A Compact 4×4 Filtering Microstrip Patch Antenna Array With Dolph-Chebyshev Power Distribution

This paper proposes a compact <inline-formula> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> filtering microstrip patch antenna array with Dolph- Chebyshev distribution implemented by multilayer structure. The proposed antenna array features good filtering performance but contains no extra filtering parts in the feeding or the radiating element. The whole antenna array consists of four <inline-formula> <tex-math notation="LaTeX">$1 \times 4$ </tex-math></inline-formula> antenna subarrays, each subarray is composed of four patch antennas which are coupled-fed by a single microstrip line. Inside the desired passband, by properly choosing the coupling area along the microstrip line, the patches are induced in-phase currents therefore achieving the excellent radiation characteristics. On the other hand, the four patches can’t be properly excited outside the passband which would enable radiation suppression performance. In order to implement a <inline-formula> <tex-math notation="LaTeX">$4 \times 4$ </tex-math></inline-formula> filtering microstrip patch antenna array with low sidelobe level (SLL), four subarrays are combined and fed by Dolphy-Chebyshev excitation method. Finally, the proposed antenna array is implemented, fabricated and measured. The maximum measured gain in the passband is 14.7 dBi, and the radiation suppression outside the passband is better than 25 dB.


I. INTRODUCTION
W ITH the development of wireless communication technology, the RF front ends are required to be miniaturized and multifunctional. Filters and antennas are the critical components in the RF front end and take large circuit area. In recent years, research efforts have been put on the co-design of the filter and antenna to meet the above requirements. In summary, there have been two main approaches to realize the co-design of the filter and antenna. The first way is to embed the filtering characteristic into the feed structure of the antenna [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], such as connecting the filter directly with the antenna which would produce large interconnection loss and take large volume [1], [2]. In [3], [4], [5], [6], the antenna is considered as the last stage or the load of the filter. Capacitively loaded loop based filter [3] and multimode resonators [4] could be integrated with an antenna. A duplexer [5] and the stubloaded resonators [6] are connected to the patch antenna to realize the filtering characteristic. In addition, power divider and baluns [7], stepped impedance resonators [8] and coupling elements [9] can also be utilized for the integration with the antenna. In [10], high quality factor filters are vertically integrated with slot antennas. The second way is to implement the filtering antenna without additional filtering circuits [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Shorting pins [11], driven patch and stacked patch [12], [13], [14], metasurface [15] are utilized to realize filtering antenna. In addition, parasitic loops [16], baluns and reflectors [17] are used to implement the filtering antenna. In [18], a broadband patch antenna with high selectivity is obtained by studying the electric and magnetic coupling between the main patch and the parasitic patches. Without complex structure, filtering response is realized by carefully selecting the coupling region between the antenna and the feed line [19]. In [20], radiation null is introduced by placing four parasitic strips close to the main patch antenna.
In this paper, a 4 × 4 filtering microstrip patch antenna array with low sidelobe level (SLL) is presented. Firstly, a 1 × 4 patch antenna subarray is studied to show the effectiveness of the proposed method. The subarray is composed of four microstrip patch antennas and a microstrip feed line which are located at different layer of the substrates. The patches are series fed by the microstrip line through the aperture on the ground layer. By choosing specific coupling area, the patches would be excited with inphase currents at the desired frequency. On the other hand, the patch antennas can't be excited properly outside the operating frequency band therefore radiation suppression is realized. Furthermore, four subarrays are combined and fed by a Dolph-Chebyshev network to implement a high gain filtering microstrip patch antenna array with low SLL. Finally, the proposed antenna array is fabricated for measurement, the measurement results verify the design.   this paper is Taconic RF-35 with thickness of 0.508 mm and relative permittivity of 3.5. Each subarray is composed of a microstrip feed line and four patch antennas. The feed line and the patch antennas are placed at the top and bottom layer, respectively. The patches are series fed by the feed line though the aperture on the middle layer of the substrate which functions as the ground. The Dolph-Chebyshev power distribution network is utilized to feed the four subarrays to form a 4 × 4 microstrip patch antenna array. Fig. 2 depicts the layout of the 1 × 4 filtering microstrip patch antenna subarray, its working mechanism would be studied and investigated first.

B. FILTERING ANTENNA ARRAY WORKING MECHANISM
According to the transmission line theory, the currents along a transmission line is periodically distributed in the sinusoidal form as shown in Fig. 3. The period is λ g , where λ g is the guided wavelength at working frequency f 0 . The areas with the same current magnitudes and phases are λ g away from each other. Then if the areas with the maximum current distribution (denoted by the grey area in Fig. 3) are chosen along the transmission line to be coupled to the patch antenna, it is expected that the excited currents on the patch antenna have the same phase as along as these adjacent coupling areas are strictly λ g away from each other (the center to center distance of the adjacent patch antenna is λ g ). Consequently, the patch antennas coupledfed by the feed line would radiate in the same direction which would produce high radiation performance inside the working frequency band.
However, the distance between the adjacent coupling area is no longer λ g outside the working frequency band which means the currents in these areas are not in phase any more. Therefore, the currents excited on the patch antenna in this case would not have the same phase either. Then radiation performance would be degraded or the radiation null would be generated when the radiation fields of   each patch antenna are out of phase. Based on the above theory, the radiation inside the working frequency band and radiation suppression outside the working frequency band of the antenna array could be realized just by tricky selection of the coupled feeding area along the transmission line. Fig. 4 illustrates the excited current distribution on the patch antenna. The patches are λ g away from the adjacent ones, λ g is the guided wavelength at 5 GHz. The physical dimensions in Fig. 2 are tabulated in Table 1. Fig. 4 (a) demonstrates the excited currents distributions at 4.3 GHz on the four patches the first two patches present out-of-phase currents compared to the third and fourth ones. The current distribution means the four patches can't radiate collaboratively and suppress each other at the far field. It's the same case in Fig. 4 (b), the current distribution show anti-phase feature at 5.7 GHz. The radiation null would be generated when the exited currents on the patches are out of phase. At 5 GHz, Fig. 4 (c) shows good in-phase current distribution on the four patches, then enhanced radiation is realized at 5 GHz whereas radiation suppression at 4.3 GHz and 5.7 GHz is realized. Fig. 5 plots the simulated S-parameter and antenna gain of the antenna subarray in Fig. 2. In Fig. 5, the impedance matching bandwidth is 100 MHz (|S 11 |<−10 dB). The maximum gain within the passband is 10.5 dBi, and radiation nulls are generated around 4.3 GHz and 5.7 GHz, respectively. Consequently, good filtering effect is realized.

