WO2021078654A1

WO2021078654A1 - Ultra wideband circular polarized radiation element with integrated feeding

Info

Publication number: WO2021078654A1
Application number: PCT/EP2020/079232
Authority: WO
Inventors: Jian Yang; Tianling Zhang; Lei Chen; Ashraf UZ ZAMAN
Original assignee: Gapwaves Ab
Priority date: 2019-10-25
Filing date: 2020-10-16
Publication date: 2021-04-29
Also published as: SE1930346A1; SE543202C2

Abstract

An antenna element (100) for an ultra-wideband circularly polarized planar array antenna. In an example embodiment, the antenna element comprises a stacked substrate layer structure, wherein a first substrate (160) comprises a radiating element (161), a second substrate (140) comprises a ground plane (141) and a transmission line arrangement (131), and a third substrate (120) comprises an electromagnetic bandgap, EBG, structure (110,121,122). The EBG structure and the transmission line arrangement may optionally form one or more inverted planar transmission line wave guides. The radiating element may comprise a bold-C spiral radiation element, suitable for millimeter-wave band operation.

Description

TITLE

ULTRA WIDEBAND CIRCULAR POLARIZED RADIATION ELEMENT WITH INTEGRATED FEEDING

TECHNICAL FIELD

The present disclosure relates to radiating antenna elements, particularly antenna elements for antenna arrays. The antenna array is suited for use in, e.g., telecommunication and radar transceivers.

BACKGROUND

Wireless communication networks comprise radio frequency transceivers, such as radio base stations used in cellular access networks, microwave radio link transceivers used for, e.g., backhaul into a core network, and satellite transceivers which communicate with satellites in orbit. A radar transceiver is also a radio frequency transceiver since it transmits and receives radio frequency signals.

Radio transceivers, in general, comprise antenna devices. An antenna device may comprise an antenna array, which in turn comprises a plurality of radiating elements. Conventionally, the element spacing in an antenna array should be smaller than one wavelength to avoid grating lobes. With this restriction, the most conventional planar array antenna designs adopt an element spacing of roughly 0.8Ahigh, to achieve high gain and obtain enough space for feeding networks, where Ahigh is the wavelength at the highest operation frequency. Tightly coupled array (TCA) antenna is another kind of antenna array employing small element spacing, i.e. an element spacing less than 0.8Ahigh. This type antenna array utilizes the mutual coupling between the radiation elements to obtain ultra-wide bandwidth for wide beam angle scanning.

A drawback of the TCA is that it is challenging to design the feeding network for the radiating elements, such as a corporate feeding network, due to the limited amount of space coming from the small element spacing. Resulting problems are unwanted radiation losses and mutual coupling between adjacent transmission lines in the feeding network. These problems are severe at millimeter-wave frequencies and become worse as the frequency increases.

There is a need for improved antenna elements for antenna arrays that, i.a., present a large bandwidth, and allow low-loss and low-leakage feeding networks.

SUMMARY

It is an object of the present disclosure to provide an improved antenna element for an antenna array. The antenna element comprises a first substrate, a second substrate, and a third substrate stacked to form a layered structure. The first substrate has a first surface facing away from the second and third substrates, wherein the first surface comprises a radiating element. A first ground plane is arranged between the first and the second substrates. The second substrate has a first surface arranged facing the third substrate, wherein the first surface comprises a transmission line arrangement comprising at least one planar transmission line. A feeding via hole arranged extending through the first substrate layer, through the first ground plane, and through the second substrate layer is configured to connect the radiating element and the transmission line arrangement. The third substrate comprises an electromagnetic bandgap, EBG, structure arranged to prevent electromagnetic propagation in a frequency band of operation from propagating from the transmission line arrangement in directions other than through the feeding via hole and along the at least one planar transmission line.

The antenna element allows for wide bandwidth transmission of electromagnetic signals and it and allows low-loss and low-leakage feeding networks. The antenna element also enables compact antenna arrays, which is beneficial for deployment and manufacturing. The stacked layered structure results in low-cost manufacturing and a robust antenna array. The EBG structure further prevents undesired back radiation. According to aspects, the antenna element further comprises a fourth substrate stacked adjacent to the first surface of the first substrate.

This way, the impedance matching of the radiating element is improved. This, in turn, leads to, i.a., improved bandwidth performance.

According to aspects, at least two of the first, second, third, and fourth substrates are separated by a prepreg layer.

This provides a simple and effective way of bonding the different substrate layers together. According to aspects, the EBG structure comprises a second ground plane and a plurality of EBG mushrooms. Each EBG mushrooms comprises a patch and a via hole, the via hole extending through the third substrate and is configured to connect the patch to the second ground plane.

This way, the EBG structure comprises an easy to manufacture and low-cost structure.

According to aspects, the EBG structure and the at least one planar transmission line together form an inverted planar transmission line wave guide.

This way, the feeding network comprises low-loss and well-isolated wave guides.

According to aspects, the radiating element is a bold-C spiral having an arcuate form extending from the feeding via hole.

This way, the radiating element may present circularly polarized radiation, and the radiation element may have a wide bandwidth impedance match and a wide bandwidth of good axial ratio (i.e. below 3-dB). The single feeding structure is another advantage, since it allows for a simple and effective feeding network, which in turn is helpful to achieve wide bandwidth performance all together of the antenna element or an array of the antenna elements. According to aspects, an array antenna comprises a plurality of the antenna elements.

This way, all positive effects of the antenna element may be utilized in conjunction with all benefits associated with an antenna array, such as improved directivity.

According to aspects, the transmission line arrangements of all antenna elements constitute a corporate feeding network in the array antenna comprising a plurality of the antenna elements.

This way, a single layer feeding network may feed all the antenna elements in the antenna array.

According to aspects, the antenna array comprises an 8 by 8 array of antenna elements.

This gives the benefits of an antenna array, as discussed above.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realizes that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described in more detail with reference to the appended drawings, where Figure 1 illustrates an example antenna array,

Figure 2 illustrates an example substrate-based inverted microstrip gap waveguide,

Figure 3 illustrates an example mushroom-like element of an electromagnetic bandgap structure and its corresponding dispersion diagram,

Figures 4(a) and (b) illustrate perspective view and a side view an example antenna element, respectively,

Figures 5(a-c) illustrate top views of an example antenna element at different cuts, Figure 6 illustrates reflection coefficients and axial ratios versus frequency for different values of the width parameter w.

