Ram Horn-Shaped Inspired Folded Compact Antenna for 4G LTE-A and WLAN Portable Mobile Applications

A compact dual-band ram horn-like folded antenna is presented in this work. The antenna is based on a ram horn-like folded strip, asymmetric microstrip feeding (AMF) technique, partial ground, and protruding stub at the ground plane. The dimension of the proposed antenna is 0.11 λ g × 0.17 λ g at 2.3 GHz (10 × 15mm 2 ). The proposed shape is achieved through the combination of two circular arcs with diﬀerent radii. The antenna operates at 2.3GHz and 5.8GHz with a measured bandwidth of 100MHz and 820 MHz, a gain of 0.62 dBi and 2.2dBi, and radiation eﬃciency of 93.67% and 99.87%, respectively. The prototype of the proposed antenna is fabricated and measured. The measured result shows a good agreement with the simulated result. The parametric study of the proposed antenna is performed and results are presented. Besides, a comparative study between the antennas proposed in this work and the state of the art is performed and presented. The proposed antenna is comparatively small in size than all the recently reported works in the literature while ensuring good radiation characteristics. Therefore, the antenna proposed in this work is a better candidate for future portable sub-6GHz ﬁfth-generation (5G), Advance Long-term Evolution (LTE-A), Worldwide Interoperability for Microwave Access (WiMAX), and Wireless Local Area Network


Introduction
e compact multiband antenna has become the darling of the wireless communication community because of the need for miniaturized devices on the part of both the manufacturers and users [1][2][3][4]. Many microwave system researchers have invested effort in realizing compact antennas such as [5][6][7][8][9][10]. Techniques such as meandering, slotting, slitting, shorting pin/plate, lumped elements, and metamaterial/ metasurface [11][12][13][14][15] have been exploited. e main goal of slotting, slitting, shorting pin/plate, and lumped element techniques is to make the antenna effective length longer than its physical length [16][17][18]. However, the meandering technique ensures maxima use of the antenna space [19]. For example, authors in [9] used a meandering strip and shorting pin to achieve a compact antenna (36 × 15 mm 2 ) operating at 2.4 GHz. e same meandering technique has been recently exploited by authors in [5][6][7][8]. Furthermore, the slotting technique has been used by authors in [2,[20][21][22][23][24][25]. For example, authors in [20] used the Audi logo-shaped as a slot on a rectangular patch on a 20 × 12 mm 2 FR-4 substrate to achieve a triband antenna operating at 3.9 GHz, 5.0 GHz, and 7.1 GHz, respectively. Also, authors in [22] use circular nested square slots comparative to an ancient coins symbol in China. e radiating patch was etched on an 88.5 × 60 mm 2 FR-4 substrate and the operating frequencies of the antenna are 1.6 GHz, 2.6 GHz, 3.7 GHz, and 5.3 GHz. In addition, authors in [14] reported the use of metamaterial for the antenna miniaturization. e authors used a double negative metamaterial based on the rectangular split ring resonator as the slot on the ground plane of the proposed antenna to achieve a 50% miniaturization. In the same light, authors in [15] have also used metasurface to achieve 67% miniaturization. In recent times, folded monopole antenna has attracted antenna design engineers due to its ability to reduce overall antenna size [25][26][27][28][29][30][31][32][33][34]. Folded monopole antenna uses the principle of space filling to miniaturize the antenna radiating patch. For example, authors in [34] proposed three folded-rectangular strip branches radiating patch etched on a 20 × 18 mm 2 FR-4 substrate.
2.3 GHz band is one of the most useful communication bands. It was designated International Mobile Telecommunication (IMT) by the ITU. Even though it was originally used for WiMAX applications, it has been generally adopted for the deployment of 4G LTE applications. is band has also been proposed for sub-6GHz 5G communication deployment due to its large TDD band spectrum, good propagation behavior, and provision of better capacity. For example, Singtel Optus, STC, and Telkom in Australia, Saudi Arabia, and South Africa, respectively, are performing 5G trials in this band. In the same light, the 5 GHz band is also one of the commonly used bands in wireless communication such as the 5 GHz band WLAN and ISM. erefore, the miniaturization of the antenna operating in these bands is of importance for future communication.
erefore, in this work, an ultracompact ram horn-like folded dual-band antenna is proposed. e proposed antenna is fed with a 50 Ω asymmetric microstrip feedline. e contribution of this work is the proposal of an ultracompact dual-band antenna suitable for LTE and WLAN applications compared to the recently published works in the open literature.
e remaining section of this paper is divided as follows. e detailed design and analysis of the proposed antenna are presented in Section 2. e results and discussion, which includes S 11 , a parametric study of some design parameters, radiation pattern, gain, and efficiency, are presented in Section 3. e proposed antenna is compared with the recently published works in the literature and presented in Section 4, and the conclusion is presented in Section 5.

