SE1930346A1 - Ultra wideband circular polarized radiation element with integrated feeding - Google Patents

Abstract

An antenna element 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 comprises a radiating element, a second substrate comprises a ground plane and a transmission line arrangement, and a third substrate comprises an electromagnetic bandgap, EBG, structure. 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 WITHINTEGRATED FEEDING TECHNICAL FIELD The present disclosure relates to radiating antenna elements, particularlyantenna 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 radiolink transceivers used for, e.g., backhaul into a core network, and satellitetransceivers which communicate with satellites in orbit. A radar transceiver isalso a radio frequency transceiver since it transmits and receives radio frequency signals.

Radio transceivers, in general, comprise antenna devices. An antenna devicemay comprise an antenna array, which in turn comprises a plurality of radiatingelements. Conventionally, the element spacing in an antenna array should besmaller than one wavelength to avoid grating lobes. With this restriction, themost conventional planar array antenna designs adopt an element spacing ofroughly 0.8Åhigh, to achieve high gain and obtain enough space for feedingnetworks, where Åhigh is the wavelength at the highest operation frequency.Tightly coupled array (TCA) antenna is another kind of antenna arrayemploying small element spacing, i.e. an element spacing less than 0.8Åhagh.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 networkfor 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 betweenadjacent 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 lt is an object of the present disclosure to provide an improved antennaelement for an antenna array. The antenna element comprises a firstsubstrate, a second substrate, and a third substrate stacked to form a layeredstructure. The first substrate has a first surface facing away from the secondand 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 arrangementcomprising at least one planar transmission line. A feeding via hole arrangedextending through the first substrate layer, through the first ground plane, andthrough the second substrate layer is configured to connect the radiatingelement and the transmission line arrangement. The third substrate comprisesEBG, electromagnetic propagation in a frequency band of operation from an electromagnetic bandgap, structure arranged to preventpropagating from the transmission line arrangement in directions other thanthrough the feeding via hole and along the at least one planar transmission line.

The antenna element allows for wide bandwidth transmission ofelectromagnetic signals and it and allows low-loss and low-leakage feedingnetworks. The antenna element also enables compact antenna arrays, whichis beneficial for deployment and manufacturing. The stacked layered structureresults 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 planeand a plurality of EBG mushrooms. Each EBG mushrooms comprises a patchand 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 planartransmission 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, andthe radiation element may have a wide bandwidth impedance match and awide bandwidth of good axial ratio (i.e. below 3-dB). The single feedingstructure is another advantage, since it allows for a simple and effectivefeeding network, which in turn is helpful to achieve wide bandwidthperformance 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 inconjunction with all benefits associated with an antenna array, such as improved directivity.

According to aspects, the transmission line arrangements of all antennaelements 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 theirordinary meaning in the technical field, unless explicitly defined othenNiseherein. All references to "a/an/the element, apparatus, component, means,step, etc." are to be interpreted openly as referring to at least one instance ofthe element, apparatus, component, means, step, etc., unless explicitly statedothenNise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated. Furtherfeatures of, and advantages with, the present invention will become apparentwhen studying the appended claims and the following description. The skilledperson realizes that different features of the present invention may becombined 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 versusfrequency for different steps in the evolution of the example bold-C spiral inFig. 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 1 1 (a) and (b) illustrate the simulated radiation patterns of the proposedunit 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 PCB prototypes with 50 Q microstrip line on TSM-DS3-CH and TSM-DS3-ULPH.

Figure 15 illustrates measured insertion losses of the microstrip line based onTSM-DS3-CH and TSM-DS3-ULPH using differential length method.

Figure 16 illustrates the measured insertion loss of 2.4-mm end launch connector.

Figures 17(a) and (b) illustrate photographs of the proposed prototype: (a) 8><8 CP array, (b) array under test.

Figure 18 illustrates measured and simulated reflection coefficients of the proposed 8 >< 8 CP array antenna.

Figure 19 illustrates measured and simulated axial ratios of the proposed 8><8 CP array antenna.

Figures 20(a-c) illustrate measured and simulated radiation patterns of theproposed 8 >< 8 CP array antenna at (a) 19 GHz, (b) 24.5 GHz and (c) 30 GHz.

Figure 21 illustrates measured and simulated gains and antenna efficienciesof the proposed 8 >< 8 CP array antenna.

Figure 22 illustrates Table I: DIMENSIONS OF THE PROPOSED CPELEMENT (UNIT: mm).