Figure 7 illustrates an evolution process of an example bold-C spiral,

Figures 8(a) and (b) illustrate reflection coefficients and axial ratios versus frequency for different steps in the evolution of the example bold-C spiral in Fig. 7,

Figures 9(a-d) illustrate current distributions on the radiating element (a) t = 0, at 20 GHz. (b) t = T/4, at 20 GHz. (c) t = 0, at 30 GHz. (d) t = T/4, at 30 GHz.

Figure 10 illustrates the simulated reflection coefficient and axial ratio of a single radiation element cell. Figures 11 (a) and (b) illustrate the simulated radiation patterns of the proposed unit cell of the bold-C spiral element at (a) 19 GHz and (b) 30 GHz.

Figure 12 illustrates the geometry of the corporate feeding network.

Figure 13 illustrates the simulated reflection coefficients of 1-to-4 power dividers in the feeding network. Figure 14 illustrates measured insertion losses of the microstrip line based on TSM-DS3-CH and TSM-DS3-ULPH using differential length method. Figure 15 illustrates the measured insertion loss of 2.4-mm end launch connector.

Figure 16 illustrates measured and simulated reflection coefficients of the proposed 8 ^c 8 CP array antenna. Figure 17 illustrates measured and simulated axial ratios of the proposed 8x8 CP array antenna.

Figures 18(a-c) illustrate measured and simulated radiation patterns of the proposed 8 ^c 8 CP array antenna at (a) 19 GHz, (b) 24.5 GHz and (c) 30 GHz.

Figure 19 illustrates measured and simulated gains and antenna efficiencies of the proposed 8 ^c 8 CP array antenna.

Figure 20 illustrates a side view of an example antenna element.

DETAILED DESCRIPTION

Aspects of the present disclosure will now be described more fully with reference to the accompanying drawings. The different devices and methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout.

The terminology used herein is for describing aspects of the disclosure only and is not intended to limit the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Abstract An ultra-wideband circularly polarized (CP) planar array antenna built by new radiation elements, the bold-C spiral elements, at millimeter- wave band is presented. Using the tightly coupled array (TCA) technique, the embedded CP bold-C spiral element in the infinite array has achieved both impedance bandwidth and 3-dB axial ratio (AR) bandwidth of 54% covering 18.1-31.5 GHz for the fixed broadside beam. Then, a feeding network is designed by using the dielectric-based inverted microstrip gap waveguide technology in order to have a compact layout. An 8x8 CP planar array is designed and prototyped. The measured results show that the impedance bandwidth of the whole array for the reflection coefficient below -10 dB is 49% from 18.2 to 30 GHz, and 3- dB AR bandwidth is 44.9% covering 19-30 GHz.

Index Terms — tightly coupled array, circularly polarized, planar array antenna, single-arm spiral, dielectric-based inverted microstrip gap waveguide.

Section I - Introduction

With the development of the fifth generation (5G) wireless communication systems, the systems in millimeter wave (mmWave) bands become a critical and inevitable part [1] Circularly polarized (CP) planar array antennas in mmWave bands have been widely demanded, because of their merits in suppressing multipath interferences and reducing polarization mismatch. There are mainly two methods to design CP planar array antennas: 1) Wideband CP elements with a corporate feeding network [2]-[5]; 2) CP elements using sequential rotation technique to overcome the limitation of the AR bandwidth [6]-[9] Actually, wideband CP elements are also needed in the latter method to avoid narrow gain bandwidth. The former method is more attractive because it utilizes simple feeding structure. However, how to obtain wideband CP elements and feeding network is a challenge.

Many CP planar array antennas in mmWave bands, reported in recent years, employed different wideband radiation elements [5], [10], [12]-[14]. In [5], the axial ratio (AR) bandwidth of the radiation element was broadened by adopting a rotated strip and metal-topped via fence. However, a 4 ^c 4 CP planar array using the element exhibited a narrow AR bandwidth of around 10%. In [10], the aperture-coupled technique was used to obtain wider bandwidth, and a Ka- band CP planar array antenna utilizing a crossed slot with four parasitic patches as the CP element was proposed with an overall bandwidth close to 20%. Furthermore, magneto-electric (ME) dipole antenna is a good choice for moderate wideband CP antenna element [11] Several ME dipole elements with slot exciting structure were used in the CP planar array antennas. For example, an 8 ^c 8 array using CP aperture-coupled ME dipole achieved 3-dB axial ratio (AR) bandwidth of 16.5% and 18.2% impedance bandwidth [12] In [13], a wideband CP ME dipole element by loading branches and truncating corners was presented and employed to form a 4 ^c 4 CP array with a microstrip line feeding network. The array shows 24.4% impedance bandwidth and 16% 3-dB AR bandwidth. The widest 3-dB AR bandwidth and impedance bandwidth of an 8 x 8 array have been achieved to 33.8% and 35.4% respectively in the literature by using stacked curl elements exhibited in [14] Our task is to design an 8 x 8 CP planar array antenna with overall bandwidth (both impedance match and AR bandwidth) exceeding 40% in the mmWave band of 20-30 GFIz, which is a very challenging task.

According to the antenna theory, the element spacing in an array antenna should be smaller than one wavelength to avoid the grating lobes. With this restriction, the most conventional planar array antenna designs adopt the element spacing as about 0.8Amgh, to achieve high gain and obtain enough space for feeding networks, where Ahigh is the wavelength at the highest operation frequency [5], [10], [12]-[14]. Another kind of array antenna employing a small element spacing, called as tightly coupled array (TCA) antennas [15]-[21], exhibit excellent wide bandwidth performance. This type antennas utilize the mutual coupling between elements to obtain ultra-wide bandwidth for wide beam angle scanning. Most reported TCAs worked with linear polarization or dual linear polarization. Few CP array designs were based on the concept of TCA. In [22], a 4 ^c 4 CP tightly coupled crossed dipole array (CP-TCCDA) was proposed to achieve ultra-wide bandwidth with the center frequency of 4 GFIz. Flowever, there was a big drawback in that design: the element radiates with bidirectional beams. Thus, the array needs absorbers placed on the bottom of the array to absorb the radiation power in one side and generate unidirectional radiation beam, which reduces the antenna gain significantly. We apply the concept of TCA to the design of an ultra-wideband fixed-beam high gain CP planar array antenna with new bold- C spiral elements in a mmWave band, with the bandwidth exceeding 40% for both AR and impedance matching.

