Open AccessArticle

A Planar Four-Element UWB Antenna Array with Stripline Feeding Network

Marek Garbaruk

Faculty of Electrical Engineering, Bialystok University of Technology, Wiejska 45d, 15-351 Białystok, Poland

Electronics 2022, 11(3), 469; https://doi.org/10.3390/electronics11030469

Submission received: 24 January 2022 / Revised: 2 February 2022 / Accepted: 3 February 2022 / Published: 5 February 2022

(This article belongs to the Topic Antennas)

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Figure 1
Geometry of proposed single prototype UWB antenna (dimensions in mm). "> Figure 2
Geometry of proposed four-element antenna arrays in yz-plane (dimensions in mm): (a) Shorter variant of UWB antenna array; (b) Longer variant of UWB antenna array. "> Figure 3
Photographs of proposed antennas: (a) Central metallic layer of shorter variant of four-element antenna array; (b) Central metallic layer of longer variant of four-element antenna array; (c) Fabricated antennas. "> Figure 4
Simulated and measured |S11| of proposed antennas. "> Figure 5
Longer variant of four-element antenna array in anechoic chamber during measurements. "> Figure 6
Simulated and measured gain of proposed antennas in direction perpendicular to antenna surface (along +/− x-axis). "> Figure 7
Simulated and measured radiation pattern of the proposed arrays in xy-plane at different frequencies: (a) 6 GHz; (b) 7.2 GHz; (c) 8.4 GHz. ">

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Abstract

This paper proposes a four-element ultrawideband (UWB) planar antenna array with elliptical-shaped radiators and a stripline excitation network designed for the 6–8.5 GHz UWB frequency band allowed in Europe by the European Commission. The designed antenna array has a symmetrical structure in which the radiators are placed along one line in the central conducting layer, arranged between two layers of a dielectric. Radiating elements are fed by the stripline excitation network that provides uniform power distribution. The dimensions of the elliptical radiators’ axes are 14 mm × 16 mm. Two variants of array are proposed. The distance between the radiators’ centers is L = 19 mm for a shorter variant and L = 24 mm for a longer one. The presented antenna array structures have a size of 81 mm × 41 mm and 96 mm × 41 mm. These arrays present a measured gain of 6.4–10.6 dBi for the shorter variant and 8.5–10.8 dBi for the longer one and a fair impedance matching. The measured |S₁₁| is less than −8.7 dB and −9.7 dB for the shorter and longer corresponding variants.

Keywords:

antennas; antenna array; 4-element; UWB applications

1. Introduction

There are plenty of examples of different ultrawideband (UWB) antennas and their applications in literature since the regulations on the spectrum use by UWB system came into existence. Although ultrawideband transmissions have been known in radiocommunication for decades, the announcement of the FCC regulations in 2002 on the use of the frequency band by UWB systems was a serious impulse for the development of UWB technology [1]. Later (2007, revised in 2019), European regulations appeared, announced by the European Commission [2]. The key difference in both regulations was the width of the main frequency band allowed for use by UWB systems with the same level −41.3 dBm/MHz of the maximum power spectral density (FCC band 3.1–10.6 GHz vs. European band 6–8.5 GHz).

The first publications on the studies concerning the design of single antennas for use in UWB systems appeared relatively quickly. The antennas with circular or elliptical shaped radiators were very popular, the natural advantage of which was the large bandwidth of the operating frequency [3,4,5,6]. In the case of specific requirements, the use of UWB antenna arrays may be highly purposeful. UWB antenna arrays are welcomed and useful in extended operating range and high sensitivity systems. Designing antenna arrays for modern radio systems that present desirable parameters in a very wide frequency band is more troublesome than designing single antennas, but the designers have been facing such problems for years.

There are many possibilities of using UWB antenna arrays and various design problems mostly in medical, radar, imaging and MIMO systems. In [7], a compact planar UWB antenna array for a radar-based breast cancer detection system in the supine position was presented. In [8] the authors presented a compact planar UWB antenna and an antenna array setup for microwave breast imaging. An experimental system for early screening of breast tumors consisting of a moveable array of improved negative-index ultrawideband antenna sensor is presented in [9]. An active slot antenna integrating a low-noise amplifier for tissue sensing arrays was proposed in [10]. A beam scanning technique was developed with a time delay for UWB arrays in [11]. The design of a grid array antenna for automotive radar sensors constructed using astroid unit cells, characterized by high gain and bandwidth, was developed in [12]. In [13], the bandwidth enhancement and frequency scanning for a UWB array antenna utilizing the novel technique of band-pass filter integration for wireless vital signs monitoring and vehicle navigation sensors was presented. In [14] a compact eight-element antenna array was presented for UWB pulsed radar in a highly reflective metallic environment. This Vivaldi antenna and array were characterized in terms of the transient energy patterns and the signal fidelity given in terms of the off-angle signal correlation.

