A Novel Printed Monopole Antenna With Folded Stepped Impedance Resonator Loading

In this article, a novel printed monopole antenna with folded stepped impedance resonator (SIR) loading is proposed, designed and fabricated with a standard printed circuit board process. The antenna comprises of a printed monopole antenna and a folded SIR with internal coupling etched on the back of the antenna. By loading this folded SIR as a near-field resonant parasitic (NFRP) element, the resonant frequency of the antenna can be reduced by the strong coupling between the folded SIR and the radiation patch of the printed monopole. After reviewing the theoretical analysis of the proposed printed monopole antenna, a prototype antenna has been fabricated and measured. The simulated results agree well with the measured results, and the prediction performance of the antenna is verified. Therefore, the proposed method in this article is a promising candidate for printed monopole antenna design.


I. INTRODUCTION
With the rapid development of wireless communication systems, antennas as a key component are facing more challenges in terms of lightweight, small size, low cost and ease manufacturing coverage [1], [2]. With the development of integrated circuit and component technology, the size of these devices is decreasing, and the demand for antenna integration is increasing. In recent years, design methods of printed monopole antennas with various structures have been proposed [3]- [10]. For instance: tapered meander line [3], folded meandering branches [4], half-cutting method [5], shorted pin connecting ground [6], meandered split-ring slot [7], loading rectangular patch [8], and coplanar waveguide (CPW) feeding monopole antenna [9], [10].
In [11], a multi-folded taper line monopole antenna proposed. The printed monopole antenna can obtain the impedance matching by folding the space between the strips. Nevertheless, the structure of the antenna is 3-D rather than 2-D, which increases the manufacturing difficulty and cost.
The associate editor coordinating the review of this manuscript and approving it for publication was Tutku Karacolak .
In [12], by half-cutting method, a printed monopole antenna is achieved, but its size is still large. In [13], [14], the meandering techniques are utilized to miniaturize the printed monopole antenna, thereby, making the antenna physically and electrically more flexibility to reduce its size. However, the antenna has narrower bandwidth than the compact monopole antenna by other techniques.
For another, near-field resonant parasitic (NFRP) element have been widely used to design printed monopole antennas [15]. In [16], by loading a pair of Egyptian axe dipole as NFRP element, the printed monopole antenna is designed to be completed. In [17], a printed monopole antenna with a capacitively-loaded loop (CLL) as NFRP element is presented. In [18], by placing a meander line that acts as an NFRP element near the printed monopole antenna, the monopole is realized. In [19], a printed monopole antenna is achieved by a single fan-shaped top-loaded monopole as an NFRP element. In [20], a printed monopole antenna with stepped impedance hairpin resonator loading as an NFRP element has been presented.
This article presents a novel design of the printed monopole antenna. By utilizing the strong coupling between the folded VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ SIR and the radiation patch of the printed monopole antenna, reduction of the resonant frequency of the antenna can be achieved. The folded SIR with internal coupling as a NFRP element is loaded on the back of the monopole antenna, so the size of the antenna is not enlarged. We have carefully reviewed and discussed theoretical analysis and simulated results of the proposed antenna in detail. Finally, the proposed printed monopole antenna is fabricated and measured. The measurement is in good agreement with the predicted results, which verify our design method.  Table 1.

