A Radial Line Slot-Array Antenna With Low Side Lobes and a Uniform-Phase, Tapered-Amplitude Aperture Field Distribution

The paper presents a circularly polarized radial line slot array (CP RLSA) antenna with a nearly optimal monotonic variation of slot length with radius, in order to simultaneously reduce antenna side-lobe levels and to make the aperture phase distribution nearly uniform. It has additional advantages of excellent radiation efficiency, high gain and good overall bandwidth. The antenna comprises a single-layer radial transverse electromagnetic quasi-TEM waveguide with radiating slots on the aperture. The amplitude tapering is implemented by gradually varying slot lengths on the antenna aperture as a function of radial distance, utilizing a slot coupling analysis. Several antenna designs were investigated, each having a physical diameter of 345 mm (<inline-formula> <tex-math notation="LaTeX">$23\lambda _{0}$ </tex-math></inline-formula>) and a thickness of 4.6 mm (<inline-formula> <tex-math notation="LaTeX">$0.3\lambda _{0}$ </tex-math></inline-formula>), where <inline-formula> <tex-math notation="LaTeX">$\lambda _{0}$ </tex-math></inline-formula> is the free-space wavelength at the operating frequency of 20 GHz. A prototype was fabricated and measured, showing a good agreement between the predicted and measured results. The sidelobe levels at the operating frequency are less than −20 dB due to tapering in the near-field amplitude distribution. The measured magnitude of the input reflection coefficient (<inline-formula> <tex-math notation="LaTeX">$S_{11}$ </tex-math></inline-formula>) of the antenna is below −10 dB within the frequency range of 19 GHz to 21 GHz. The antenna has a measured peak directivity and a peak gain of 34.3 dBic and 33.8 dBic, respectively. Both the measured 3dB directivity bandwidth and 3dB gain bandwidth are 5.4%. Aperture efficiency is 48% and radiation efficiency is 94.2% at the operating frequency. The fabricated antenna also successfully fulfilled the condition of circular polarization with an axial ratio bandwidth exceeding 5%.


I. INTRODUCTION
A n unprecedented growth and dependency on wireless devices and components have steered a global effort to provide on-the-move connectivity to onboard mobile platforms using satellites and other high-altitude platforms. A key components of this communication model is an highly efficient front-end antenna system capable of steering its beam in a large angular range. Such beam-steering capability is The associate editor coordinating the review of this manuscript and approving it for publication was Davide Comite . essential when the satellite is not stationary relative to the communicating ground terminal.
Traditionally,the most widely used and low-cost solution for receiving satellite services such as satellite television (TV) is based on parabolic dishes. These parabolic dishes are affordable and efficient, but have serious constraints due to large volume when it comes to using on moving platforms such as a train or an aeroplane [1]- [4]. Furthermore, it is extremely challenging to swiftly track a satellite or re-align a parabolic dish to a satellite when the platform is moving. For these reasons only high-end solutions are available for on-the-move connectivity applications and low-cost reflector dishes are often used for stationary platforms.
RLSA antennas are considered one of the most attractive and versatile candidates as the base antenna for antenna beam steering systems based on Near-Field Meta-Steering [4]- [6] because of their promising features such as high gain, high efficiency, extremely low profile, and planar configuration [1], [7]- [19]. This paper presents an extremely highgain radial-line slot array (RLSA) antenna that is useful as the base antenna in such systems. In order to address typical issues of side-lobes, the proposed CP RLSA base antenna has been designed with an optimal tapered amplitude distribution and a uniform phase distribution to maintain excellent farfield pattern quality. The novelty of the paper is the creation of an optimal amplitude taper in the antenna near field while maintaining a nearly uniform near-field phase distribution and by doing so, achieving significantly low side lobe levels in the far field while maintaining a high antenna gain and a large 3dB gain bandwidth.
The RLSA antennas are parallel plate waveguide antennas with a radiating slot pattern on the top plate and have been investigated in the past as an alternative to parabolic dishes [1], [2], [8], [20]- [26]. It is because of their extremely low height, they can conveniently be installed or even concealed on moving platforms. Several RLSA designs reported in literature focused on improving gain, efficiency and return loss performance [9], [23], [27]- [35]. On the other hand, a thorough investigation on RLSA side-lobe reduction methods is yet to be conducted [10], [36]- [44]. This paper addresses this need by developing a method that substantially reduces CP-RLSA side-lobe levels and at the same time enhances phase coherence across the aperture.
The aim of this paper is to develop a highly directive CP RLSA antenna that not only has extremely low sidelobes with excellent pattern quality but also has near-field radiation characteristics needed for beam steering applications. A strong emphasis has been, therefore, placed on achieving uniform aperture phase distribution along with optimally tapered amplitude field distribution on a plane parallel to the antenna aperture. A controlled amplitude distribution is achieved by manipulating the radiating slot lengths on the RLSA aperture, using a slot coupling analysis. Phase distribution is obtained by appropriately selecting the proper in-phase slot layout on the top radiating plate. The paper is arranged such that the operating principle and configuration of the antenna are explained in Section II. The analysis of electric near-field distributions and design examples are presented in Section III. Fabrication and measurement results are given in Section IV to validate the concept.