Ultrawideband antenna arrays for MIMO UWB systems are also very popular in the literature. Their concept often differs from the concept of classic antenna arrays, but they are in fact systems containing many radiators and can be considered as antenna arrays. A compact printed UWB slot antenna consisting of two modified coplanar waveguides for MIMO diversity applications was described in [15]. Another example of antenna with two coplanar stripline-fed staircase-shaped radiating elements, with high isolation, is proposed for portable UWB MIMO systems in [16]. In [17] an ultra-compact frequency reconfigurable UWB MIMO antenna with four radiators that are capable of rejecting WLAN signals on demand by activating the PIN diodes is presented. A generic design method of spiral MIMO antenna arrays for short-range ultrawideband imaging application and its focusing property were discussed in [18]. In [19], the authors developed a compact uniplanar 4-port MIMO antenna array with rejecting band and polarization diversity. Several fabricated prototypes of 8 × 4 tightly coupled dipoles in linearly polarized phased arrays were presented in [20]. Tightly coupled dipoles in 11 × 11 UWB arrays with integrated baluns were also discussed in [21]. In [22] UWB MIMO array installed around a polystyrene block in the 3D-octagonal arrangement system was proposed for 3D non-planar applications. In [23], a UWB metasurface-based beam-switching antenna system was proposed. A four-element planar UWB antenna requiring no decoupling circuit for MIMO system was developed in [24]. In [25] an ultrawideband MIMO antenna system composed of two radiating elements with an improved isolation by using slotted stubs is presented. An eight-element UWB MIMO antenna with a deployed inductor capacitor stub on the ground plane for 3G/4G/5G networks was proposed in [26]. Another design of MIMO UWB antenna composed of two offset microstrip-fed elements with a band-notched function was analyzed in [27]. Finally, a wideband neutralization line was proposed in [28] to reduce the mutual coupling of a compact MIMO UWB antenna. Moreover, in the case of antenna array designing, an additional problem of optimizing the ratio between the power deposited over a given area and the whole transmitted power may arise. The synthesis of fields able to maximize the power radiated in an arbitrary portion of the visible space was discussed in [29]. Effective antenna arrays, including UWB arrays, can therefore be utilized for high beam efficiency transmissions. This overview presents that there is a great interest in the design of UWB antenna arrays and many design approaches can be found.

In this paper there are presented constructions of two variants of the four-element UWB planar antenna array designed for modern UWB radio systems. The antenna design and simulated and measured results are shown. Each array variant contains a stripline power divider, which is an excitation network providing a uniform distribution of excitations of all radiators. These feeding networks contain in their structure a few staircase-shaped sections. A new concept in this design is placing a metallic layer with the radiators between two layers of dielectric in the stripline structure. That results in getting a fully two-layer symmetrical structure. At the same time, in the part of the array containing the excitation network, its shielding is obtained. The antenna arrays have been designed to operate in the main band (6–8.5 GHz) allowed by the European Commission for use by UWB systems in Europe [2]. The obtained results were compared with the results achieved for a single prototype UWB antenna, with the same dimensions of the radiator and the length of excitation section, which was a reference antenna in the design of four-element UWB arrays.

The paper is organized as follows: the design of the proposed UWB prototype antenna and two variants of the four-element antenna array are presented in Section 2. The results of simulation and measurements are discussed in Section 3. The conclusion of the work is presented in Section 4.

2. Antenna Design

In order to design the 4-element antenna array considered in this work, a single UWB prototype antenna was firstly designed. Its design specifies the dimensions of the radiator and the length of the excitation line connecting the antenna input with the radiator. It was decided to choose an elliptical-shaped radiator due to the possibility of obtaining a good impedance matching in a wide frequency operation range.

Figure 1 shows the structure of a single UWB prototype antenna with the orientation of the adopted coordinate system. This single antenna contains an elliptical-shaped radiator with axis lengths of 14 mm for a shorter (horizontal) axis and 16 mm for a longer (vertical) one, fed by the stripline.