B. DESIGN OF FOLDED SIR
The basic principle of the NFPR antenna is to use parasitic elements in the near field of the antenna to reduce the resonant frequency by the coupling between the parasitic elements and the antenna [15]. In our design, the proposed folded SIR is used as NFPR. In Fig. 2, the folded SIR and its equivalent circuit are presented. Two basic SIRs are connected in parallel by internal coupling, so the equivalent circuit includes two admittance Y is and a J -inverter. Where, Y is can be obtained according to empirical formulas [21], From the basic theory of filter design [22], J-inverter can establish admittance relationship, where, the values of J s can be calculated using the generalized expressions for parallel coupled-lines with arbitrary coupling length in [21].
Y b is the input admittance of Y s in parallel with Y is and can be written as follows where, Z 1 , Z 2 , θ 1 , and θ 2 are the characteristic impedances and electrical lengths, respectively. From formula (3), we can understand that the input impedance Y b can be obtained by specifying the required J s . In our design, J s is realized by internal coupling of the folded SIR, in other words, the gap S 1 .  Fig. 3, the radiation patch of the printed monopole is represented by a lossy shunt resonator composed of R a , L a and C a . Where, R a equivalent to the radiation resistance of the printed monopole antenna. The gap between the radiation patch of printed monopole antenna and folded-line SIR is considered equivalently as J -inverter. As mentioned above, the admittance Y a can be obtained by Y b and J -inverter, where, similarly, the values of J can be calculated using the expression in [21]. Then, the admittance of the 'traditional' printed monopole antenna (i.e., without the folded SIR) is extracted by the electromagnetic simulation software CST. As shown in Fig. 2, Y c can be obtained.
where, G a and B a is the real part and imaginary part of admittance Y c , respectively. Therefore, Y a , Y c and Y in have established a parallel relationship, For convenience, the impedances Z a , Z b , Z c and Z in , which are the reciprocal of the admittance, will be analyzed. The specific sizes of the antenna have been tabulated in Table 1. As showed in Fig. 4, the calculated results of impedance Z a , Z b , Z c and Z in are presented. As shown in Fig. 4, the imaginary part of the impedance Z b is capacitive (the imaginary part is negative), since passes through the J -inverter, the imaginary part of the obtained impedance Z a is inductive (the imaginary part is positive), indicating that their impedance forms a J -inverter. Z in is obtained by capacitance coupling (i.e., inverter J ) between the SIR and printed monopole. Therefore, it also can be seen from Fig. 4 that the resonant frequency decreases from 3.50 GHz to 2.40 GHz, and printed monopole antenna is achieved.
Based on the basic knowledge of filter design, the coupling between the resonators shift the resonant frequencies to the low frequency and high frequency, respectively. The stronger the coupling strength, the more obvious the deviation.
As mentioned above, the resonant frequency varies with the gap between the antenna and the folded SIR. In Fig. 1, we define parameter h 1 as the gap between the antenna and the folded SIR.
Several simulations have been carried out to explain this phenomenon. Under simulation, the size parameters of the proposed antenna are set as follows: l 1 = 37.0 mm, l 1a = 21.2 mm, l 1d = 5.0 mm, l 11 = 14.9 mm, l 12 = 13.5 mm, l 13 = 9.5 mm, l 14 = 11.0 mm, w 1 = 54.0 mm, w 1a = 3.8 mm, w11 = 3.6 mm, w12 = 3.4 mm, and s1 = 0.5 mm. Fig. 5 shows the return loss varied with different thickness h 1 , which can be used to tune the resonant frequency of the monopole antenna. Meanwhile, the simulated S-parameters of a traditional printed monopole antenna without such NFRP is also plotted in Fig. 5. As can be seen from Fig. 5, the resonant frequency of the antenna loaded by the folded SIR continuously moves to the lower frequency band with the decreases of the parameter h 1 . That is to say, the stronger the coupling between the folded SIR and printed monopole antenna (the smaller the gap), the lower the resonant frequency of the monopole antenna. Obviously, in Fig. 5, compared to that of the traditional printed monopole antenna, the resonant frequency of monopole antenna with folded SIR is dramatically reduced. VOLUME 8, 2020

D. IMPROVED VERSION OF THE PROPOSED ANTENNA
Based on the analysis above, the radiation pattern of the proposed antenna has been simulated and drawn in Fig. 6. The E-plane pattern of the traditional printed monopole antenna (i.e., remove the folded SIR with internal coupling) at 2.4 GHz has also been simulated and drawn in Fig. 6 for comparison. It can be seen that the maximum radiation direction of the proposed E-plane pattern shifts a certain angle, which is caused by the asymmetry configuration of the antenna.
The radiation pattern can be corrected by symmetrically loading another identical folded SIR on the top of the printed monopole antenna. The physical layout of an improved antenna and its equivalent circuit are shown in Fig. 7 and Fig. 8, respectively. Compared to that in Fig.3, the equivalent circuit has one more parallel resonant circuit as shown in Fig.8.  Similarly, the E-plane pattern of the improved antenna is simulated and shown in Fig. 9. The specific sizes of the antenna are listed in Table 2.  As seen from the simulated results in Fig. 9, the E-plane radiation pattern is consistent to that of the traditional printed monopole antenna.
As seen from the simulated results in Fig. 9, the E-plane radiation pattern is consistent to that of the traditional printed monopole antenna.

III. MEASURED RESULTS
In order to verify the designed printed monopole antenna, two sample monopole antennas have been fabricated and measured. After optimization by electromagnetic simulation software CST, the size parameters of the antenna are listed in Table 1 and Table 2, respectively. Meanwhile, the photographs of the fabricated antennas are shown in Fig.10.
The reflection coefficient and radiation pattern were measured by Keysight E5071C vector network analyzer and SATIMO near-field antenna measurement system, respectively.
Both the simulated and measured S-parameters are shown in Fig.11. As can be seen from Fig.11, the center frequency of the proposed antenna is about 2.51 GHz, and the corresponding value of S 11 is about -19dB. At the same time, the center frequency of the improved antenna is about 1.85 GHz, and the corresponding value of S 11 is about -20dB.  The results of simulation and measurement are in good agreement.
As shown in Fig. 12, the simulated and measured gain of the improved monopole antenna are proposed. The measured results show that the antenna has a relatively stable gain over the operating frequency range of 1.6 GHz to 2.2 GHz. The maximum gain is about 1.67dBi, which agree well with the simulated result.
As shown in Fig. 13 and Fig. 14, E-plane and H-plane radiation pattern of the proposed antenna and the VOLUME 8, 2020  improved antenna at 2.51 GHz and 1.85GHz are given, respectively.
As can be seen from the Fig. 13 and Fig. 14, the antenna has good omnidirectional radiation characteristics over the operating frequency band and low cross polarization performance.

IV. CONCLUSION
This article presents a novel printed monopole antenna with folded SIR loading. By utilizing the coupling between the folded SIR and the radiation patch of the printed monopole antenna, the resonant frequency of the antenna can be reduced. The omnidirectional radiation patterns are also presented to verify the satisfactory performance of the proposed antenna. Ultimately, a prototype printed monopole antenna is designed, manufactured and measured. The simulated results are in good agreement with the measured results, which provide a valuable verification for our proposed design method.