II. ANTENNA CONFIGURATION
The configuration of the CP RLSA antenna is shown in Fig. 1. It is made of two parallel conducting plates. The feed point is at the center of the bottom plate. These plates form a radial waveguide with radiating slots on its upper plate. The bottom plate behaves as a ground plane. The antenna is fed at the center by a coaxial probe and it forms a rotationally symmetric outward travelling quasi-TEM wave in the radial waveguide. The slots are cut on the upper conductor and the power gradually radiates from these slots as the wave propagates from the center towards outer edge. The slots consist of many pairs; each one of which is a unit radiator that radiates a circular polarized electric field. The slots are arrayed along a spiral pattern to achieve constructive interference in the boresight direction. The spacings between adjacent slot pairs in radial and spiral directions are represented by S ρ and S φ , respectively, as shown in Fig. 1. To suppress the grating lobes, the waveguide is filled with a combination of dielectric material and air. The dielectric material filling the waveguide creates a slow wave and minimizes reflection towards the coaxial transmission line [2], [8]. An area of radius λ g /2 around the center on the upper plate is left unslotted to allow the outward travelling wave to settle before radiating. When the height of the radial waveguide is limited to be less than one half guide wavelength, the only possible symmetric waveguide mode that can propagate within the radial waveguide is the TEM mode. The coaxial feed excites this mode within the radial waveguide. Electromagnetic power is fed from a coaxial transmission line into the center of the radial slow-wave waveguide by the feed probe. The feed has a disk head, shown in Fig. 1 (b), which converts the electromagnetic power from a TEM coaxial mode into a quasi-TEM radial waveguide.
The radiating plate, depending on aperture size, may have several hundreds of unit radiators, as shown in Fig. 1. A radiating unit consists of two slots that are placed orthogonally and have same length (L) and width, to radiate two orthogonal electric-field components with equal magnitude. To radiate circular polarization, the two slots in a unit radiator are physically separated by a distance of λ g /4 in order to create a phase shift of 90 • , where λ g is the guided wavelength. The first slot in a radiating unit is oriented to an angle of 45 • with respect to the current flow line (a line drawn from the center VOLUME 8, 2020 of the radiating surface towards the edge), whereas the second slot in the radiating unit is oriented to an angle of 45 • + θ, where θ is the angle between the two slots in a radiating unit. The RLSA aperture amplitude distribution is controlled by the lengths of radiating slots. A more detailed explanation of slot layout design of CP RLSA antennas is given in [7]. It was followed in this work.