Figure 23 illustrates Table ll: COMPARISON BETWEEN THE PROPOSEDAND REPORTED CP ELEMENTS.

Figure 24 illustrates: Table III: COMPARISON WITH THE PROPOSED ANDREPORTED PLANAR ARRAY ANTENNAS.

Figure 25 illustrates a side view of an example antenna element.

DETAILED DESCRIPTION Aspects of the present disclosure will now be described more fully withreference 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 onlyand 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 newradiation elements, the bold-C spiral elements, at millimeter- wave band ispresented. Using the tightly coupled array (TCA) technique, the embedded CPbold-C spiral element in the infinite array has achieved both impedancebandwidth and 3-dB axial ratio (AR) bandwidth of 54% covering 18.1 -31 .5 GHzfor the fixed broadside beam. Then, a feeding network is designed by usingthe dielectric-based inverted microstrip gap waveguide technology in order tohave a compact layout. An 8><8 CP planar array is designed and prototyped.The measured results show that the impedance bandwidth of the whole arrayfor 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 communicationsystems, the systems in millimeter wave (mmWave) bands become a criticaland inevitable part [1]. Circularly polarized (CP) planar array antennas inmmWave bands have been widely demanded, because of their merits insuppressing 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 Iimitation of theAR bandwidth [6]-[9]. Actually, wideband CP elements are also needed in thelatter method to avoid narrow gain bandwidth. The former method is moreattractive 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]. ln [5], theaxial ratio (AR) bandwidth of the radiation element was broadened by adoptinga rotated strip and metal-topped via fence. However, a 4 >< 4 CP planar arrayusing the element exhibited a narrow AR bandwidth of around 10%. ln [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 parasiticpatches as the CP element was proposed with an overall bandwidth close to20%. Furthermore, magneto-electric (ME) dipole antenna is a good choice formoderate wideband CP antenna element [11]. Several ME dipole elementswith slot exciting structure were used in the CP planar array antennas. Forexample, an 8 >< 8 array using CP aperture-coupled ME dipole achieved 3-dBaxial ratio (AR) bandwidth of 16.5% and 18.2% impedance bandwidth [12]. ln[13], a wideband CP ME dipole element by loading branches and truncatingcorners was presented and employed to form a 4 >< 4 CP array with a microstripline feeding network. The array shows 24.4% impedance bandwidth and 16%3-dB AR bandwidth. The widest 3-dB AR bandwidth and impedance bandwidthof an 8 >< 8 array have been achieved to 338% and 35.4% respectively in theliterature by using stacked curl elements exhibited in [14]. Our task is to designan 8 >< 8 CP planar array antenna with overall bandwidth (both impedancematch and AR bandwidth) exceeding 40% in the mmWave band of 20-30 GHz, which is a very challenging task.

According to the antenna theory, the element spacing in an array antennashould be smaller than one wavelength to avoid the grating lobes. With thisrestriction, the most conventional planar array antenna designs adopt theelement spacing as about 0.8Åhigh, to achieve high gain and obtain enough space for feeding networks, where Åhigh is the wavelength at the highest operation frequency [5], [10], [12]-[14]. Another kind of array antennaemploying a small element spacing, called as tightly coupled array (TCA)antennas [15]-[21], exhibit excellent wide bandwidth performance. This typeantennas utilize the mutual coupling between elements to obtain ultra-widebandwidth for wide beam angle scanning. Most reported TCAs worked withlinear polarization or dual linear polarization. Few CP array designs werebased on the concept of TCA. ln [22], a 4 >< 4 CP tightly coupled crossed dipolearray (CP-TCCDA) was proposed to achieve ultra-wide bandwidth with thecenter frequency of4 GHz. However, there was a big drawback in that design:the element radiates with bidirectional beams. Thus, the array needsabsorbers placed on the bottom of the array to absorb the radiation power inone side and generate unidirectional radiation beam, which reduces theantenna gain significantly. We apply the concept of TCA to the design of anultra-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 TCAis a difficult task due to the relatively small element spacing of TCAs. Substrateintegrated waveguide (SIW) or microstrip line technology, which are mostlyemployed 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 networkdifficult to achieve bandwidth above 40%. For microstrip line technology, it isvery flexible to design a UWB feeding networks. However, it has somedrawbacks in mmWave bands, such as the unwanted radiation losses andmutual coupling between adjacent lines. Therefore, the dielectric-basedinverted microstrip gap waveguide (DIMGW) technology is adopted in our workfor the feeding network design. Dielectric-based IMGW, which is based on gapwaveguide technology [22]-[27], is also called as substrate integrated gapwaveguide (SIGW) [28]-[32]. lt can be easily fabricated by multi-layer printedcircuit board (PCB) technology. ln the present disclosure, a UWB fixed-beam CP planar TCA array antennawith bold-C spiral elements is designed, fabricated and measured. This planararray antenna has achieved a bandwidth of about 45% for both the impedancematching and the AR, much wider than those of the existing mmWave planararray antennas in the literature. We have done some modifications andoptimization in the final design by considering the fabrication constraints andthe manufacture cost, which are presented here. The novelties of this workinclude the followings. i) The tightly coupled bold-C spiral array has beenintroduced and optimized to achieve UWB performance for the CP fixed-beamin mmWave bands. ii) A simple compact feeding structure has been introducedfor the bold-C spiral elements. iii) A feeding network using DIMGW technologyhas been designed with a compact size. iv) A simple multi-layer PCB structurefor the whole array antenna has been designed and realized for a low-cost manufacture.