A two-layer, symmetrical structure of the antenna was obtained. Between the two layers of the dielectric there is a central conductive middle layer, while on the outer dielectric sides there are two identical areas of ground planes placed exactly one above the other in the plane of the antenna surface (yz-plane). The elliptical radiator is connected to the central conductor of the stripline structure and is located between two dielectric layers each 1.575 mm thick, ε_r = 2.2 (for fabrication of the considered antennas Rogers Duroid 5880 substrate with the same parameters was used). The total thickness of this two-layer structure is 3.15 mm. A 50 Ω feeding line is employed to feed the radiator. The calculated width of the excitation line is W = 2.61 mm, and its length equals 20 mm. The width of this excitation line was calculated in the stripline impedance calculator integrated with software used for a computer simulation. This line is shielded on both sides by two external ground planes with a slightly shorter length, equal to 19.44 mm. This shift of the upper border of the reference ground plane with respect to the lower end of the radiator has a positive effect on the impedance matching of the prototype antenna.

The boundaries of the whole single UWB antenna in its upper part (containing the radiator) are moved by 5 mm from the boundaries of the radiator. The overall dimensions of the single antenna are 24 mm × 41 mm. The antenna input is at its bottom edge. In the computer simulation a port dedicated for stripline structures was employed as the antenna input. The SMA connector mounted to fabricated antennas was not included in computer models. The presented dimensions of the radiator and the whole structure of the single antenna provide a nearly omnidirectional shaped radiation pattern and a good impedance matching.

Based on the structure of the single UWB prototype antenna, two variants of four-element UWB planar antenna array have been developed (Figure 2). The four-element arrays presented in this paper contain four radiators of the same elliptical shape and the same orientation as the radiator of the single prototype antenna. The radiators are positioned in a straight line along the y-axis at equal distances (the surfaces of all antennas are oriented in the yz-plane, as the single prototype antenna shown in Figure 1). Their positioning and orientation in relation to the top and bottom border of the entire array structure (in relation to the substrate boundaries) is analogous to the prototype UWB antenna. The boundaries of the entire antenna array structure are moved by 5 mm upwards and in the left and right directions from the first and the last radiator.

For two considered antenna array variants the centers of the radiators are positioned in the distance: L = 19 mm (shorter variant) and L = 24 mm (longer variant). The distances between the radiators’ edges on the y-axis are thus D = 5 mm and D = 10 mm, respectively. The electrical length between the centers of neighboring radiating elements varies throughout the operation frequency range from 0.38λ at 6 GHz to 0.54λ at 8.5 GHz for the shorter variant and from 0.48λ at 6 GHz to 0.68λ at 8.5 GHz for the longer variant (λ is the free-space wavelength). The overall dimensions of both variants of the four-element UWB antenna arrays are 81 mm × 41 mm for the shorter variant and 96 mm × 41 mm for the longer variant. The single input of each array variant is placed in the middle of the bottom edge of each array variant structure. Ports dedicated to stripline structures were used again as the arrays’ inputs in the computer analysis without modeling SMA connectors. The length of the excitation section measured along the z-axis, from the bottom of the ground plane to the bottom edges of the radiators, is 20 mm. The length of the ground planes in this direction is shortened by 0.54 mm to 19.44 mm as for the single prototype antenna.

The radiating elements are fed by symmetrical multi-section broadband power dividers which were developed as the stripline structures, as for the single antenna. Figure 2 shows the structures of the central conducting layer of both variants of the four-element arrays, including the radiators with the feeding network. In the beginning part of the feeding network structure, staircase-shaped sections can be distinguished. In the design of the entire array structure, this approach resulted in a better impedance matching of the array in the operating frequency band. The widths of all subsequent sections of feeding network are marked as W1–W6. All remaining dimensions fully describing the geometry of the designed networks have also been denoted (those dimensions that result from the symmetry of the array structure have not been marked).

The uniform power distribution of the radiators is adopted in the development of both variants of excitation networks. This power distribution allows the direction of the maximum radiation to be perpendicular to the surface of the antenna array, both in the positive and negative direction of x-axis. Two feeding structures for both array variants include dividing parts that provide a uniform power division between all corresponding sections and radiators. They also provide equal electrical lengths from antenna arrays inputs to all radiators.

In the area where the section of width W2 passes into two lines of width W3, the input power is divided equally into two parts. It is similar in places where sections with W5 widths change into sections with W6 widths. The uniform power distribution of the radiators was obtained under the condition of full symmetry of the designed arrays structures. In the design of both variants of antenna arrays there is the symmetry of the feeding network and also the symmetry of the entire structure of the antenna array with respect to the z-axis passing through the center of the first section of width W1 to which the input port is connected (a symmetry line S1 in Figure 2). At the same time, the extension of the axis of symmetry of the section W5 in the direction of the z-axis (a symmetry line S2 in Figure 2) is located exactly halfway between the first and the second radiator of the array on both sides of the overall structure, respectively.

