A Novel Negative Meander Line Design of Microstrip Antenna for 28 GHz mmWave Wireless Communications

The increasing applications for nomadic computing have experienced enormous development over the preceding decade. This has eventually caused the lack of bandwidth. Therefore, to accomplish the need of consumers, compact antennas shall be designed for mmWave wireless communications. Consequently, this paper presents a novel negative meander line based microstrip antenna system being composed of inductors (L) and capacitors (C). A detailed impedance analysis of the configuration is reported. The effects of changing the radiating element’s width and length on the resonant frequency have been studied. The finalized arrangement involved 243 sq. mm area and functioned at 28 GHz with a bandwidth of 2.16 GHz. At resonant frequency, the system exhibited gain and efficiency values of 8.40 dBi and 83.51%, respectively. Furthermore, the proposed design demonstrated better bandwidth and gain capabilities in comparison with the conventional microstrip patch antenna and meander line antenna.


Introduction
The development of LTE-Advanced was predominantly driven by need for internet services [1], [2]. Still, superior capacity networks are desired to accommodate the demands of massive population. In addition, new data services and traffic types are emerging to support concepts like the e-health or smart grid. The next-generation system drivers are expected to be more diverse by supporting these applications seamlessly together with existing internet and voice services. Furthermore, the future devices are predicted to adopt new portions of spectrum where much more bandwidth is available such as the 28 GHz mmWave band.
The antenna is regarded as a critical component in any wireless communication system due to its capability to transmit or receive electromagnetic waves [3]. The most conventional version of an antenna is the microstrip arrangement, which involves a metallic patch and a ground plane printed on either side of a dielectric substrate. These printed antennas are of active interest because of their effortless manufacturing procedure, mechanical robustness and all that. However, they suffer from some limitations including large size, narrow bandwidth and low gain. Research clarifies that wide antenna bandwidth can be accomplished by employing several techniques for instance, optimizing impedance matching [4], mounting parasitic elements [5], incorporating thick substrates [6], integrating multiple resonances [7], exercising low effective permittivity substrate [8], and employing reactive impedance surface [9]. However, these methods experience a few notable complexities like spurious coupling, larger extent, reduced efficiency and low gain.
Recently, scholars have presented several printed antenna design procedures for mmWave wireless communications. Thomas et al.
[2] demonstrated a method of combining rectangular and circular patch to achieve a gain of 6.62 dBi only. Similarly, Jandi et al.
[10] utilized a rectangular patch. In this antenna system however with compact dimension, the resultant bandwidth and efficiency did not appear to be optimum. In another approach, Yoon and Seo [11] exercised multiple slots in the radiating patch but found that a bandwidth of 3.35 GHz and a gain of 13 dBi can be achieved at the cost of increased board area.
Generally, the conductors in a meander line approach are fashioned by a sequence of sets of right-angled bends to obtain configuration simplicity, large bandwidth and to reduce the radiating element's overall size [12][13][14][15][16][17]. However, this arrangement suffers from poor radiation efficiency [18]. An efficient solution to overcome this shortcoming and enhance the transmission magnitude is to etch periodic slots on the metal plate [19]. Therefore, this paper presents a novel miniature design of radiating elements

Design and Impedance Analysis
The geometry of negative meander line antenna system is shown in Fig. 1(a) and 1(b). CST Microwave Studio software [29] has been utilized for simulation purposes. Taking into account the two adjacent unit cells illustrated in Fig. 1(c), the two microstrip line patches will be at potentials ±V contributing to an increase in the gap capacitance (C). The current flow through the microstrip line assembly acts as an inductor with inductance (L). The equivalent circuit of the adjacent microstrip line patches can be indicated as specified in Fig. 1(d). Duroid RO4003C microwave substrate with a thickness of 0.813 mm and dielectric permittivity of 3.38 was utilized to construct the system as depicted in Fig. 1(e). Their design parameters are tabulated in Tab. 2. A 2.4 mm series blunt post connector, model no. HPC4312-12, manufactured by A-Info Inc was used for the antenna prototype.
The value of capacitance can be determined with the help of Fig. 1(c) where, a line z = 0 is assumed at the middle i.e. between the microstrip line structures. Considering the 1 mm distance d between microstrip line patches, the gap capacitance per unit length can be estimated from the subsequent equation [30], [31], and z 1 , z 2 are the z-coordinates of the edges of the microstrip line as depicted in Fig. 1 After neglecting the higher order terms of K(z), the above equation can be written as, Considering the length of every single microstrip line to be s with half of the electric field in air and the remaining field in the dielectric substrate [31], the total capacitance can be obtained using the following expression, The surface impedance η = jν from the equivalent circuit representation in Fig. 1(d) can be plotted by using the successive equations [31],