The Detailed Description is organized as follows. The configuration and thedesign of the bold-C spiral radiation element, the TCA geometry and theDIMWG feeding network elaborated in Section ll. The influence of PCB'scopper foil on the loss is investigated in Section lll. The simulations andmeasurements of the 8 >< 8 array are illustrated in Section IV. Section V draws the conclusions of this work.

Section ll - Antenna Desiqn and Confiquration Fig. 1 shows an example 8 >< 8 array comprising the disclosed antenna element100. Fig. 25 shows an example antenna element 100. lt is appreciated thatany array size comprising a plurality of antenna elements 100 is possible. Theexample array in Fig. 1 consists of four layers of stacked substrates. Betweenadjacent substrates is prepreg FR28 for the lamination of multi-layer PCB. Onthe top (substrate 4) is Taconic TLY-5 with a relative dielectric constant of 2.2and 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 11 as 1.016, 0.254 and 1.524mm, respectively. The bold-C spiral elements areetched on the top of substrate 1, and the distance spacing between theadjacent elements is 5.7 mm in both X- and y-directions. The corporate feedingnetwork is designed with the DIMGW technology, which is composed of themicrostrip line, a perfect electrical conductor, PEC, ground (the second groundplane) and an electromagnetic bandgap, EBG, structure. The elements areconnected to the feeding network by feeding vias through substrate 1, the first ground plane and substrate 2.

Fig. 25 shows one embodiment of the disclosed antenna element. Morespecifically, Fig. 25 shows an antenna element 100 for an antenna array. Theantenna element comprises a first substrate 160, a second substrate 140, anda third substrate 120 stacked to form a layered structure. The first substratehas a first surface facing away from the second and third substrates, whereinthe first surface comprises a radiating element 161 . A first ground plane 141 isarranged between the first and the second substrates. The second substrate140 has a first surface arranged facing the third substrate, wherein the firstsurface comprises a transmission line arrangement 131 comprising at leastone planar transmission line. A feeding via hole 151 arranged extendingthrough the first substrate layer 160, through the first ground plane 141, andthrough the second substrate layer 140 is configured to connect the radiatingelement 161 and the transmission line arrangement 131. The third substrate120 comprises an electromagnetic bandgap, EBG, structure 110,121,122arranged to prevent electromagnetic propagation in a frequency band ofoperation from propagating from the transmission line arrangement 131 indirections 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. 25, at leasttwo of the first, second, third, and fourth substrates 120,140,160,180 areseparated by a prepreg layer 130,150,170. ln the present disclosure, a multi layered printed circuit board (PCB), or layered structure, comprises stacked substrate layers. The substrate is sometimes 12 called core. A substrate comprises a non-conductive material. Two exampleproperties of a substrate are the dielectric constant (also called relativepermittivity, an example value is 2.2) and dielectric loss (often characterized interms of a loss tangent, an example value is 0.004). An example of a substrateis FR4, which is a woven epoxy resin impregnated fiberglass cloth. Anotherexample of a substrate is insulated metal substrate. A substrate may have athickness 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 thicknessin the order of 10-100 micrometers, but other thicknesses are also possible.The conductive foil may be arranged (e.g. by etching) to form differentstructures, such as a radiating element, planar transmission lines, or a groundplane. ln a multilayer PCB, multiple substrates may be attached together usingprepreg layers in between the substrate layers. Prepreg (or pre-preg) is shortfor pre-impregnated composite fiber, and is used as an adhesive, i.e. forbonding different layers together. An example of a prepreg is uncured epoxyresin-based substrate, such as uncured FR4 or FR28. The thickness of aprepreg may be in the order of 0.1-10 mm, but other thicknesses are alsopossible. A via hole (orjust via or vertical interconnect access) is an electricalconnection that may extend through a single or through multiple substratelayers. The via may therefore connect different layers of conductive foil layerstogether. A via may comprise a hole, which is made conductive byincorporating a conductive tube or by electroplating the hole. The diameter ofa via hole can, e.g., be in the span 0.1-10 mm, but other diameters are also possible. ln Fig. 25, it is noted that the radiating element 161 may comprise a conductivefoil laminated to the first substrate 160. The first ground plane 141 maycomprise a conductive foil laminated to the second substrate 140. The groundplane 141 may alternatively or in combination comprise a conductive foillaminated to the first substrate 160. The transmission line arrangement 131 may comprise a conductive foil laminated to the second substrate 140. 13 A planar transmission line is a transmission lines wherein the electricalconductor (e.g. copper) is flat. Examples of planar transmission line are microstrip, stripline and coplanar waveguide.

