Super‐wideband two‐arm antenna for future generations of mobile communications

In this paper, a modified rhombic‐shaped wide‐flare two‐arm antenna is proposed for super‐wideband applications of the next generations of mobile technologies. To enhance antenna input impedance and to improve its performance, the copper arms are lithographed on a dielectric substrate with multilayers and fed through a wideband balun that is designed to feed the balanced two‐arm antenna through the conventional (unbalanced) coaxial line. The proposed antenna is fabricated for experimental validation of the simulation results. It is shown through electromagnetic simulation as well as experimental measurements that the proposed antenna is operational in the range of frequencies ( 2.3‐23 GHz $2.3 \mbox{-} 23\unicode{x0200A}\mathrm{GHz}$ ). Parametric study is performed to obtain the optimum design of the proposed antenna. The distributions of the surface current on the antenna arms at different frequencies are presented and discussed for more understanding of the antenna operation over the entire frequency band. It is shown that the antenna has a radiation efficiency of greater than 96%, percentage bandwidth of 164%, ratio bandwidth of 10, bandwidth‐to‐dimension ratio of 1360, and maximum gain that exceeds 5 dBi $5\unicode{x0200A}\text{dBi}$ .


| INTRODUCTION
Antennas with a ratio bandwidth (RBW) of greater than 10 : 1 are known as super-wideband (SWB) antennas.When compared with the ultra-wideband (UWB) technology, the SWB technology has more facilities for high data rate transmission, can realize higher channel capacity, and achieve better time accuracy.][3][4][5] Many recent publications have been interested in SWB antennas.For example, in Azim et al., 3 an SWB antenna of a planar structure with two asymmetric ground planes to operate over the frequency band of  is presented. Also, a small planar SWB antenna over a frequency-selective surface is introduced in Kundu and Chatterjee 6 to operate from 2.8 to 40 GHz. In Balani et al., 7 a compact planar monopole patch antenna is proposed for SWB operation from 1.2 to 47 GHz.
In the present work, an SWB antenna of high efficiency is introduced.The antenna is constructed as a two-arm planar structure on multilayered dielectric substrate.Three design techniques are combined together to produce an SWB antenna with high efficiency over the entire frequency band.First, each of the antenna arms is shaped like a rhombus with a wide-flare angle.Second, each arm is capacitively coupled, near its end, to a parasitic circular sectorial element.Third, the dielectric substrate is made of the layered substrate with different dielectric constants to create a graded dielectric profile, where the upper and lower layers have very low loss tangent and are much thinner than the middle layer.Also, the shape of the substrate is cut exactly the same shape of the conducting antenna arms.Fourth, the proposed balanced two-arm antenna is fed using a broadband balun which is connected from one side to the antenna and from the other side to conventional coaxial unbalanced line.The employment of these four techniques in one design results in an antenna that is SWB with almost 100% radiation efficiency along the operating range of frequencies.
After the present section, the paper is arranged as mentioned below.In Section 2, the proposed antenna design is explained.Section 3 introduces the most important numerical and experimental results concerning the assessment of the antenna performance regarding the bandwidth, radiation patterns, gain, and radiation efficiency.Section 4 presents a comparison between the two-arm antenna and antennas found in the literature.In Section 5, the conclusions of the work are conducted.

| ANTENNA DESIGN
To attain the SWB operation, the proposed antenna has the structure of two planar arms with a modified rhombus shape of wide-flare angle and blended corners as shown in Figure 1A,B.Near the end of each arm, a parasitic element of circular sectorial shape is capacitively coupled to the arm through a narrow gap.The purpose of these parasitic elements is to increase the operating bandwidth.The conducting arms of the antenna and the parasitic elements are supported on a three-layer substrate whose structure is depicted in Figure 1C.One arm is lithographed on the uppermost layer of the substrate whereas the other arm is lithographed on the lowermost layer.The substrate material for the top and bottom layers is Rogers RO3003 with ε r1 = 3 and thickness h = 0.13mm efficiency.The substrate is cut exactly the same shape and dimensions of the copper arms and the circular pads of the balun for minimizing the losses in the dielectric material and thereby increasing the radiation efficiency.As the proposed antenna has a balanced two-arm structure, a balun is required to feed by the conventional (unbalanced) coaxial line.This balun should provide wideband impedance matching to cover the desired SWB operation within the operating range of frequencies from 2.3 to 23 GHz.The proposed balun consists of two annular pads with diameters D L and D U .The bottom pad of the balun connects the coaxial line clad copper to the lower arm of the antenna whereas the upper pad connects the coaxial line core to the upper arm of the antenna as shown in Figure 1D.