The feeding network is another key part of UWB CP planar array antennas. Obviously, the design of a corporate feeding network with the concept of TCA is a difficult task due to the relatively small element spacing of TCAs. Substrate integrated waveguide (SIW) or microstrip line technology, which are mostly employed in the reported designs, are not suitable here. For SIW technology, there is no enough space for a feeding network based on SIW in our TCA case, and in addition, the bandwidth limitation of SIW makes the feeding network difficult to achieve bandwidth above 40%. For microstrip line technology, it is very flexible to design a UWB feeding networks. Flowever, it has some drawbacks in mmWave bands, such as the unwanted radiation losses and mutual coupling between adjacent lines. Therefore, the dielectric-based inverted microstrip gap waveguide (DIMGW) technology is adopted in our work for the feeding network design. Dielectric-based IMGW, which is based on gap waveguide technology [22]-[27], is also called as substrate integrated gap waveguide (SIGW) [28]-[32] It can be easily fabricated by multi-layer printed circuit board (PCB) technology.

In the present disclosure, a UWB fixed-beam CP planar TCA array antenna with bold-C spiral elements is designed, fabricated and measured. This planar array antenna has achieved a bandwidth of about 45% for both the impedance matching and the AR, much wider than those of the existing mmWave planar array antennas in the literature. We have done some modifications and optimization in the final design by considering the fabrication constraints and the manufacture cost, which are presented here. The novelties of this work include the followings i) The tightly coupled bold-C spiral array has been introduced and optimized to achieve UWB performance for the CP fixed-beam in mmWave bands ii) A simple compact feeding structure has been introduced for the bold-C spiral elements iii) A feeding network using DIMGW technology has been designed with a compact size iv) A simple multi-layer PCB structure for the whole array antenna has been designed and realized for a low-cost manufacture.

The Detailed Description is organized as follows. The configuration and the design of the bold-C spiral radiation element, the TCA geometry and the DIMWG feeding network elaborated in Section II. The influence of PCB’s copper foil on the loss is investigated in Section III. The simulations and measurements of the 8 ^c 8 array are illustrated in Section IV. Section V draws the conclusions of this work. Section - Antenna Design and Configuration

Fig. 1 shows an example 8 x 8 array comprising the disclosed antenna element 100. Fig. 20 shows an example antenna element 100. It is appreciated that any array size comprising a plurality of antenna elements 100 is possible. The example array in Fig. 1 consists of four layers of stacked substrates. Between adjacent substrates is prepreg FR28 for the lamination of multi-layer PCB. On the top (substrate 4) is Taconic TLY-5 with a relative dielectric constant of 2.2 and a loss tangent of 0.0009, and the thickness of 1.016mm. Taconic TSM- DS3 with a relative dielectric constant of 3.0 and a loss tangent of 0.0014 is used for the other substrates. The thickness of the substrates 1 -3 are chosen as 1.016, 0.254 and 1.524mm, respectively. The bold-C spiral elements are etched on the top of substrate 1, and the distance spacing between the adjacent elements is 5.7 mm in both x- and y-directions. The corporate feeding network is designed with the DIMGW technology, which is composed of the microstrip line, a perfect electrical conductor, PEC, ground (the second ground plane) and an electromagnetic bandgap, EBG, structure. The elements are connected to the feeding network by feeding vias through substrate 1 , the first ground plane and substrate 2.

Fig. 20 shows one embodiment of the disclosed antenna element. More specifically, Fig. 20 shows an antenna element 100 for an antenna array. The antenna element comprises a first substrate 160, a second substrate 140, and a third substrate 120 stacked to form a layered structure. The first substrate has a first surface facing away from the second and third substrates, wherein the first surface comprises a radiating element 161 . A first ground plane 141 is arranged between the first and the second substrates. The second substrate 140 has a first surface arranged facing the third substrate, wherein the first surface comprises a transmission line arrangement 131 comprising at least one planar transmission line. A feeding via hole 151 arranged extending through the first substrate layer 160, through the first ground plane 141 , and through the second substrate layer 140 is configured to connect the radiating element 161 and the transmission line arrangement 131. The third substrate 120 comprises an electromagnetic bandgap, EBG, structure 110,121 ,122 arranged to prevent electromagnetic propagation in a frequency band of operation from propagating from the transmission line arrangement 131 in directions other than through the feeding via hole and along the at least one planar transmission line.

Optionally in the disclosed antenna element, with reference to Fig. 20, at least two of the first, second, third, and fourth substrates 120,140,160,180 are separated by a prepreg layer 130,150,170.

In the present disclosure, a multi layered printed circuit board (PCB), or layered structure, comprises stacked substrate layers. The substrate is sometimes called core. A substrate comprises a non-conductive material. Two example properties of a substrate are the dielectric constant (also called relative permittivity, an example value is 2.2) and dielectric loss (often characterized in terms of a loss tangent, an example value is 0.004). An example of a substrate is FR4, which is a woven epoxy resin impregnated fiberglass cloth. Another example of a substrate is insulated metal substrate. A substrate may have a thickness in the order of 0.1 to 10 mm, but other thicknesses are also possible. A substrate layer may have a conductive foil (e.g. copper) arranged (e.g. laminated) on both or either side of it. The conductive foil may have a thickness in the order of 10-100 micrometers, but other thicknesses are also possible. The conductive foil may be arranged (e.g. by etching) to form different structures, such as a radiating element, planar transmission lines, or a ground plane. In a multilayer PCB, multiple substrates may be attached together using prepreg layers in between the substrate layers. Prepreg (or pre-preg) is short for pre-impregnated composite fiber, and is used as an adhesive, i.e. for bonding different layers together. An example of a prepreg is uncured epoxy resin-based substrate, such as uncured FR4 or FR28. The thickness of a prepreg may be in the order of 0.1-10 mm, but other thicknesses are also possible. A via hole (or just via or vertical interconnect access) is an electrical connection that may extend through a single or through multiple substrate layers. The via may therefore connect different layers of conductive foil layers together. A via may comprise a hole, which is made conductive by incorporating a conductive tube or by electroplating the hole. The diameter of a via hole can, e.g., be in the span 0.1-10 mm, but other diameters are also possible.