Dielectric-based lnverted Microstrip Gap Wavequide (DIMGW) Fig. 2 illustrates the geometry of DIMGW, which consists of two stackedsubstrates and a prepreg. The microstrip line is on the top of mushroom-likeEBG structure and below the first ground plane. Thus, the mushroom-like EBGstructure with artificial magnetic conductor property and the PEC ground (thesecond ground plane) with a distance smaller than a quarter wavelength makea parallel plate waveguide where no waves propagate. With the microstrip line,the Quasi-TEM wave can propagate between the PEC ground and the stripline, which is referred to as dielectric-based inverted microstrip gap waveguide(DIMGW). Therefore, the mutual suppressed by this gap waveguide structure, which is very helpful to the coupling between adjacent lines is 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-TEMmode, and less losses than the microstrip line due to no radiation loss. lnaddition, the whole structure of DIMWG can be easily fabricated by multi-layerPCB process and integrated with RF components. lt is appreciated that themicrostrip line can be replaced by any planar transmission line, such ascoplanar transmission line. ln the disclosed antenna element 100, withreference to Fig. 25, the EBG structure 110,121,122 and the at least one planartransmission line may together form an inverted planar transmission line waveguide. Furthermore, in the disclosed antenna element 100, with reference toFig. 25, the EBG structure comprises a second ground plane 110 and aplurality of EBG mushrooms. Each EBG mushrooms comprises a patch 122and a via hole 121. The via hole extends through the third substrate 120 andis configured to connect the patch 122 to the second ground plane 110. Thepatch 122 and/or the second ground plane 110 may comprise conductive foilslaminated to respective surfaces on the third substrate 120. The thickness of the third substrate is preferably smaller than a quarter of the operational 14 wavelength of the highest frequency in the frequency band, in order for theEBG 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 afrequency band of operation from propagating from the transmission linearrangement 131 in directions other than through the feeding via and along theat least one planar transmission line. A frequency band is an interval offrequencies 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. lt is possible to replace the third substrate 120 comprising an EBG structurewith an air based EBG structure. Such air based EBG structure may, forexample, be based on conductive repetitive protruding elements extendingfrom 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 EBGunit cell with the ground plate are optimized to achieve a stopband covering asmuch as possible over about 20-30 GHz. The final optimized dimensionparameters, including the period, the diameters of via and pad, are given inTable I (Fig. 22). The dispersion diagram of the unit cell, obtained using theeigenmode solver of CST Microwave Studio, is shown in Fig. 3. lt is seen that a stopband covering 15.4-36.25 GHz is achieved.

Bold-C Spiral Radiation Element The geometry ofthe proposed example bold-C spiral radiation element is givenin Fig. 4. As it is known, a single-arm multi-turn spiral antenna can be fed by asimple feeding structure with excellent wideband performance with a bit largesize [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 beenmodified significantly and optimized for UWB impedance match and low ARperformance. The final optimized spiral structure, referred to as the bold-Cspiral in this paper. ln the disclosed antenna element 100, with reference toFig. 25, 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 stackedsubstrates. TLY-5 is used on the top of the element as superstrate to improvethe impedance matching characteristic. lt is appreciated that TLY-5 is just oneout of many possible substrates for improving matching. Therefore, withreference to Fig. 25, the disclosed antenna element 100 may optionallycomprise a fourth substrate 180 stacked adjacent to the first surface of the firstsubstrate 160.