All dimensions of both variants of feeding networks were determined in the optimization process of their whole geometry. The impedance of the first sections connected to the array input sockets was set as 50 Ω (W1 = 2.61 mm for the selected substrate as for the single prototype antenna), the other sections with widths from W2 to W6 were optimized. The adopted optimization criterion was to obtain |S₁₁| < −10 dB in the 6–8.5 GHz operation band. Computer simulations of the analyzed structures were carried out in the IE3D/Hyper Lynx 3D EM program, a 3D electromagnetic simulator using method of moments (MoM).

It can be noticed that the determined widths of the subsequent sections of the feeding network of both array variants are relatively wide. This is due to the values of parameters of the substrate that was selected for the antennas’ design and their fabrication. The use of a thinner dielectric or a dielectric with a higher value of ε_r would result in narrower widths of the successive sections. In that case the entire structure of the feeding networks should be optimized to achieve the desired level of impedance matching.

The single antenna and two four-element antenna arrays were fabricated by the author. SMA-connectors were used as inputs. The center conductive layer with a mosaic of radiators and feeding excitation network was etched on one side of the first dielectric layer. Its other external side, considering the structure of the entire antenna array, contains the first ground reference plane. The second layer of dielectric contains only the second ground plane on its external side. After soldering the SMA-connector, both dielectric layers were tightly stuck together. The metallization thickness of all conducting layers is 35 µm, however in the computer project and simulation the thickness was assumed as 0 mm.

Figure 3a,b presents the central metallic layers of both array variants and photos of the ready-made antennas are shown in Figure 3c.

3. Simulated and Measured Results

The experimental verification of S-parameters and radiation patterns of the proposed UWB prototype antenna and both variants of the four-element antenna arrays were conducted. The simulated and measured results were compared.

The measurements of |S₁₁| were made with the Agilent N5230A vector network analyzer. The results of the simulations and measurements of |S₁₁| of the proposed antennas are shown in Figure 4. In the case of the single prototype UWB antenna a good impedance matching was achieved. The simulated |S₁₁| is less than −21.5 dB and measured less than −16.6 dB. Considering the four-element antenna arrays, the impedance matching is slightly worse. The shorter variant of the antenna array shows the simulated |S₁₁| less than −10.5 dB and measured less than −8.7 dB, although the simulated |S₁₁| is larger than −10 dB only in part of the frequency band above 8.1 GHz. For the longer variant of the four-element antenna array the simulated |S₁₁| is less than −9.7 dB and measured is also less than −9.7 dB. There is a good agreement between the simulated and measured results.

The radiation characteristics were measured in an anechoic chamber in the 6–8.5 GHz frequency band with 0.2 GHz frequency steps. The photography of the wider variant of the four-element UWB antenna array during the measurements in the anechoic chamber is presented in Figure 5. The radiation patterns were measured in the horizontal xy-plane (the measured antenna was mounted vertically in yz-plane, as a computer model presented in Figure 1). During each measurement procedure two orthogonal components of the gain were measured in E-plane and H-plane and then total gain was calculated.

Figure 6 shows the simulated and measured frequency characteristics of the gain determined in the direction perpendicular to the antenna surface (both in the positive and negative direction of the x-axis due to the array symmetry). The simulated gain (solid line in Figure 6) of the single UWB prototype antenna has a value in the range 1–2.1 dBi in the frequency operating band 6–8.5 GHz while the measured gain (dashed line in Figure 6) differs from the simulated one by a maximum of 1.3 dB. In the case of the four-element antenna arrays, the simulated gain for the shorter variant changes in the operating band in the range 6.7–8.9 dBi and for the wider variant in the range 8.5–9.3 dBi. The measured gain has slightly higher values. For corresponding UWB arrays the measured ranges of gain are: 6.4–10.6 dBi and 8.5–10.8 dBi. The differences between the simulated and measured results are caused mostly by errors during measurements of the radiation patterns. The difference in the simulated gain of proposed arrays in Figure 6 decreases from 1.8 dB at 6 GHz to 0.3 dB at 8.5 GHz and the measured from 2.1 dB to 0.4 dB, respectively.