In Fig. 20, it is noted that the radiating element 161 may comprise a conductive foil laminated to the first substrate 160. The first ground plane 141 may comprise a conductive foil laminated to the second substrate 140. The ground plane 141 may alternatively or in combination comprise a conductive foil laminated to the first substrate 160. The transmission line arrangement 131 may comprise a conductive foil laminated to the second substrate 140.

A planar transmission line is a transmission lines wherein the electrical conductor (e.g. copper) is flat. Examples of planar transmission line are microstrip, stripline and coplanar waveguide.

Dielectric-based Inverted Microstrip Gap Waveguide (DIMGW)

Fig. 2 illustrates the geometry of DIMGW, which consists of two stacked substrates and a prepreg. The microstrip line is on the top of mushroom-like EBG structure and below the first ground plane. Thus, the mushroom-like EBG structure with artificial magnetic conductor property and the PEC ground (the second ground plane) with a distance smaller than a quarter wavelength make a parallel plate waveguide where no waves propagate. With the microstrip line, the Quasi-TEM wave can propagate between the PEC ground and the strip line, which is referred to as dielectric-based inverted microstrip gap waveguide (DIMGW). Therefore, the mutual coupling between adjacent lines is suppressed by this gap waveguide structure, which is very helpful to the wideband feeding network design in a very limited space layout. As a result, the DIMGW has a more compact size than SIW because of its Quasi-TEM mode, and less losses than the microstrip line due to no radiation loss. In addition, the whole structure of DIMWG can be easily fabricated by multi-layer PCB process and integrated with RF components. It is appreciated that the microstrip line can be replaced by any planar transmission line, such as coplanar transmission line. In the disclosed antenna element 100, with reference to Fig. 20, the EBG structure 110,121 ,122 and the at least one planar transmission line may together form an inverted planar transmission line wave guide. Furthermore, in the disclosed antenna element 100, with reference to Fig. 20, the EBG structure comprises a second ground plane 110 and a plurality of EBG mushrooms. Each EBG mushrooms comprises a patch 122 and a via hole 121 . The via hole extends through the third substrate 120 and is configured to connect the patch 122 to the second ground plane 110. The patch 122 and/or the second ground plane 110 may comprise conductive foils laminated to respective surfaces on the third substrate 120. The thickness of the third substrate is preferably smaller than a quarter of the operational wavelength of the highest frequency in the frequency band, in order for the EBG structure to function properly as an artificial magnetic conductor (on the surface with the patches the third substrate).

The EBG structure is arranged to prevent electromagnetic propagation in a frequency band of operation from propagating from the transmission line arrangement 131 in directions other than through the feeding via and along the at least one planar transmission line. A frequency band is an interval of frequencies between a lower cutoff frequency and a higher cutoff frequency. The antenna element is arranged to transmit and receive electromagnetic signals in the frequency band of operation.

The EBG structure is not necessarily based on mushroom-like patches and via holes. Other PCB based EBG structures are also possible. It is possible to replace the third substrate 120 comprising an EBG structure with an air based EBG structure. Such air based EBG structure may, for example, be based on conductive repetitive protruding elements extending from the second ground plane 110 towards the second substrate 140. An example of such repetitive structure is taught by US15/311 , 128.

Further referencing Fig. 3, the dimensions of the example mushroom-like EBG unit cell with the ground plate are optimized to achieve a stopband covering as much as possible over about 20-30 GFIz. The final optimized dimension parameters, including the period, the diameters of via and pad, are given in Table I. The dispersion diagram of the unit cell, obtained using the eigenmode solver of CST Microwave Studio, is shown in Fig. 3. It is seen that a stopband covering 15.4-36.25 GFIz is achieved.

TABLE I

DIMENSIONS OF THE PROPOSED CP ELEMENT (UNIT: mm)

Bold-C Spiral Radiation Element

The geometry of the proposed example bold-C spiral radiation element is given in Fig. 4. As it is known, a single-arm multi-turn spiral antenna can be fed by a simple feeding structure with excellent wideband performance with a bit large size [34] Thus, the single-arm spiral structure is adopted here as the element for our design. However, different from the structure in [34], in order to have a compact wideband spiral element for our design, the spiral structure has been modified significantly and optimized for UWB impedance match and low AR performance. The final optimized spiral structure, referred to as the bold-C spiral in this paper. In the disclosed antenna element 100, with reference to Fig. 20, the radiating element 161 is a bold-C spiral having an arcuate form extending from the feeding via hole 151 .

The example bold-C spiral, as shown in Fig. 4, consists of four stacked substrates. TLY-5 is used on the top of the element as superstrate to improve the impedance matching characteristic. It is appreciated that TLY-5 is just one out of many possible substrates for improving matching. Therefore, with reference to Fig. 20, the disclosed antenna element 100 may optionally comprise a fourth substrate 180 stacked adjacent to the first surface of the first substrate 160.

The top views of substrates 1-3 are shown in Fig. 5 and the final optimized dimensions of the example bold-C spiral element are given in Table I. The bold-C spiral elements are etched on the top surface of substrate 1 and each fed by a single via through substrate 2, the ground and substrate 1 . The single feeding structure is one of the advantages of the proposed bold-C spiral element, which makes the corporate network simple and helpful to achieve wide bandwidth performance within the limited space due to the concept of TCA.