A. SLOT COUPLING ANALYSIS
As the wave travels outwards in the radial waveguide, energy radiates through slot pairs. The reflections from the two slots in a unit radiator cancel each other as they are λ g /4 distance apart and the internal field can be expressed as [19], where α is the coupling coefficient, which represents internal field attenuation. The slow wave factor ζ indicates the wavelength reduction of the radially outward traveling wave due to slot coupling. Under the travelling wave operation, the incident power can be divided into slot radiation and transmitted power as pictorially represented in Fig. 2. The radiation from slots can be modeled as attenuation of internal power and can be written as (1e −2αS ρ ), and the transmitted power as e −2αS ρ [19]. If the attenuation due to slot radiation is higher in the middle of aperture, then the amplitude of the cylindrical wave and the aperture illumination will be steeply tapered. Therefore, slot coupling is the most important factor that controls the aperture field distributions of RLSAs, and consequently, the gains and far-field radiation patterns of RLSA antennas. An RLSA antenna can be designed to produce a particular field distribution for lower sidelobes or higher aperture directivity. The field distribution of a circular planar antenna can be analytically expressed as [45], where, ρ is the radial distance and a is radius of the circular aperture. Equation (2) generates a uniform field distribution over the aperture when n = 0. As the value of n increases the distribution becomes tapered towards the edge. To achieve a specific objective, for example, radiation characteristics of the planar circular arrays with uniform and tapered field distributions are given in Table 1. From the Table 1, it can be seen that a uniform aperture distribution (n = 0) theoretically has highest directivity, whereas a radially taper squared aperture distribution (n = 2) leads to a lowest side lobe levels (SLLs) in circular planar arrays. If all the unit radiators arrayed on the CP RLSA aperture have equal lengths and widths, then each slot couples almost a constant proportion of the radial power. As the power is fed at the centre of the CP RLSA, more energy will radiate from the slots close to the center and less energy will radiate from those closer to the edges. This will reduce the power intensity of the outward travelling quasi-TEM wave by a factor of 1/ √ ρ through the coupling of the radiating slots, which is not favourable in terms of boresight gain. Hence a proper slot coupling analysis is necessary to control illumination over the CP RLSA antenna aperture. One possible method is to keep the slot density constant (S ρ × S φ = constant) on the aperture and control the energy coupled by the unit radiators from the internal waveguide field to the radiating field.
This investigation is focused on creating a tapered amplitude distribution with proper slot coupling analysis for improving the radiation pattern quality by reducing the side lobe levels. The distribution is obtained by manipulating the slot lengths on the antenna aperture. The slot length can be written as a function of radial distance according to where δ is a constant factor that depends on the operating frequency, ρ is the radial distance, and α is the coupling coefficient, which is the rate at which the slot length increases with radius, i.e. ∂L slot /∂ρ. δ is optimised before creating the slot patterns. The term (ρ × α) acts as a coupling factor. For a particular distribution, the coupling coefficient α needs to be optimised. As the radial distance increases, the coupling factor increases, leading to increasing slot lengths. By varying the slot lengths, we can control the proportion of energy coupled from the internal field to the radiating field, and hence the aperture taper.

III. INVESTIGATION ON NEAR-FIELD DISTRIBUTION
To investigate the influence of aperture field distribution on far-field performance, several CP RLSA antennas were designed. Different distributions were obtained by varying the slot length on the antenna aperture. Equation 3 was used to vary the slot lengths. The antennas were designed at 20 GHz following the design procedure explained in Section II and all of them have an aperture size of 23λ 0 (345 mm). The common design parameters of the antennas are summarized in Table 2.
All were designed to provide right hand circular polarization (RHCP). The spiral geometry is made with 13 rings of radiating slots. The waveguide was filled with Taconic TLY-5 dielectric material ( r = 2.2) and air ( r = 1). A 50 disk ended feed probe was used to feed EM power at the center of the waveguide. The design parameters were kept constant for all the designs except the slot distribution on the top radiating plate. The slot layouts on the surface were created using a custom Visual Basic interface with CST Microwave Studio. The plot of slot length (L slot ) as a function of radius is shown in Fig. 3 for five values of α. The value of δ is constant and set to an optimal value of 4.3. α is varied from 0.002 to 0.01 with a step size of 0.002. As it can be seen from the figure that as the value of α increases, the slot lengths increase. The near-field amplitude distributions and the far-field radiation patterns at 20 GHz for different values of α are shown in Fig. 4 (a) and Fig. 4 (b), respectively.
The electric near-field amplitudes of E x are taken on a plane that is parallel to the XZ plane and located 1λ 0 (15 mm) above the top radiating plate. As it can be seen from Fig. 4 (a), for α = 0.002, the amplitude distribution is strongly tapered. As the value of α increases, the amplitude tapering weakens. Values of α in the range of 0.004 and 0.006 provide nearly a radial taper squared distributions, whereas α in the range    α (= 0.004), the phase fluctuation throughout the aperture is only 45 degrees. Such a small phase variation is very much acceptable over the aperture and does not significantly affect radiation performance. Later, we have chosen this value of α for prototyped antenna design explained in the next section, as it provides a nearly uniform phase distribution and low side lobes.