The top views of substrates 1-3 are shown in Fig. 5 and the final optimizeddimensions of the example bold-C spiral element are given in Table I (Fig. 22).The bold-C spiral elements are etched on the top surface of substrate 1 andeach fed by a single via through substrate 2, the ground and substrate 1. Thesingle feeding structure is one of the advantages of the proposed bold-C spiralelement, which makes the corporate network simple and helpful to achievewide bandwidth performance within the limited space due to the concept ofTCA.

The concept of TCA is adopted here in the design of bold-C spiral array for afixed-beam planar array antenna. Usually, the element spacing in TCAs is lessthan 0.5Åhigh to avoid scanning anomalies in high frequency. Considering theaim of a fixed beam array antenna here, the element spacing is chosen as0.57Åhigh to obtain more space for the feeding network. The single-arm spiralelement with periodic boundary, i.e. the unit cell, was optimized to achieve UWB characteristics. Fig. 6 shows the variation of the simulated activereflection coefficient and AR with the element spacing w. lt is seen that theelement spacing w has nearly no influence on the impedance matching performance while the AR performance deteriorates with the element spacing 16 reducing. This phenomenon proves that a wideband CP element can beachieved by using the concept of TCA.

Four steps are shown in Fig. 7 to explain the design process of the proposedbold-C spiral element. Step 1 is an 1/4-turn single-arm square spiral. ln Step2, a triangular chamfer is implemented in the second corner of the spiral. lnStep 3, two other triangular chamfers are carved in the beginning of the spiral.ln Step 4, the fourth triangular chamfer is carved in the first corner. Fig. 8shows the simulated results of the unit cell in Steps 1-4. lt can be seen that theunit cell in Step 1 has an impedance bandwidth from 16 to 26.2 GHz and anarrow 3-dB AR bandwidth from 16.4 to 23 GHz. After cutting the chamfer inthe second corner of the spiral of Step 1, the AR bandwidth is enlargedcovering 16.6-26.9 GHz, and the impedance matching is also slightlyimproved. The third step is changing the beginning structure of the spiral withtwo more triangular chamfers, and the AR bandwidth is improved further,covering 16-27.5 GHz. After carving the last triangular chamfer in the firstcorner, the operating band is moved to the high frequency, and the overallbandwidth is improved. This phenomenon is a result of current flowing pathbeing more and more circular with each step and the width of the spiral makingthe wideband performance since the current paths for high frequencies andlow frequencies are along the inner circle and the outer circle, respectively.The simulated current distributions on the spiral at different time and frequencypoints are given in Fig. 9 to explain the generation of the circular polarizationof the proposed CP element. lt is seen from Fig. 9 (a) that att = 0 the resultanttotal current is along +y-direction at 20 GHz. At t = T/4, the direction of theresultant total current is changed to +x-direction. Thus, the current changes ina clockwise direction from t = 0 to t = T/4, which generates a left-hand circularpolarization (LHCP) wave. Similarly, this phenomenon also occurs at 30 GHz.Fig. 10 shows the simulated results of the proposed bold-C element, includingthe 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 ARbandwidth record of the existing mmWave CP planar array designs in the literature. Considering the impedance matching characteristics, the overall 17 bandwidth is 50.7% covering 18.1-30.4 GHz. Fig.11 presents the simulatedradiation 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 reportedelements in the literature are summarized in Table ll (Fig. 23). Apparently, thebold-C spiral element achieves the widest bandwidth above 50% for both ARand impedance match. lt is appreciated that any array size comprising a plurality of antenna elements100 is possible. Although the element spacing in TCAs is typically less than0.5Åhlgh, it is possible to arrange a plurality antenna elements 100 in an arrayin a span of element spacing, e.g., from 0.1Åhigh to 2Åhagh. Other elementspacing lengths are also possible. radar transceiver According to aspects, a telecommunication or comprises at least one antenna element 100. lt is noted that the bold-C spiral element may be useful for other structuresthan the disclosed antenna element 100, as in, e.g., a two layered PCBstructure. Thus, according to aspects, there is herein disclosed a bold-C spiralelement comprising a first substrate 160. A first surface on the first substratecomprises a radiating element 161, the element having an arcuate formextending from a feeding via hole 151 as exemplified in Fig. 7. The bold-Cspiral element is shaped like the letter 'C', i.e., it extends from the via hole in afirst direction, and then turns clockwise by approximately 45 degreed in threeturns to form the 'C' shape. Figure 7 describes a method for generating thebold-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 layer140 stacked adjacent to a second surface of the first substrate 160, whereinthe second substrate has a first surface arranged facing away from the firstsubstrate. A ground plane 141 is optionally arranged between the first and thesecond substrates. The first surface of the second substrate optionallycomprises a transmission line arrangement 131 comprising at least one planartransmission line. The feeding via hole optionally extends through the first substrate layer 160, through the first ground plane 141, and through the 18 second substrate layer 140, and is thereby configured to connect the radiatingelement 161 and the transmission line arrangement 131. The bold-C spiralelement may optionally comprise a fourth substrate 180 stacked adjacent tothe first surface of the first substrate 160. ln the bold-C spiral element,optionally at least two of the first, second, and fourth substrates 140,160,180are separated by a prepreg layer 150,170.