The concept of TCA is adopted here in the design of bold-C spiral array for a fixed-beam planar array antenna. Usually, the element spacing in TCAs is less than 0.5Ahigh to avoid scanning anomalies in high frequency. Considering the aim of a fixed beam array antenna here, the element spacing is chosen as 0.57Ahigh to obtain more space for the feeding network. The single-arm spiral element with periodic boundary, i.e. the unit cell, was optimized to achieve UWB characteristics. Fig. 6 shows the variation of the simulated active reflection coefficient and AR with the element spacing w. It is seen that the element spacing w has nearly no influence on the impedance matching performance while the AR performance deteriorates with the element spacing reducing. This phenomenon proves that a wideband CP element can be achieved by using the concept of TCA.

Four steps are shown in Fig. 7 to explain the design process of the proposed bold-C spiral element. Step 1 is an 1 /4-turn single-arm square spiral. In Step 2, a triangular chamfer is implemented in the second corner of the spiral. In Step 3, two other triangular chamfers are carved in the beginning of the spiral. In Step 4, the fourth triangular chamfer is carved in the first corner. Fig. 8 shows the simulated results of the unit cell in Steps 1 -4. It can be seen that the unit cell in Step 1 has an impedance bandwidth from 16 to 26.2 GHz and a narrow 3-dB AR bandwidth from 16.4 to 23 GHz. After cutting the chamfer in the second corner of the spiral of Step 1 , the AR bandwidth is enlarged covering 16.6-26.9 GHz, and the impedance matching is also slightly improved. The third step is changing the beginning structure of the spiral with two more triangular chamfers, and the AR bandwidth is improved further, covering 16-27.5 GHz. After carving the last triangular chamfer in the first corner, the operating band is moved to the high frequency, and the overall bandwidth is improved. This phenomenon is a result of current flowing path being more and more circular with each step and the width of the spiral making the wideband performance since the current paths for high frequencies and low frequencies are along the inner circle and the outer circle, respectively. The simulated current distributions on the spiral at different time and frequency points are given in Fig. 9 to explain the generation of the circular polarization of the proposed CP element. It is seen from Fig. 9 (a) that at t = 0 the resultant total current is along +y-direction at 20 GHz. At t = T/4, the direction of the resultant total current is changed to +x-direction. Thus, the current changes in a clockwise direction from t = 0 to t = T/4, which generates a left-hand circular polarization (LHCP) wave. Similarly, this phenomenon also occurs at 30 GHz. Fig. 10 shows the simulated results of the proposed bold-C element, including the active reflection coefficient and AR. The proposed element achieves a 3- dB AR bandwidth of 54% from 18.1 to 31.5 GHz, which breaks the AR bandwidth record of the existing mmWave CP planar array designs in the literature. Considering the impedance matching characteristics, the overall bandwidth is 50.7% covering 18.1 -30.4 GHz. Fig.11 presents the simulated radiation patterns of unit cell of the bold-C spiral element at 19 and 30 GHz. The performances of the proposed bold-C spiral element and some reported elements in the literature are summarized in Table II. Apparently, the bold-C spiral element achieves the widest bandwidth above 50% for both AR and impedance match.

TABLE II

COMPARISON BETWEEN THE PROPOSED AND REPORTED CP ELEMENTS

It is appreciated that any array size comprising a plurality of antenna elements 100 is possible. Although the element spacing in TCAs is typically less than 0.5Ahigh, it is possible to arrange a plurality antenna elements 100 in an array in a span of element spacing, e.g., from 0.1 Ahigh to 2Ahigh. Other element spacing lengths are also possible.

According to aspects, a telecommunication or radar transceiver comprises at least one antenna element 100.

It is noted that the bold-C spiral element may be useful for other structures than the disclosed antenna element 100, as in, e.g., a two layered PCB structure. Thus, according to aspects, there is herein disclosed a bold-C spiral element comprising a first substrate 160. A first surface on the first substrate comprises a radiating element 161, the element having an arcuate form extending from a feeding via hole 151 as exemplified in Fig. 7. The bold-C spiral element is shaped like the letter ‘C’, i.e. , it extends from the via hole in a first direction, and then turns clockwise by approximately 45 degreed in three turns to form the Ό’ shape. Figure 7 describes a method for generating the bold-C shape. The bold-C spiral element may optionally have the shape parameters according to Fig. 5(a-c). The bold-C spiral element may optionally comprise a second substrate layer 140 stacked adjacent to a second surface of the first substrate 160, wherein the second substrate has a first surface arranged facing away from the first substrate. A ground plane 141 is optionally arranged between the first and the second substrates. The first surface of the second substrate optionally comprises a transmission line arrangement 131 comprising at least one planar transmission line. The feeding via hole optionally extends through the first substrate layer 160, through the first ground plane 141 , and through the second substrate layer 140, and is thereby configured to connect the radiating element 161 and the transmission line arrangement 131. The bold-C spiral element may optionally comprise a fourth substrate 180 stacked adjacent to the first surface of the first substrate 160. In the bold-C spiral element, optionally at least two of the first, second, and fourth substrates 140,160,180 are separated by a prepreg layer 150,170.

Corporate feeding network

A corporate feeding network for the 8 ^c 8 bold-C spiral planar array antenna is designed with DIMGW. A similar design has been presented in [35], so we describe the main design procedure briefly here for the convenience for the readers. Fig. 12 shows the geometry of the 1 -to-64 feeding network, which is composed of sixteen type 1 , four type 2 and one type 3 of 1 -to-4 equally split power dividers and a transition from the dielectric-based IMGW to the microstrip line. Multi-step impedance transformers are used in 1 -to-4 power dividers to broaden the impedance match bandwidth. Thanks to the merits of the DIMGW which suppresses the mutual coupling between closely adjacent transmission lines, a good performance has been achieved. Fig. 13 presents the simulated reflection coefficients of these three types of 1-to-4 power dividers. It can be observed that the bandwidth with the reflection coefficient less than -15 dB is covering 18.2-35.2 GFIz. In an antenna array comprising the disclosed antenna element 100, with reference to Fig. 20, the transmission line arrangements 131 of all antenna elements constitute a corporate feeding network. It is appreciated that the one or more transmission line arrangements may be arranged in other configurations than a corporate feeding network, such as multiple feeding networks for groups of antenna elements or beam steering networks. Optionally, the one or more transmission line arrangements comprise RF components, such as capacitors, resistors and inductors. Integrated RF components are also possible, such as integrated chips, ICs.