IV. FABRICATION AND MEASUREMENTS A. FABRICATED PROTOTYPE
One of the designs discussed above was fabricated to validate the concept. The parameters of the prototype are given in Table 2. The antenna was simulated using the time-domain solver of CST Microwave Studio to predict the near and farfield radiation characteristics. The front and back views of the fabricated prototype are illustrated in Fig. 6. The printed slot pattern was etched on a Taconic TLY-5 ( r = 2.2) laminate. The slots were etched from a 0.035 mm thick copper cladding attached to the laminate. All slots have a width of 1 mm. Slot lengths vary as described above from 4.324 mm (inner slot) to 4.98 mm (outer slot). The ground plane is made with Aluminum. The Taconic TLY-5 laminate and bottom aluminum plate have holes around the edges. Nylon screws and spacers were used to hold the laminate with the Aluminium plate. The aluminum plate has a hole in the center for the 50 coaxial connector. The connector has a disk-ended feed probe as shown in Fig. 6 (b). The disk head is 2 mm thick and 2.8 mm wide, made with Aluminum and glued on the top of the feed pin.

B. INPUT MATCHING
The magnitude of the input refection coefficient |S 11 |, measured using an Agilent PNA-X N5242A vector network analyzer, is plotted in Fig. 7. The magnitude of reflection coefficient is less than -10 dB in the measured frequency range from 19 GHz to 21 GHz. This CP RLSA antenna has a return loss bandwidth greater than 10%.

C. NEAR-FIELD DISTRIBUTION
The numerical simulation data was post processed to extract relevant amplitude and the phase distributions of the electric field at the operating frequency of 20 GHz, which are shown in Figure 8. The electric near-field patterns of E x and E y are taken in the XZ plane extending from the top surface of the antenna to a distance of 1λ 0 (15 mm above the radiating plate). The figure shows a radial taper amplitude 208536 VOLUME 8, 2020   distribution. 2D and 1D views of the near-field phase distribution of the antenna at 20 GHz are shown in Figure 9 and Figure 10, respectively. It is evident from the phase distribution that antenna radiates nearly planar and symmetric phase fronts, demonstrating the uniformity in the aperture phase distribution.

D. GAIN, AXIAL RATIO AND EFFICIENCY
The far-field patterns were measrued in an NSI near-field spherical antenna range. The gain was measured using the gain comparison method with a WR-51 standard gain horn. The antenna beam peak is in the boresight direction. The  measured peak gain and directivity in the frequency range from 19 GHz to 21 GHz are plotted in Fig. 11. The predicted gain and directivity are also included in the same figure. The predicted peak directivity is 33.8 dBic and predicted peak gain is 33 dBic at 20 GHz. The predicted 3dB gain bandwidth is 6%, from 19.4 GHz to 20.6 GHz. The measured peak directivity is 34.3 dBic and measured peak gain is 33.8 dBic at 19.4 GHz. Here, one can observe a 3% frequency shift toward lower frequencies in the measured results. This shift can be attributed to manufacturing tolerances of the parts and components used in assembling the prototype. The antenna has a measured 3dB gain bandwidth of 5.4%, which extends from 18.95 GHz to 20 GHz.
The predicted and measured axial ratio of the antenna is shown in Figure 12. The axial ratio is less than 3dB, satisfying the standard circular polarization condition, and the 3dB axial ratio bandwidth is more than 5%. Better than expected results obtained in axial ratio measurements are due to fabrication tolerances. The measured aperture efficiency of the prototype is 48%. The radiation efficiency is 94.2% at 20 GHz. Note that in this design no absorber is used, therefore the losses are very low, and efficiency is excellent. The overall efficiency varies between 82% to 92% in the frequency band from 19 GHz to 21 GHz. VOLUME 8, 2020