Corporate feedinq networkA corporate feeding network for the 8 >< 8 bold-C spiral planar array antenna isdesigned with DIMGW. A similar design has been presented in [35], so wedescribe the main design procedure briefly here for the convenience for thereaders. Fig. 12 shows the geometry of the 1-to-64 feeding network, which iscomposed of sixteen type 1, four type 2 and one type 3 of 1-to-4 equally splitpower dividers and a transition from the dielectric-based IMGW to themicrostrip line. Multi-step impedance transformers are used in 1-to-4 powerdividers to broaden the impedance match bandwidth. Thanks to the merits ofthe DIMGW which suppresses the mutual coupling between closely adjacenttransmission lines, a good performance has been achieved. Fig. 13 presentsthe simulated reflection coefficients of these three types of 1-to-4 powerdividers. lt can be observed that the bandwidth with the reflection coefficientless than -15 dB is covering 18.2-35.2 GHz. the disclosed antenna element 100, with reference to Fig. 25, the transmission ln an antenna array comprising line arrangements 131 of all antenna elements constitute a corporate feeding network. lt is appreciated that the one or more transmission line arrangements may bearranged in other configurations than a corporate feeding network, such asmultiple feeding networks for groups of antenna elements or beam steeringnetworks. Optionally, the one or more transmission line arrangementscomprise RF components, such as capacitors, resistors and inductors.

Integrated RF components are also possible, such as integrated chips, lCs. 19 Section lll - Influence of PCB's Copper Foil on Loss PCB manufacturers, such as Rogers and Taconic, offer high frequency circuitsubstrates with various types of copper foils, such as standardelectrodeposited copper foils, rolled copper foils, resistive copper foils, etc.produced by different manufacturing processes. Different types of copper foilshave different surface roughness, which affects the conductor losses oftransmission lines in mmWave bands significantly. ln order to have accuratedata for the transmission line loss for our design, the insertion losses of themicrostrip lines on Taconic TSM-DS3 with two different copper foils arefabricated and measured. One copper foil designation is CH, which iscommonly used. The other one is ULPH, which has the smoothest surfaceamong the various types of copper foils. The thickness of substrate TSM-DS3for obtaining insertion loss data by measurements is selected as 0.254 mmwhich is the same as that of substrate 2 in the array antenna.

As shown in Fig. 14, two pairs of PCB prototypes with 50-Q characteristicimpedance microstrip lines on both TSM-DS3-CH and TSM-DS3-ULPH arefabricated with silver plating and lengths of 20 mm and 50 mm in both pairs.Two 2.4-mm Southwest end launch connectors are deployed formeasurements. With different lengths, the insertion losses of the microstriplines are obtained from the measured S parameters. Fig. 15 shows themeasured insertion losses of the microstrip line based on TSM-DS3-CH andTSM-DS3-ULPH. lt can be seen that the insertion loss with TSM-DS3-ULPHis much lower due to its smoother copper foil surface. Also, the measured lossof 2.4-mm end launch connector is presented in Fig. 16. The loss varies from0.18 to 0.36 dB over the frequency range of 15-35 GHz. ln consequence, theefficiency of the planar antenna will be improved if TSM-DS3-ULPH is used. lnthis work for the optimal trade-off between the cost and the performance, theTSM-DS3-ULPH is applied to the feeding network on substrate 2, and theTSM-DS3-CH is applied to radiation elements and mushroom pin structure onsubstrate 1 and 3. The total cost of one 8 >< 8 array antenna prototype as described below would be less than ê20 if 10000 pieces were ordered.