Section III - Influence of PCB's Copper Foil on Loss

PCB manufacturers, such as Rogers and Taconic, offer high frequency circuit substrates with various types of copper foils, such as standard electrodeposited copper foils, rolled copper foils, resistive copper foils, etc. produced by different manufacturing processes. Different types of copper foils have different surface roughness, which affects the conductor losses of transmission lines in mmWave bands significantly. In order to have accurate data for the transmission line loss for our design, the insertion losses of the microstrip lines on Taconic TSM-DS3 with two different copper foils are fabricated and measured. One copper foil designation is CH, which is commonly used. The other one is ULPFI, which has the smoothest surface among the various types of copper foils. The thickness of substrate TSM-DS3 for obtaining insertion loss data by measurements is selected as 0.254 mm which is the same as that of substrate 2 in the array antenna.

Twopairs of PCB prototypes with 50-W characteristic impedance microstrip lines on both TSM-DS3-CFI and TSM-DS3-ULPFI are fabricated with silver plating and lengths of 20 mm and 50 mm in both pairs. Two 2.4-mm Southwest end launch connectors are deployed for measurements. With different lengths, the insertion losses of the microstrip lines are obtained from the measured S parameters. Fig. 14 shows the measured insertion losses of the microstrip line based on TSM-DS3-CFI and TSM-DS3-ULPFI. It can be seen that the insertion loss with TSM-DS3-ULPFI is much lower due to its smoother copper foil surface. Also, the measured loss of 2.4-mm end launch connector is presented in Fig. 15. The loss varies from 0.18 to 0.36 dB over the frequency range of 15-35 GFIz. In consequence, the efficiency of the planar antenna will be improved if TSM-DS3-ULPH is used. In this work for the optimal trade-off between the cost and the performance, the TSM-DS3-ULPH is applied to the feeding network on substrate 2, and the TSM-DS3-CH is applied to radiation elements and mushroom pin structure on substrate 1 and 3. The total cost of one 8 x 8 array antenna prototype as described below would be less than €20 if 10000 pieces were ordered.

Section IV - Measurement and Discussion

With the consideration of multi-layer PCB process requirements, a wideband LHCP (Left Hand Circularly Polarized) 8 x 8 planar array antenna with the bold- C spirals elements has been designed and fabricated. The Taconic prepreg FR28 is used for bonding the substrates. The thickness of FR28 used is 0.124 mm for bonding TLY-5, substrate 1 and 2, 0.248 mm between substrate 2 and 3. The external coppers of PCB are plated with pure gold without nickel, which makes the 10 mm microstrip line achieve a good transmission performance. The size of the 8 ^c 8 CP array antenna is 45.6x55.6x4.4 mm³. For measurement, a 2.4-mm Southwest end launch connector is deployed.

The reflection coefficient of the proposed CP array antenna is measured using Agilent E8363B vector network analyzer. Fig. 16 shows the measured and simulated reflection coefficients, which agree with each other quite well. The measured and simulated impedance matching bandwidths with the reflection coefficient less than -10 dB are 49% from 18.2 to 30 GHz and 52.8% covering 17.87-30.7 GHz, respectively. Fig. 16 illustrates the measured and simulated ARs of the array. The measured 3-dB AR bandwidth is 44.9% from 19 to 30 GHz, while the simulated one is 49.2% covering 18.6-30.75 GHz. Considering the impedance and AR bandwidth, the overall bandwidth is 44.9% covering 19-30 GHz.

The radiation patterns of the array antenna are measured in an anechoic far- field chamber. The measured and simulated CP radiation patterns in xz and yz planes of the array at 19, 24.5 and 30 GHz are shown in Fig. 18. Good agreement can be observed between the measured and the simulated LHCP radiation patterns. In addition, the low backlobe level has been achieved by using DIMGW. With eliminating the loss of 2.4-mm end launch connector shown in Fig. 15, the measured and simulated gains and efficiencies are shown in Fig. 19. The maximum measured gain is 20.9 dBi at 26.5 GFIz. The difference between the measured and simulated gains from 28 to 30 GFIz may be caused by the difference between the simulated and measured ARs at 28- 30 GFIz, and the real insertion loss of the transmission line which is frequency- dependent (see the measurement in Fig. 14) while in the simulations the loss tangent of the substrates is constant over the frequency. The total efficiency is calculated with the measured gain and the antenna aperture of 45.6x45.6 mm². It is seen that the measured efficiency is above 40% over 18-29 GFIz. The performance of the proposed and several reported CP planar array antennas are summarized in Table III. The proposed CP array antenna exhibits the widest overall bandwidth of 44.9%, which considers the impedance and AR bandwidth. In addition, the minimum total antenna efficiencies (e_ant=e_misMatch e_ap e_rad) are also shown in Table III. Although the proposed CP array antenna has a lower maximum gain than that in [12], [14] due to a smaller aperture caused by the concept of TCA, its minimum total antenna efficiency within the working band is larger than 34%, which is higher than that in the most of the other designs.

TABLE III

COMPARISON WITH THE PROPOSED AND REPORTED PLANAR ARRAY ANTENNAS

Section V - Conclusion

A fixed-beam wideband CP planar array antenna using bold-C spiral elements and the TCA technology in mmWave band has been proposed and verified with experiments. With the periodic boundary of tight element spacing, the bold-C spiral element exhibits an excellent performance, including the impedance matching bandwidth above 54% and 3-dB AR bandwidth of 54% covering 18.1-31.5 GHz. Although the tight element spacing brings a challenging task in the design of wideband feeding network, a good performance is obtained with the merits of the DIMGW. The proposed 8 x 8 CP planar array antenna covers an overall bandwidth 44.9% from 19-30 GHz for both impedance and AR, which extends the bandwidth of CP planar array antennas in mmWave bands significantly compared to the existing CP planar array technologies in the literature.

