.3 GHz. Moreover, as the shorting pin can improve the resonant frequency of patch mode at the lower band, the two resonances get close when d increases. At last, d is chosen to be 15.0 mm as a compromise.

3. Experimental Results

A prototype of the proposed dual-band dual-polarized antenna is manufactured and measured, as shown in Figure 11. Two pairs of 3 dB couplers are designed to test the S-parameters and realized gains of the two ports at both lower and upper bands. Thus, the difference between simulated and measured results mainly comes from manufacturing and testing errors, as well as the introduction of 3 dB couplers.

Figure 12 shows the measured S-parameters of the proposed antenna. The proposed antenna operates at 1.86 GHz to 2.52 GHz and 3.26 GHz to 3.72 GHz for |S₁₁| < −10 dB. Besides, the measured port isolation levels at the two operation bands are higher than 38.4 dB and 36.2 dB, respectively. As Figure 13 shows, the measured radiation gains are 8.9 ± 0.9 dBi and 10.8 ± 1.2 dBi at the two bands, which are close to the simulated results. In addition, insertion loss of the antenna is added due to the introduction of 3 dB couplers. The measured gains are a bit lower than the simulated ones, as well as the radiation efficiency. The measured radiation patterns of the proposed antenna at 2.2 GHz and 3.5 GHz are plotted in Figure 14. Stable and unidirectional radiation patterns can also be observed with low cross-polarization levels at the two working bands.

4. Discussion

A comparison between the proposed antenna and reference antennas is made, as

Table 1. Comparison of dual-band antennas.

Ref.	Radiator Size (λc)	Bandwidth	Polar	Peak Gain (dBi)	Iso. (dB)
[6]	0.29 × 0.29 × 0.097	22%/19.3%	RHCP/LP	8.5/10.8	>40.2/>32.8
[7]	0.48 × 0.48 × 0.085	11.4%/5.1%	±45°/±45°	8.3/10.5	>40/-
[8]	0.49 × 0.49 × 0.13	12.3%/7.6%	±45°/±45°	8.7/9.8	>37/>38
[12]	0.1382 × 0.025	1.6%/2.2%	LP/LP	10.2/10.0	-/-
[15]	0.1362 × 0.31	18.38%/7.01%	LP/LP	6.44/8.98	>36/-
[17]	0.212 × 0.17	10.1%/3.7%	LP/LP	1.18/0	-/-
Pro.	0.46 × 0.46 × 0.12	30.1% 13.2%	VP/HP VP/HP	9.8 12.0	>38.4 >36.2

shows (λc is the wavelength at the center frequency of the lower band). The proposed antenna shows a wider operation band than the other references except for [6] at the upper band. Although the proposed antenna has a higher profile than that of [6,7], the realized gains at the two bands are the highest, especially in designs using dielectric resonators. In general, the proposed dual-band dual-polarized antenna reveals a low profile, wide bandwidth, and high radiation gains.

5. Conclusions

In this paper, a differentially fed, dual-band, dual-polarized patch antenna is proposed. By introducing CDR as the feeding structure of the patch antenna, two additional resonances are obtained, as well as high port isolation levels and gains. Further, the impedance matching performance is greatly enhanced by loading four shorting pins. Measured results show that the proposed antenna has dual wideband from 1.86 GHz to 2.52 GHz and 3.26 GHz to 3.72 GHz for |S₁₁| < −10 dB, which can cover B1/B34/B39/B40 band for 4G wireless communication and N1/N7/N25/N34/N38/N39/N40/N41/N78/N84 band for 5G wireless communication. In addition, the measured gains at the two operation bands are 8.9 ± 0.9 dBi and 10.8 ± 1.2 dBi, respectively, which is 1–2 dB higher than that of conventional dual-band dual-polarized antenna designs. Moreover, the radiator has a very compact size of 0.46λ × 0.46λ × 0.12λ, which indicates a promising candidate for 4G/5G wireless communication systems, such as base stations.