| Geometrical parameters of the proposed antenna
The detailed geometrical parameters of the proposed design are presented in Figure 1.The optimum design dimensions are listed in Table 1.These optimum dimensions have been obtained through a complete parametric study through electromagnetic simulation to realize the best antenna performance.Some examples of the most important simulation results concerned with the parametric study that has been achieved to get the optimum design of the proposed antenna are presented and discussed in Section 2.2.

| Parametric study for optimum design of the antenna
In this section, the effects of the most important geometrical parameters of the antenna on its performance are studied and their best values are obtained.The most effective parameters of the antenna arms are the antenna length, L B (twice the arm length), and the flare angle, Ψ.On the other hand, the most effective balun parameters are the diameters of the upper and lower circular pads, D U and D L , respectively, and the vertical separation, L P between them.In the following paragraphs, the effects of these five parameters on the antenna performance are investigated through electromagnetic simulation.
The frequency response of reflection coefficient magnitude, S | | 11 , is presented in Figure 2 for different values of the antenna length, L B .Regarding the impedance-matching frequency band, it is shown that the lower and higher frequencies are decreased with increasing L B ; that is, the entire frequency band is shifted to the left.Also, it is found that the maximum RBW is 10 and is obtained for L = 5.7cm B .The effect of the flare angle, ψ, on the frequency response of reflection coefficient magnitude, S | | 11 , is presented in Figure 3.It is shown that the impedancematching frequency band of the proposed antenna is affected by changing ψ such that the lower and higher frequencies are increased with increasing ψ; that is, the entire frequency band is shifted to the right.Also, it is found that the RBW has its maximum value (about 10) when ψ = 125°.
The effect of changing the vertical separation, L P , between the upper and lower circular pads of the balun (the substrate height) on the frequency response of S | | 11 is investigated through electromagnetic simulation where the results are presented in Figure 4.It is shown that the lower frequency of the impedance-matching frequency band is almost unaffected by changing L P .However, the higher frequency is significantly decreased with increasing L P .It is found that the maximum RBW is obtained for L = 2.6mm P and the resulting frequency band is 2.3-23GHz.
As described in Section 2.1, the diameters D U and D L of the upper and lower circular pads of the feeding balun, respectively, affect the impedance-matching bandwidth of the two-arm antenna.They should be carefully set to The best values for the geometrical parameters are presented in Figure 1 for the optimum design of the proposed antenna.minimize the magnitude of the reflection coefficient, S | | 11 , over the desired frequency band for impedance matching.In Figures 5 and 6, it is shown that changing the diameters D U and D L affects the value of S | | 11 rather than the lower and higher frequency limits of the frequency band.It is shown that setting D U = 1.9 mm and D = 6mm L results in perfect impedance matching over the desired frequency band .
Achieving a complete parametric study including the effects of the five parameters as presented above and the other dimensional parameters indicated in Figure 1, the optimum design of the antenna is obtained.The final values of the geometrical parameters of the antenna are those listed in Table 1.

| SIMULATION RESULTS AND EXPERIMENTAL VALIDATION
This section presents the results of the practical measurements and the evaluation of the proposed antenna performance regarding bandwidth, gain, and efficiency.Some of the electromagnetic simulations using CST are validated by comparison to the experimental results.