Section IV - Measurement and Discussion With the consideration of multi-layer PCB process requirements, a widebandLHCP (Left Hand Circularly Polarized) 8 >< 8 planar array antenna with the bold-C spirals elements has been designed and fabricated. The Taconic prepregFR28 is used for bonding the substrates. The thickness of FR28 used is 0.124mm for bonding TLY-5, substrate 1 and 2, 0.248 mm between substrate 2 and3. Fig. 17 shows the fabricated prototype. The external coppers of PCB areplated with pure gold without nickel, which makes the 10 mm microstrip lineachieve a good transmission performance. The size of the 8 >< 8 CP arrayantenna is 45.6><55.6><4.4 mm3. For measurement, a 2.4-mm Southwest endlaunch connector is deployed.

The reflection coefficient of the proposed CP array antenna is measured usingAgilent E8363B vector network analyzer. Fig. 18 shows the measured andsimulated reflection coefficients, which agree with each other quite well. Themeasured and simulated impedance matching bandwidths with the reflectioncoefficient less than -10 dB are 49% from 18.2 to 30 GHz and 52.8% covering17.87-30.7 GHz, respectively. Fig. 18 illustrates the measured and simulatedARs of the array. The measured 3-dB AR bandwidth is 44.9% from 19 to 30GHz, while the simulated one is 49.2% covering 18.6-30.75 GHz. Consideringthe impedance and AR bandwidth, the overall bandwidth is 44.9% covering19-30 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 andyz planes of the array at 19, 24.5 and 30 GHz are shown in Fig. 20. Goodagreement can be observed between the measured and the simulated LHCPradiation patterns. ln addition, the low backlobe level has been achieved byusing DIMGW. With eliminating the loss of 2.4-mm end launch connectorshown in Fig. 16, the measured and simulated gains and efficiencies areshown in Fig. 21. The maximum measured gain is 20.9 dBi at 26.5 GHz. Thedifference between the measured and simulated gains from 28 to 30 GHz may be caused by the difference between the simulated and measured ARs at 28- 21 GHz, and the real insertion loss of the transmission line which is frequency-dependent (see the measurement in Fig. 15) while in the simulations the losstangent of the substrates is constant over the frequency. The total efficiency iscalculated with the measured gain and the antenna aperture of 45.6><45.6mm2. lt is seen that the measured efficiency is above 40% over 18-29 GHz.

The performance of the proposed and several reported CP planar arrayantennas are summarized in Table lll (Fig. 24). The proposed CP arrayantenna exhibits the widest overall bandwidth of 44.9%, which considers theimpedance and AR bandwidth. ln addition, the minimum total antennaefficiencies (eafiemlsmalch eap erad) are also shown in Table lll (Fig. 24).Although the proposed CP array antenna has a lower maximum gain than thatin [12], [14] due to a smaller aperture caused by the concept of TCA, itsminimum total antenna efficiency within the working band is larger than 34%, which is higher than that in the most of the other designs.

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

References 22 [1] M. K. Samimi and T. S. Rappaport, “3-D Millimeter-Wave statisticalchannel model for 5G wireless system design,” IEEE Trans. Microw. TheoryTechn., vol. 64, no. 7, pp. 2207-2225, Jul. 2016. [2] C. Liu, Y. Guo, X. Bao, and S. Xiao, “60-GHz LTCC integrated circularlypolarized 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 andridge 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 patchantenna 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 ratiobandwidth enhancement of 60-GHz substrate integrated waveguide-fedcircularly 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 widebandcircularly polarized cavity-backed stacked patch antenna array for millimeter-wave applications,” IEEE Trans. Antennas Propag., vol. 66, no. 10, pp. 5170-5179, Oct. 2018. [7] Y. Zhang, W. Hong, and R. Mittra, “45 GHz wideband circularly polarizedplanar antenna array using inclined slots in modified short-circuited SIW,” IEEETrans. 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 circularlypolarized antenna array with substrate integrated cavity excitation,” IEEEAntennas 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 spiralantenna array for millimeter-wave applications,” IEEE Trans. AntennasPropag., vol. 65, no. 2, pp. 636-643, Feb. 2017. 23