E. RADIATION PATTERNS
The antenna has stable radiation patterns in the measured frequency band. The radiation pattern measurements were carried out in a NSI spherical near-field range at AusAMF. The predicted 3D radiation pattern at 20 GHz is shown in Fig. 13. The cross-polar level at 20 GHz is 20.5 dB below the co-polar level in the boresight direction. The predicted radiation patterns of the antenna, on φ = 0 • plane and φ = 90 • plane, are plotted in Fig. 14 at six frequencies within the gain bandwidth. These patterns clearly show directive beams pointing towards boresight with significantly low side lobe levels. It can be seen that the SLLs in both φ = 0 • and φ = 90 • planes remain below -20 dB. Antenna performance figures for the given six frequencies are summarized in Table 4.
The measured radiation patterns at four different frequencies, on φ = 90 • plane and φ = 0 • plane, respectively, are shown in Fig. 15 and Fig. 16 along with the radiation pattern envelopes (RPE) of Class-1 and Class-2 antennas, as specified by ETSI. It is evident from the figures that the prototype has good radiation pattern qualities with significantly lower SLLs and have met the requirements of the envelope specified for ETSI Class-1 and Class-2 antennas except for the angles over 75 • on φ = 0 • plane, where the measured pattern exceeded Class-2 RPE.   Table 5 lists the performance figures of the prototyped CP RLSA and compares with some previously published RLSA antennas. As it can be seen from the table that the proposed CP RLSA antenna has the lowest side lobe levels and highest antenna efficiency compared to the previously reported RLSAs. Antenna efficiency is high because no absorber was used in this design. The new antenna also has 208538 VOLUME 8, 2020   a larger measured 3dB gain bandwidth compared to the previous RLSAs. Furthermore, the height of the new CP RLSA antenna is the second lowest in terms of wavelengths at the center frequency.

V. CONCLUSION
In this paper, we demonstrated a method to control the nearfield amplitude distribution of CP RLSA antennas to improve far-field performance while making the near-field phase distribution nearly uniform. The tapered amplitude distribution was achieved using a slot coupling analysis. This method relies on the fact that the energy coupled by a radiating slot to space depends on slot length and internal field strength, and by changing slot lengths on the radiating surface, different near-field amplitude distributions can be achieved. The measured prototype, which is 4.6 mm (0.3λ 0 ) thick, has a peak gain of 33.8 dBic, an improved 3dB gain bandwidth of 5.4%, a return loss bandwidth of more than 10% and total antenna efficiency of 84.4%. Measurement results have shown that by using the proposed strategy, amplitude distribution can be controlled, which leads to significant reduction in side lobe levels. At 20 GHz, the side lobe levels of the measured prototypes are less than -20 dB. Dr. Afzal has received several awards and scholarships, including a meritbased scholarship in six out of eight semesters during the undergraduate degree, a scholarship of complete fee waiver during the postgraduate degree, and the international Macquarie Research Excellence (iMQRES) scholarship towards Doctorate study from Macquarie University. He received a competitive travel grant in 2015 to present my research work at a flagship conference under the Antennas and Propagation Society (APS) in Vancouver, Canada. He assisted in preparing several grant applications, including a successful ARC Discovery Grant in 2018. He was third CI in a team of five who received a grant of more than $20K from the German Academic Exchange Service in a funding scheme ''Australia-Germany Joint Research Co-Operation Scheme.'' He is currently working on the development of satellite-terminal antenna technology and has research interests in electromagnetic phaseshifting structures, frequency selective surfaces, and similar metamaterials for microwave and millimeter-wave antenna applications.