Figure 7 shows the simulated and measured radiation patterns in the xy-plane at three frequencies from the operating band of the antenna arrays: 6, 7.2 and 8.4 GHz. A high agreement of the simulated and measured characteristics is observed. Due to the double symmetry of the designed antenna array structures, the gain characteristics are symmetrical in the antenna alignment yz-plane and also in the xz-plane. There is a noticeable rise in the value of each antenna array gain with increasing the frequency and with increasing the spacing L between radiators. It is also observable that the growth of the spacing between the radiators does not increase the gain very much. The increase of the gain is inhibited by the growth of the sidelobes, especially visible in the upper part of the frequency operation band. The same relationship is observed in the results presented in Figure 6. Considering the maximum gain for both variants of the array in Figure 7 it can be observed that the largest difference between the simulated and measured gain is 0.3 dB at 6 GHz, 0.4 dB at 7.2 GHz and 1.7 dB at 8.4 GHz. This difference rises with an increase in frequency. It may occur mostly from two factors. Firstly, in the computer simulation (MoM), the meshing was performed automatically and is the same for all analyzed frequencies. Thus, in cases of increases in frequency, the size of the cells in relation to the wavelength decreases, which may turn into a slight decrease in the precision of the calculation. Secondly, the measurement uncertainty is difficult to estimate precisely. It may be influenced, among others, by errors in the calibration of the measurement system or by the influence of a rigid coaxial cable used for mounting antennas during measurements (visible in Figure 5).

The simulated and measured radiation patterns of the designed UWB antenna arrays are characterized by a fairly high level of sidelobes, especially noticeable in the upper part of the frequency operation band for the wider array variant. The level of sidelobes can be minimized by the design of the feeding network that provides a heterogeneous distribution of excitations at the inputs of the radiators, which will be the subject of the future research.

4. Conclusions

In this paper the four-element UWB antenna array in two variants of the spacing between the radiating elements is proposed. Both designed variants are excited by multi-section stripline feeding network with the uniform power distribution. The structure of the feeding network contains staircase-shaped sections that improve impedance matching. Two external ground planes fully shield the structure of the excitation network. The characteristics of |S₁₁| and radiation patterns are presented, both simulated and measured. For comparison, the |S₁₁| characteristics are also shown for the UWB single prototype antenna, which has the same sized radiator as the antenna arrays.

The simulated and measured results for both variants of the UWB antenna arrays present a similar value of |S₁₁|; less than −10 dB in almost the whole operation band. The single UWB antenna shows better impedance matching than the proposed antenna arrays, but the measured gain is less by 4.2–9.1 dB depending on the frequency and the array spacing variant. The wider variant of the antenna array shows a slightly higher gain than the shorter one. The simulation and measurement results for all considered structures show a good agreement.

Funding

This research was realized in Bialystok University of Technology, Poland, and supported by the Polish Ministry of Education and Science under Rector’s Project WZ/WE-IA/1/2020.

Conflicts of Interest

The author declares no conflict of interest.

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Figure 1. Geometry of proposed single prototype UWB antenna (dimensions in mm).

Figure 2. Geometry of proposed four-element antenna arrays in yz-plane (dimensions in mm): (a) Shorter variant of UWB antenna array; (b) Longer variant of UWB antenna array.

Figure 3. Photographs of proposed antennas: (a) Central metallic layer of shorter variant of four-element antenna array; (b) Central metallic layer of longer variant of four-element antenna array; (c) Fabricated antennas.

Figure 4. Simulated and measured |S₁₁| of proposed antennas.

Figure 5. Longer variant of four-element antenna array in anechoic chamber during measurements.

Figure 6. Simulated and measured gain of proposed antennas in direction perpendicular to antenna surface (along +/− x-axis).

Figure 7. Simulated and measured radiation pattern of the proposed arrays in xy-plane at different frequencies: (a) 6 GHz; (b) 7.2 GHz; (c) 8.4 GHz.

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Garbaruk, M. A Planar Four-Element UWB Antenna Array with Stripline Feeding Network. Electronics 2022, 11, 469. https://doi.org/10.3390/electronics11030469

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Garbaruk M. A Planar Four-Element UWB Antenna Array with Stripline Feeding Network. Electronics. 2022; 11(3):469. https://doi.org/10.3390/electronics11030469

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Garbaruk, Marek. 2022. "A Planar Four-Element UWB Antenna Array with Stripline Feeding Network" Electronics 11, no. 3: 469. https://doi.org/10.3390/electronics11030469

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A Planar Four-Element UWB Antenna Array with Stripline Feeding Network

Abstract

1. Introduction

2. Antenna Design

3. Simulated and Measured Results

4. Conclusions

Funding

Conflicts of Interest

References

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