References

[1] M. K. Samimi and T. S. Rappaport, “3-D Millimeter-Wave statistical channel model for 5G wireless system design,” IEEE Trans. Microw. Theory Techn., vol. 64, no. 7, pp. 2207-2225, Jul. 2016. [2] C. Liu, Y. Guo, X. Bao, and S. Xiao, “60-GHz LTCC integrated circularly polarized helical antenna array,” IEEE Trans. Antennas Propag., vol. 60, no. 3, pp. 1329-1335, Mar. 2012.

[3] B. Cao, Y. Shi, and W. Feng, “W-band LTCC circularly circularly- polarized antenna array with mixed U-type substrate integrated waveguide and ridge gap waveguide feeding networks,” IEEE Antennas Wireless Propag. Lett, 2019. (Early Access)

[4] H. Sun, Y. Guo, and Z. Wang, “60-GHz circularly polarized U-slot patch antenna array on LTCC,” IEEE Trans. Antennas Propag., vol. 61, no. 1, pp. 430-435, Jan. 2013. [5] Y. Li, Z. N. Chen, X. Qing, Z. Zhang, J. Xu, and Z. Feng, “Axial ratio bandwidth enhancement of 60-GHz substrate integrated waveguide-fed circularly polarized LTCC antenna array,” IEEE Trans. Antennas Propag., vol. 60, no. 10, pp. 4619-4626, Oct. 2012.

[6] H. Xu, J. Zhou, K. Zhou, Q. Wu, Z. Yu, and W. Hong, “Planar wideband circularly polarized cavity-backed stacked patch antenna array for millimeter- wave applications,” IEEE Trans. Antennas Propag., vol. 66, no. 10, pp. 5170- SI 79, Oct. 2018.

[7] Y. Zhang, W. Hong, and R. Mittra, “45 GHz wideband circularly polarized planar antenna array using inclined slots in modified short-circuited SIW,” IEEE Trans. Antennas Propag., vol. 67, no. 3, pp. 1669-1680, Mar. 2019.

[8] J. Zhu, S. Li, S. Liao, and Q. Xue, “60 GHz wideband high-gain circularly polarized antenna array with substrate integrated cavity excitation,” IEEE Antennas Wireless Propag. Lett, vol. 17, no. 5, pp. 751-755, May 2018.

[9] Q. Zhu, K.-B. Ng, and C. H. Chan, “Printed circularly polarized spiral antenna array for millimeter-wave applications,” IEEE Trans. Antennas Propag., vol. 65, no. 2, pp. 636-643, Feb. 2017.

[10] J. Wu, Y. J. Cheng, and Y. Fan, “Millimeter-wave wideband high-efficiency circularly polarized planar array antenna,” IEEE Trans. Antennas Propag., vol. 64, no. 2, pp. 535-541 , Feb. 2016.

[11] Luk, K.M., Wong, H.: ‘A new wideband unidirectional antenna element’, Int. J. Microw. Opt. Technol., 2006, 1 , (1 ), pp. 35-44.

[12] Y. J. Li and K. M. Luk, “A 60-GHz wideband circularly polarized aperture- coupled magneto-electric dipole antenna array,” IEEE Trans. Antennas Propag., vol. 64, no. 4, pp. 1325-1333, Apr. 2016.

[13] Z. Gan, Z.-H. Tu, Z.-M. Xie, Q.-X. Chu, and Y. Yao, “Compact wideband circularly polarized microstrip antenna array for 45 GHz application,” IEEE Trans. Antennas Propag., vol. 66, no. 11 , pp. 6388-6392, Nov. 2018.

[14] Q. Wu, J. Hirokawa, J. Yin, C. Yu, H. Wang, and W. Hong, “Millimeter- wave planar broadband circularly polarized antenna array using stacked curl elements,” IEEE Trans. Antennas Propag., vol. 65, no. 12, pp. 7052-7062, Dec. 2017. [15] J. T. Logan, R. W. Kindt, M. Y. Lee, and M. N. Vouvakis, “A new class of planar ultrawideband modular antenna arrays with improved bandwidth,” IEEE Trans. Antennas Propag., vol. 66, no. 2, pp. 692-701 , Feb. 2018.

[16] S. S. Holland and M. N. Vouvakis, “The planar ultrawideband modular antenna (PUMA) array,” IEEE Trans. Antennas Propag., vol. 60, no. 1 , pp. 130-140, Jan. 2012.

[17] H. Zhang, S. Yang, S. Xiao, Y. Chen, S. Qu, and J. Hu, “Ultrawideband phased antenna arrays based on tightly coupled open folded dipoles,” IEEE Trans. Antennas Propag., vol. 18, no. 2, pp. 378-382, Feb. 2019.

[18] W. Zhou, Y. Chen, and S. Yang, “Dual-Polarized tightly coupled dipole array for UHF-X-Band satellite applications,” IEEE Trans. Antennas Propag., vol. 18, no. 3, pp. 467-471 , Mar. 2019.

[19] J. Zhong, A. Johnson, E. A. Alwan, and J. L. Volakis, “Dual-Linear polarized phased array with 9:1 bandwidth and 60° scanning off broadside,” IEEE Trans. Antennas Propag., vol. 67, no. 3, pp. 1996-2001 , Mar. 2019.

[20] D. K. Papantonis and J. L. Volakis, “Dual-Polarized tightly coupled array with substrate loading,” IEEE Antennas Wireless Propag. Lett., vol. 15, pp. 325-328, 2016.

[21] R. Xia, S. Qu, S. Yang, and Y. Chen, “Wideband wide-scanning phased array with connected backed cavities and parasitic striplines,” IEEE Trans. Antennas Propag., vol. 66, no. 4, pp. 1767-1775, Apr. 2018.

[22] P. -S. Kildal, A. U. Zaman, E. Rajo-lglesias, E. Alfonso, and A. Valero- Nogueira, “Design and experimental verification of ridge gap waveguide in bed of nails for parallel-plate mode suppression,” IET Microw., Antennas Propag., vol. 5, no. 3, pp. 262-270, 2011.

[23] A. Vosoogh, P. Kildal, and V. Vassilev, “Wideband and high-gain corporate-fed gap waveguide slot array antenna with ETSI Class II radiation pattern in V-band,” IEEE Trans. Antennas Propag., vol. 65, no. 4, pp. 1823- 1831 , Apr. 2017.