III. MICROSTRIP ANTENNA ARRAY WITH DOLPH-CHEBYSHEV POWER DISTRIBUTION
The above analysis verifies that filtering characteristics could be obtained by selecting specific coupling area along the transmission line to feed the patch antenna. However, since the power along the transmission line attenuates exponentially so that higher gain could not be realized by just simply adding more radiating elements along the feed line. In this section a Dolph-Chebyshev power distribution network is utilized to feed the previously proposed 1 × 4 filtering microstrip patch antenna subarray. Four subarrays are combined and fed by the Dolph-Chebyshev network so that higher gain and low SLL could be realized.
The schematic of the Dolph-Chebyshev network with 4 uniformly spaced subarrays is presented in Fig. 6. The network provides power distribution which is symmetrical about the center of the array. When SLL of the antenna array is chosen to be −26 dB, the current distribution of the network could be determined as [21]: For the subarrays on the both sides of the center, the adjacent subarrays are connected by a network which is composed of four λ g /4 transmission line with impedance of Z 0 , and Z 1 , respectively. In order to achieve the amplitude distribution in (1), it is necessary to analyze the relationship between Z 0 and Z 1 . The four λ g /4 series connected transmission line network could be modeled as shown in Fig. 7, Z A represents the impedance of each subarray. The relationship between the currents and voltages of the two port of the network could be expressed by the ABCD matrix as: Then the currents i 1 and i 2 across the Z A could be related xzas According to (1), then Z 1 = 0.6855 × Z 0 = 34.27 since Z 0 = 50 , and the impedance value Z im of the impedance transformer in Fig. 6 is 35.35 . The physical dimensions in Fig. 6 could be determined as W2 = 1.98 mm, W3 = 1.12 mm, W4 = 1.9 mm.
Based on the above analysis, the design procedure for the proposed filtering antenna array could be explained as: 1) Design the individual radiating patch for the working frequency f 0 ; 2) Place 4 radiating patches along the feed line with center to center intervals of λ g to form the 1 × 4 filtering microstrip patch antenna subarray. The coupling between the feed line and the patches could be tuned by adjusting the aperture dimensions; 3) Design the Dolph-Chebyshev power distribution network following the equation (1)

IV. IMPLEMENTATION AND MEASUREMENT
To verify the feasibility of the design approach proposed above, a filtering microstrip patch antenna array with Dolph-Chebyshev power distribution is implemented and fabricated with a three metal layer structure bonded by two substrates as shown in Fig. 1. The fabricated antenna array is shown in Fig. 8. The comparisons between the simulation and measurement are presented in Fig. 9. In Fig. 9 (a), the measured impedance bandwidth (|S 11 | < −10 dB) is 100 MHz when the center frequency is chosen to be 5 GHz, and the fractional bandwidth is 2%. The proposed antenna array could be applied to the scenario which requires compact RF front-end and narrow bandwidth such as the NB-IoT (Narrow Band Internet of Things). In Fig. 9 (b), the measured maximum gain is 14.7 dBi while the simulated gain is 15.2dBi. The tiny disagreement may be attributed to the fabrication inaccuracy and SMA connector loss. The gain suppression levels at the lower-and upper-stopband are both better than 25 dB which matches the simulation well. Due to the multiple radiation nulls generated at the stopband, excellent roll-off effect is achieved.
At 5 GHz, the co-pol and x-pol realized gain patterns in the xz and yz plane are depicted in Fig. 10 (a) and (b), respectively. In the xz plane, the x-pol level is lower than -45 dB. In the yz plane, the SLL is 25 dB lower than the peak gain in the operating band due to the utilization of the Dolph-Chebyshev power distribution network. Table 2 compares the proposed work with some previous publications.

V. CONCLUSION
In this paper, a 4 × 4 filtering microstrip patch antenna array is proposed. The microstrip feed line, ground metal and the patch antenna are placed at different layers of the substrates. The patch antennas are coupled-fed by the feed line through the aperture of the ground. By choosing the coupling area which can excite in phase currents on the patches inside the working frequency band and out-of-phase currents on the patches outside the passband, then high performance radiation inside the passband and radiation suppression outside the passband is achieved. To further reduce the SLL of the antenna array, a Dolph-Chebyshev power distribution network is utilized to feed the array. The proposed antenna array shows 14.7 dBi measured gain at 5 GHz, radiation suppression level outside the passband is better than 25 dB.