| Antenna fabrication and measurement of the reflection coefficient
A preliminary model of the designed SWB antenna is fabricated for experimental validation of the antenna achievements.A coaxial connector of SMA type is soldered at the antenna input as presented in Figure 7A.The ground copper of the SMA is soldered to the lower circular pad of the feeding balun whereas the inner copper pin is soldered to the upper circular pad.The reflection coefficient of the antenna is measured by the vector network analyzer model Keysight N9918A as shown in Figure 7B.
The magnitude of S 11 is presented in Figure 7C.Both the practical measurements and the electromagnetic simulation show that the impedance-matching frequency band (for S | | < −10dB

| Surface current distribution
To operate over the superwide frequency band (2.3-23 GHz), the antenna should have different radiation mechanisms to cover such a wideband of the frequency.For deeper insight and more understanding of the antenna operation, the current distributions are presented and discussed at different frequencies over the operational frequency band.Figure 8 shows the surface current distributions on the antenna arms at the frequencies 5, 8, 14, and 22 GHz.First of all, it is clear that the current level at the feeding port is almost constant at the four frequencies, which indicates that the current level is almost constant over the entire frequency band.This ensures that the impedance is perfectly matched over this band.At 5 GHz, the current distribution exhibits first-order variation along the antenna arms as the current maintains the same direction over the arms as shown in Figure 8A.At 8 GHz, the current direction is reversed once in each arm, Figure 8B, indicating second-order variation along the antenna.At 14 GHz, the current direction is reversed twice along each arm indicating third-order variation as shown in Figure 8C.Finally, Figure 8D shows that the direction of the current is reversed three times along each arm indicating fourth-order variation at 22 GHz.It is clear that the peak value of the surface current is always at the feed point to maintain the impedance matching irrespective of the frequency.Also, it is shown that the parasitic element at the end of each arm helps extend the area of the surface current especially at the frequencies around the middle of the operational band; see Figure 8B,C, to maintain the impedance matching at these frequencies.
In view of the above discussion of the surface current distribution, it becomes clear that the proposed antenna has got its broadband radiation characteristics owing to the geometrical design that endowed the antenna with its ability to change the mode order of the current distribution (and, hence, the near field) with varying the frequency over the entire band.

| Far-field patterns
The far-field radiation patterns in the principle planes are given in Figure 9A,B.It is clear that the far-filed patterns calculated by simulation come in good agreement with the experimentally measured patterns.Owing to their uniformity, such radiation patterns are suitable for many applications as single radiators, elements of antenna arrays, and elements for multiple-input and multiple-output antenna systems to provide various types of antenna diversity for the next generations of mobile communications.

| Antenna gain and efficiency
The variation of the maximum gain and the radiation efficiency of the SWB antenna over the frequency band (2.  are presented in Figure 10A,B, respectively. The experimental measurements show good agreement with the electromagnetic simulations. Oing to the optimized design, the maximum gain is greater than 2.5dBi at the lower frequencies of the impedance-matching frequency band and ranges between 4 and 5dBi over most of the operational frequencies of this band.On the other hand, the proposed SWB antenna has superb radiation efficiency which is greater than 99% for the lower frequencies and is maintained above 96% along the operating frequency range from 2.3 to 23 GHz.

1.
The central layer is paper with ε r2 = 2.3 and thickness h = 2.7mm.The purpose of the layered structure of the substrate is to enhance the operating bandwidth and the radiation F I G U R E 1 Proposed antenna design indicating the geometrical parameters.(A) Upper view, (B) lower view, (C) multilayered substrate, and (D) feeding line connector mounted to the antenna via the broadband balun.

F I G U R E 3
Frequency response of the reflection coefficient magnitude, S | | 11 , for different values of the flare angle ψ.F I G U R E 4 Frequency response of the reflection coefficient magnitude, S | | 11 , for different values of the vertical separation, L P , between the upper and lower circular pads of the balun.F I G U R E 5 Frequency response of the reflection coefficient magnitude, S | | 11 , for different values of the diameter, D U , of the upper circular pad of the impedance-matching balun.F I G U R E 6 Frequency response of the reflection coefficient magnitude, S | | 11 , for different values of the diameter, D L , of the lower circular pad of the impedance-matching balun.

7
Practical measurement of the impedance matching of the designed SWB antenna.(B) Fabricated antenna, (A) connection of the SWB antenna to the VNA port for measuring S | | 11 , and (C) reflection coefficient magnitude S | | 11 over the operating frequency band.SWB, super-wideband; VNA, vector network analyzer.

F I G U R E 8
Current distribution of the conducting arms of the proposed super-wideband antenna at (A) 5 GHz, (B) 8 GHz, (C) 14 GHz, and (D) 22 GHz.Radiation patterns obtained at 12GHz (central frequency) by both simulation and measurement in the planes.(A) ϕ = 0°a nd (B) θ = 90°.