[24] A. Farahbakhsh, D. Zarifi, and A. U. Zaman, “A mmWave wideband slot array antenna based on ridge gap waveguide with 30% bandwidth,” IEEE Trans. Antennas Propag., vol. 66, no. 2, pp. 1008-1013, Feb. 2018. [25] J. Liu, A. Vosoogh, A. U. Zaman, and J. Yang, “A slot array antenna with single-layered corporate-feed based on ridge gap waveguide in the 60 GHz band,” IEEE Trans. Antennas Propag., vol. 67, no. 3, pp. 1650-1658, Mar. 2019.

[26] M. Ferrando-Rocher, J. I. Herranz-Herruzo, A. Valero-Nogueira, B. Bernardo-Clemente, A. U. Zaman, and J. Yang, “8x8 Ka-band dual-polarized array antenna based on gap waveguide technology,” IEEE Trans. Antennas Propag, vol. 67, no. 7, pp. 4579-4588, Jul. 2019.

[27] T. Zhang, L. Chen, S. M. Moghaddam, A. U. Zaman, and Jian. Yang, “Wideband dual-polarized array antenna on dielectric-based inverted microstrip gap waveguide,” In 13th European Conference on Antenna and Propagation (EuCAP), Krakow, Poland, 2019, pp. 1 -3

[28] J. Liu, J. Yang, and A. U. Zaman, “Study of dielectric loss and conductor loss among microstrip, covered microstrip and inverted microstrip gap waveguide utilizing variational method in millimeter waves,” In Proc. IEEE Int. Symp. Antennas Propag. (ISAP), Busan, Korea (South), 2018, pp. 1 -2.

[29] D. Shen, C. Ma, W. Ren, X. Zhang, Z. Ma, and R. Qian, “A low-profile substrate-integrated-gap-waveguide-fed magnetoelectric dipole,” IEEE Antennas Wireless Propag. Lett., vol. 17, no. 8, pp. 1373-1376, Aug. 2018.

[30] N. Bayat-Makou and A. A. Kishk, “Millimeter-Wave substrate integrated dual level gap waveguide horn antenna,” IEEE Trans. Antennas Propag, vol. 65, no. 12, pp. 6847-6855, Dec. 2017.

[31] C. Ma, Z. Ma, and X. Zhang, “Millimeter-Wave circularly polarized array antenna using substrate-integrated gap waveguide sequentially rotating phase feed,” IEEE Antennas Wireless Propag. Lett., vol. 18, no. 6, pp. 1124-1128, Jun. 2019.

[32] J. Zhang, X. Zhang, D. Shen, and A. A. Kishk, “Packaged microstrip line: a new quasi-TEM line for microwave and millimeter-wave applications,” IEEE Trans. Microw. Theory Techn., vol. 65, no. 3, pp. 707-719, Mar. 2017.

[33] T. Zhang, L. Chen, S. M. Moghaddam, A. U. Zaman, and J. Yang, “Ultra- wideband Circularly Polarized Planar Array Antenna Using Single-Arm-Spiral Elements and Dielectric-based IMGW,” In Proc. IEEE Int. Symp. Antennas Propag. (ISAP), Xi’an, China, 2019, 27-30 October.

[34] H. Nakano, R. Satake, and J. Yamauchi, “Extremely low-profile, single arm, wideband spiral antenna radiating a circularly polarized wave,” IEEE Trans. Antennas Propag, vol. 58, no. 5, pp. 1511-1520, May. 2010.

[35] T. Zhang, L. Chen, S. M. Moghaddam, A. U. Zaman, and J. Yang, “Ultra- wideband linearly polarized planar bowtie array antenna with dielectric-based inverted microstrip gap waveguide,” IET Microw., Antennas Propag., submitted.

Claims

1. An antenna element (100) for an antenna array, the antenna element comprising a first substrate (160), a second substrate (140), and a third substrate (120) stacked to form a layered structure, wherein the first substrate has a first surface facing away from the second and third substrates, the first surface comprising a radiating element (161 ); wherein a first ground plane (141 ) is arranged between the first and the second substrates; wherein the second substrate (140) has a first surface arranged facing the third substrate, the first surface comprising a transmission line arrangement (131 ) comprising at least one planar transmission line; wherein a feeding via hole (151 ) arranged extending through the first substrate layer (160), through the first ground plane (141), and through the second substrate layer (140) is configured to connect the radiating element (161 ) and the transmission line arrangement (131 ); wherein the third substrate (120) comprises an electromagnetic bandgap, EBG, structure (110,121 ,122) arranged to prevent electromagnetic propagation in a frequency band of operation from propagating from the transmission line arrangement (131 ) in directions other than through the feeding via hole and along the at least one planar transmission line.

2. The antenna element (100) according to claim 1 , comprising a fourth substrate (180) stacked adjacent to the first surface of the first substrate (160).

3. The antenna element (100) according to any previous claim, wherein at least two of the first, second, third, and fourth substrates (120,140,160,180) are separated by a prepreg layer (130,150,170).

4. The antenna element (100) according to any previous claim, wherein the EBG structure comprises a second ground plane (110) and a plurality of EBG mushrooms, wherein each EBG mushrooms comprises a patch (122) and a via hole (121 ), the via hole extending through the third substrate (120) and is configured to connect the patch (122) to the second ground plane (110).

5. The antenna element (100) according to any previous claim, wherein the EBG structure (110,121,122) and the at least one planar transmission line together form an inverted planar transmission line wave guide.

6. The antenna element (100) according to any previous claim, wherein the radiating element (161) is a bold-C spiral having an arcuate form extending from the feeding via hole (151 ).

7. An array antenna comprising a plurality of antenna elements (100) according to any previous claim.

8. The antenna array according to claim 7, wherein the transmission line arrangements (131) of all antenna elements constitute a corporate feeding network.

9. The antenna array according to claim 8, wherein the antenna array comprises an 8 by 8 array of antenna elements (100).

10. A telecommunication or radar transceiver comprising at least one antenna element according any of claims 1 -6.