Magnitude of S11
The operating principle of negative meander line antenna system was studied by concentrating on four categories (A, B, C and D) that we have defined. For a Category-A Antenna, the overall dimension of the system was decided by varying the number N of capacitive fingers from one to nine (Fig. 2). Here, the width (A2) and length (B2) of the inductive finger were kept stable at 8 mm and 7 mm, respectively. The desired result of 28 GHz operating frequency was yielded when nine capacitive fingers along with one inductive finger were employed. A rise in the total inductance was observed with an increase in the wire's length. Even though adding fingers witnessed a decrease in the overall capacitance.
For a Category-B Antenna, the total size (27 × 9 mm²) of the system was kept constant, while the width (A2) of capacitive or inductive fingers was allowed to vary. Nine distinct sizes from 1 mm to 9 mm were considered (Fig. 3). An optimum outcome was observed for A2 = 8 mm.
For a Category-C Antenna, the length (B1) and width (A2) of capacitive fingers were fixed to 1 mm and 8 mm, respectively. Also, the width (A2) of inductive finger was set to 8 mm. However, the length (B2) of the inductive finger was varied from 1 mm to 9 mm (Fig. 4). It is evident from the magnitude of S11 curves that B2 = 7 mm is ideal for 28 GHz resonance.
For a Category-D Antenna, the absolute extent of system was set to 243 mm 2 . However, parametric simulation analyses were performed for deciding the length (B1) of capacitive fingers. Four unique cases were examined notably 0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm (Fig. 5). The arrangement with B1 = 1.0 mm showed magnitude of S11 value of -23.866 dB at 28 GHz and covered the frequencies of 27.039 GHz to 29.199 GHz.
Briefly, a comparable effect on the overall capacitance was observed for all the categories of antenna. The results illustrated in Figs. 2 to 5 prove that the anticipated   . The finest results were accomplished for (4, 13.5) mm. Consequently, an antenna was constructed by employing these optimum dimensions. Their measurements were performed by utilizing Agilent's N5245A PNA-X Microwave Network Analyzer (Fig. 6).

E-Field Vector and Surface Current Distributions
The simulated e-field vector and surface current distributions of the concluded configuration at 28 GHz are shown in Fig. 7(a) and 7(b), respectively. A coaxial feed probe was employed to excite the system and yield the charge distributions. This converted the TEM mode into the parallel plate mode across the probe aperture and thereby confined the higher-order mode's evanescent waves. Furthermore, the zero-order mode propagated away to excite the antenna's TM10 resonant mode. A charge density was witnessed around the edges due to the repulsive force between these positive charges. In this way, the fringing fields produced by the charges led to radiation. Briefly, the vital electric fields were produced from the capacitive fingers whereas the ground plane participated in suppressing the back radiation which ultimately improved the impedance matching of the radiating element.

Performance Comparison of the Proposed Design with the Conventional Antenna Systems
Generally, meander line antennas are designed by bending the arms of a microstrip dipole at right angles to establish an electrically larger radiating element in a smaller area [12], [13]. In order to evaluate the performance of the proposed negative meander line antenna, a conventional microstrip patch antenna and a meander line antenna was designed using CST Microwave Studio software [19]. The typical configuration of a meander line antenna, described in Fig. 10(a) and Tab. 2, was obtained by replacing the microstrip line patches of the proposed antenna with slots and vice versa. Additionally, a conventional microstrip patch antenna, illustrated in Fig. 10(c), was designed by employing equations mention in [32][33][34]. Duroid RO4003C with a thickness of 0.813 mm and dielectric permittivity of 3.38 was utilized as the substrate. It is evident from the results shown in Fig. 10

Conclusion
In this paper, a novel negative meander line antenna was presented to be used in the 28 GHz mmWave frequency band. The system occupied 27 × 9 mm 2 of board area and overall height of 0.813 mm. The manufactured antenna was tested in a laboratory which demonstrated an accurate simulation and measurement outcomes agreement. The results confirmed a superior bandwidth and gain values compared with the conventional microstrip patch antenna and the meander line antenna. In addition, the proposed system involved a simple layout which ultimately confirms an appreciable choice for incorporation with the recent compact devices.