Antenna Design and Analysis
e radiating patch of the proposed antenna is a strip folded in a ram horn-like shape, fed with a 50 Ω asymmetric microstrip feedline, and a partial ground with a protruding stub made up the ground plane, as shown in Figure 1. e ram horn-like shape is realized through the concatenation of two slit rings, as shown in Figure 1(c). e antenna is built on a 10 × 15 mm 2 duroid5880 substrate of 1.57 mm thickness. e detailed configuration is given in Figure 1, and the optimized parameters are given in Table 1.
e distance between the edge of the radiating patch and the substrate is 1 mm at the two sides and 0.75 mm at the top. e frequency of a quarter-wavelength strip can be determined by using equation (1) [2,35]. erefore, the expected fundamental mode resonance of the strip can be predicted. e equivalent (total) length of the strip that made up the ram horn is determined from equation (2). e first term at the RHS of equation (3) denotes the circumference of a circle, while the second term denotes the length of the slit of ith ring: where and where e AMF width (w f ) is calculated using the standard microstrip line equations (5)-(9) [35,36]: Since the value of the characteristic impedance is known in this case to be 50 Ω, the width of the microstrip line can be calculated using 2 International Journal of Antennas and Propagation where where L t is the equivalent length of the unfolded strip, L R i is the length of the ith arc-strip, S i is the arc gap of the ith ring, R i is the inner radius of the ith ring, φ i is the angle subtended by ith arc, w f is the width of the microstrip feedline, h is the thickness of the dielectric, Z 0 is the line characteristic impedance (50 Ω), ε r is the permittivity of the substrate, and ε eff is the effective permittivity.

Reflection Coefficient (S11).
e fabrication of the prototype of the proposed ram horn-like folded antenna is shown in Figure 2, which demonstrates the practicability of the proposed antenna radiating patch shape. Figure 3 shows the measured and simulated S 11 results of the antenna proposed in this work. It can be observed that, for both simulated and measured results, two modes are excited which implies that the proposed antenna is a dual-band antenna. It can be observed that, for simulation, the two resonances are 2.3 GHz and 5.7 GHz with a -10 dB bandwidth of 70 MHz and 481 MHz and a return loss of 15.42 dB and 17.6 dB, respectively. It can be observed that there is a shift toward the lower frequency, while the resonance at the upper frequency of the measured S11 is maintained. at is, the measured resonances are 2.3 GHz (2.298 GHz) and 5.8 GHz, respectively. It can also be observed that the measured −10 dB bandwidth is 100 MHz and 820 MHz at lower and upper bands, respectively. In terms of return loss, the measured reflection coefficient at the lower and the upper bands is 13.3 dB and 23.36 dB, respectively. e variation in the results could be traced to the effect of cable and connector losses as well as the fabrication errors. It can be deduced that this antenna can be used in WiMAX and 5 GHz band WLAN applications. In other to study the effect of the ground plane and its protruding stub, the parametric study is done and presented in the next section.

Parametric Study.
e parametric study of the proposed ram horn-inspired shape antenna is carried out to understand the impact of some design parameters and justify the optimized values of the parameter presented in Table 1. e parametric study specifically focused on the ground plane effect on the resonance behavior of the antenna proposed. Figure 4 shows the effect of w g on the resonance behavior of the proposed antenna. It can be observed that an increase in the value of w g leads to a tunning effect towards lower frequency at the upper band. It can also be noticed that, as w g increases from 6 mm, aside from shifting the resonant frequency downward, the impedance matching is also enhanced until 7 mm after which the reflection coefficient starts increasing. 5.8 GHz is the desired frequency at the upper band; therefore, 8.5 mm is taken as the optimized value of g w in the proposed configuration. It can also be observed that variation in w g has no significant frequency tunning effect at the lower band, but it has a pronounced effect on its impedance matching, as shown in Figure 4. e tunning and impedance matching effect of w g is more pronounced at the upper band of the proposed ram horn-like folded antenna than at the lower band. Conclusively, g w can be used to both tune the upper band resonant frequency as well as the impedance matching of both lower and upper bands.