[10] J. Wu, Y. J. Cheng, and Y. Fan, “Millimeter-wave wideband high-efficiencycircularly 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. AntennasPropag., 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 widebandcircularly polarized microstrip antenna array for 45 GHz application,” IEEETrans. 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, “MiIIimeter-wave planar broadband circularly polarized antenna array using stacked curlelements,” 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 ofplanar ultrawideband modular antenna arrays with improved bandwidth,” IEEETrans. Antennas Propag., vol. 66, no. 2, pp. 692-701, Feb. 2018.

[16] S. S. Holland and M. N. Vouvakis, “The planar ultrawideband modularantenna (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, “Ultrawidebandphased antenna arrays based on tightiy coupled open folded dipo|es,” IEEETrans. Antennas Propag., vol. 18, no. 2, pp. 378-382, Feb. 2019.

[18] W. Zhou, Y. Chen, and S. Yang, “Dual-Polarized tightly coupled dipolearray 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-Linearpolarized phased array with 9:1 bandwidth and 60° scanning off broadside,”IEEE Trans. Antennas Propag., vol. 67, no. 3, pp. 1996-2001, Mar. 2019. 24

[20] D. K. Papantonis and J. L. Volakis, “Dual-Polarized tightly coupled arraywith 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 phasedarray 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-Iglesias, E. Alfonso, and A. Valero-Nogueira, “Design and experimental verification of ridge gap waveguide in bedof nails for parallel-plate mode suppression,” |ET Microw., Antennas Propag.,vol. 5, no. 3, pp. 262-270, 2011.

[23] A. Vosoogh, P. Kildal, and V. Vassilev, “Wideband and high-gaincorporate-fed gap waveguide slot array antenna with ETSI Class II radiationpattern 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 slotarray antenna based on ridge gap waveguide with 30% bandwidth,” IEEETrans. 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 withsingle-layered corporate-feed based on ridge gap waveguide in the 60 GHzband,” IEEE Trans. Antennas Propag., vol. 67, no. 3, pp. 1650-1658, Mar.2019.

[26] M. Ferrando-Rocher, J. I.

Bernardo-Clemente, A. U. Zaman, and J. Yang, “8><8 Ka-band dual-polarized Herranz-Herruzo, A. Valero-Nogueira, B. array antenna based on gap waveguide technology,” IEEE Trans. AntennasPropag, 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 invertedmicrostrip gap waveguide,” In 13th European Conference on Antenna andPropagation (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,” ln 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-profiledipole,” IEEEAntennas Wireless Propag. Lett., vol. 17, no. 8, pp. 1373-1376, Aug. 2018.[30] N. Bayat-Makou and A. A. Kishk, “Millimeter-Wave substrate integrateddual 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 substrate-integrated-gap-waveguide-fed magnetoelectric antenna using substrate-integrated gap waveguide sequentially rotating phasefeed,” 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,” IEEETrans. 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-SpiralElements and Dielectric-based IMGW,” ln Proc. IEEE Int. Symp. AntennasPropag. (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,” IEEETrans. 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-basedinverted microstrip gap waveguide,”submitted.

IET Microw., Antennas Propag.,

Claims (10)

CLAIIVIS

1. An antenna element (100) for an antenna array, the antenna elementcomprising a first substrate (160), a second substrate (140), and a thirdsubstrate (120) stacked to form a layered structure, wherein the first substrate has a first surface facing away from the second andthird substrates, the first surface comprising a radiating element (161); wherein a first ground plane (141) is arranged between the first and the secondsubstrates; wherein the second substrate (140) has a first surface arranged facing the thirdsubstrate, 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 substratelayer (160), through the first ground plane (141), and through the secondsubstrate layer (140) is configured to connect the radiating element (161) andthe transmission line arrangement (131); wherein the third substrate (120) comprises an electromagnetic bandgap,EBG, structure (110,121,122)propagation in a frequency band of operation from propagating from the arranged to prevent electromagnetictransmission 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 fourthsubstrate (180) stacked adjacent to the first surface of the first substrate (160).

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

5. The antenna element (1 OO) according to any previous claim, wherein theEBG 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 (1 OO) according to any previous claim, wherein theradiating 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 linearrangements (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 (1 OO).

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