Effect of L g .
As can be seen in Figure 5, L g significantly affects the impedance matching of the proposed ram horn-like folded antenna at both bands. It can be observed that as L g increases, the reflection coefficient increases which implies that 1 mm has the best impedance matching at both frequency bands as shown in Figure 5. Nonetheless, to ensure good ground plane support for the proposed antenna, 1.5 mm has been taken as the optimized value in the proposed configuration. As can be noticed from Figure 5, the highest suitable length of the ground plane to achieve a 10 dB return loss in both frequency bands is 2 mm. erefore, ground length has a tremendous effect on the impedance matching of the proposed structure at both frequencies. Figure 6 shows how S 11 changes with L gs . It can be observed that an increase in L gs not only reduces the impedance matching at both bands but also results in frequency tunning at the upper band. As shown in Figure 6, to ensure a 10 dB return loss at the lower band, the maximum value of L gs should be 4 mm. Although 1 mm has the best impedance matching at the upper band, it does not have the best S 11 at the lower band. erefore, for a reasonable reflection coefficient at both resonant frequencies, 2 mm is choosen as the optimal value in this paper.

Effect of w gs
. w gs does not have a significant effect on the resonant frequency of the proposed antenna, but a small variation can be seen in the reflection coefficient of the antenna as it increases. Although 0.5 mm has the best S 11 at both frequencies, this is more pronounced at the lower band as can be seen in Figure 7. It can be observed that 1 mm has a comparative S 11 with 0.5 mm at both resonant frequencies.
Besides, to avoid significant fabrication error and to further enhance the ground plane support of the proposed antenna, 1 mm was chosen as the optimized value in the proposed configuration. Figure 8(a), the radiation pattern at E-plane and H-plane is bidirectional and omnidirectional, respectively, at 2.3 GHz with a peak of −1.5 dB at 10 o and a 3 dB beamwidth of 70 o . e E-plane and H-plane radiation pattern at 5.8 GHz is a dumbbell and omnidirectional pattern, respectively, with a peak of 2.36 dB at 180 o and a 3 dB beamwidth of 130 o , as shown in Figure 8(b). As shown in Figure 9, a peak gain of 0.62 dBi and 2.2 dBi is achieved at the lower and upper bands, respectively. e peak efficiency at the lower and the upper band is 93.67% and 99.87%, respectively, as shown in Figure 9. With this analysis, the ram horn-like folded antenna proposed shows an acceptable performance despite its compactness.

Current Distribution.
To understand the mode of operation of the proposed antenna, the vector current distributions at the two resonant frequencies are plotted and analyzed. As can be seen in Figure 10(a), a high current concentration is noticed on the folded strip and the feeding branch except at the strip-tip where there is mode cancelation at 2.3 GHz. It can also be observed that the ground    International Journal of Antennas and Propagation significant current around the edge of the ground plane which shows that the ground plane plays an important role in the resonance at the upper band. is is in agreement with the parametric study presented before.

Comparative Study
To validate the compactness of the ram horn-like folded antenna proposed in this work, a comparative analysis along with the recently published works is carried out and presented in Table 2. Guided wavelength size at the lowest resonant frequency has been used for normalization purposes. It can be observed that the proposed antenna is comparatively compact than the recently reported works in literature. It can also be observed that the proposed antenna shows competitive performance in terms of size, gain, and efficiency when compared with the recent works, as shown in Table 2.

Conclusion
A compact ram horn-like folded dual-band antenna for LTE-A and WLAN applications is proposed in this work. e proposed antenna design is based on a folded monopole, partial ground, AMF, and protruding stub. e overall antenna footprint is 15 × 10 mm 2 , and it operates at 2.3 GHz and 5.8 GHz with a gain of 0.62 dBi and 2.2 dBi, and radiation efficiency of 93.67% and 99.87%, respectively. e comparative analysis of the antenna proposed in this work with the state of the art shows that the proposed antenna is the smallest yet maintaining good radiation characteristics. erefore, the antenna proposed in this work is a competitive candidate for future portable wireless communication.
Data Availability e data supporting the findings of this study are all presented within the article.

Conflicts of Interest
ere are no conflicts of interest among the authors.