Different Designs of Dual-Focus Perforated Transmitarray Antenna in Near / Far-Field Region

In this paper, the design of single-focused and dual-focused transmitarray antennas in the near-field (NF) or/and far-field (FF) applications are proposed. A unit-cell element consists of a dielectric square box with four identical holes is used. The phase compensation of the unit-cell elements depends on changing the hole radii according to the position of the unit-cell element, the feeding horn, and the beam direction. The radiation characteristics of 13×13 unit-cell elements transmitarray antenna focused in the nearand the far-field regions are investigated. A single structure, dual-focused transmitarray is designed using a chess-board arrangement of the unit-cell elements. Double-focused transmitarray antennas for FF/FF, FF/NF, and NF/NF are designed and investigated. The phase distributions and the corresponding hole radii for different configurations of the dual-focused transmitarray antennas are presented. The radiation characteristics of different array configurations are investigated and analyzed using a full-wave simulator CST Microwave Studio.


Introduction
One of the major trends in antenna design is to enhance antenna gain for a variety of applications.Different types of high gain antennas such as, parabolic reflector, dielectric lens, and phased array are investigated [1].These antennas provide high gain radiation characteristics, however, these antennas are bulky, heavy, and need complex feeding networks [2,3].The reflectarray and transmitarray antennas are safe alternatives to parabolic reflectors and lenses because of their planar low profile, simple manufacturing process, and low cost especially for beam shaping applications [4,5].The transmitarray antenna consists of unit-cell elements arranged in a planar structure, and is illuminated by an incident wave from horn antenna.The antenna unit-cell elements are tuned in order to collimate the transmitted wave in a directed focused beam [6].Different methods that used to control the phase compensation of the transmitarray elements are discussed in details in [7][8][9].The radiation characteristics of different transmitarray designs using dipoles, microstrip patch, and dielectric resonator antenna are investigated in [10][11][12][13][14]. Multifocal optical lenses are artificial lenses designed to provide simultaneous focus at different distances in contrast to mono-focal lenses which only one focal point [15].Focused beam systems are borrowed from optics such as spherical optical lenses.Nearfield focused transmitarray antenna is a good choice for many applications due to low profile, low cost, and compact structure.Near-field focused transmitarray transforms the incident plane wave into a spherical wave that focused at a focal point [16].The design of the near-field (NF) focused antenna is based on tuning the phase of the field, along the aperture of the antenna in order to obtain a locally plane wavefront at a focal point [17].NF-focused antennas have different applications such as microwave remote sensing, and medical application [18,19].NF-focused microstrip arrays are designed for gate-access control and industrial heating [20,21].Radio-frequency identification (RFID) are introduced as a proper application for NF-focused antennas for fixed readers [22][23][24].In this paper, a dual-focused transmitarray antennas with different focal locations are introduced using a single structure.The unit-cell element consists of a perforated dielectric box with identical four holes.Double-focused transmitarrays in the far-field and near-field regions are presented.The chess-board unit-cell element arrangement is applied for dual-focused transmitarray design.The radiation characteristics of different transmitarray configurations are analyzed using (Computer Simulation Technology) Microwave Studio CST [25].

Transmitarray unit-cell element design
The shape of the perforated dielectric unit-cell element for transmitarray antenna design has been introduced in [26] as shown in Fig. 1a.The unit-cell element consists of a square dielectric box, with an arm length L = 15 mm, thickness h = 15 mm, and dielectric constant, εr =12 (HiK500F material).Each unit-cell element has four identical circular holes with radius r.The required phase compensation of each unit-cell element is achieved by varying the hole radius, using the waveguide simulator model [26].The waveguide simulator is excited by a normal incidence plane wave for simplicity (This premise holds for incident plane wave with oblique angle up to 40 points).The variations of the transmission coefficient phase and magnitude versus the hole radius, r, at 10 GHz are shown in Fig. 1b.The transmission coefficient phase variation from 0 to 340 degrees with transmission coefficient magnitude variation from 0 to -5.3 dB is achieved.The hole radius changed from 0.2 to 3.75 mm.The results are calculated using the CST software and are compared with that calculated using High Frequency Structural Simulator (HFSS).Satisfactory agreement between results is obtained.

FF-NF-focused transmitarray antenna
The transmitarray is considered as a 2-D planar array with spatial feeding horn with its aperture located at (xf, yf, zf).The phase of each unit-cell element depends on its position in the array (xij,yij), the direction of the transmitted beam (  , ∅  ), and is given by: where   is the propagation constant, dij is the distance from the feed point to the ij th element in the array.The NFfocusing is achieved by adding an extra phase to the unit-cell elements to collimate the beam at a focal point at a distance Ro from the array aperture as shown in Fig. 2. The extra phase shift of the ij th element in the NF-focused transmitarray is calculated from [27], The total phase for the NF-focus array is obtained by adding Eq. 1 to Eq. 3 for each unit-cell element.The design of 13×13 unit-cell elements perforated transmitarray with total area of 195×195 mm 2 to collimate field at far-field (FF) at 10 GHz applications is shown in Fig. 3.The design of 13×13 unitcell elements perforated transmitarray with the same area to collimate field at near-field (NF) is shown in Fig. 4. A linearly polarized horn antenna is used to feed the transmitarray with dimensions Ri = 25 mm, li × wi = 11.4×22.8mm 2 and thickness t = 0.6 mm.The horn antenna is placed at a distance F = 175.5 mm from the transmitarray aperture.The transmitarray has a symmetric configuration.
The phases and values of holes radii are symmetric.The phase distribution on the first quadrant of the FF-focused transmitarray and the NF-focused transmitarray at Ro = 310 mm with the corresponding hole radii are listed in Table 1.The E-plane and H-plane radiation patterns are presented in Fig. 5.The FF-focused transmitarray introduces a maximum gain of 23.45 dB compared to 17.26 dB for the NF-focused transmitarray.The maximum gain is reduced for the NFfocused transmitarray due to the collimating of the radiated power in the NF.The side lobe level (SLL) is -15.7 dB/-18.6 dB with half-power beamwidth (HPBW) of 7.6/8.3degrees in the E-plane/H-plane for the FF-focused transmitarray.The side lobe level (SLL) is -8 dB/-8 dB with half-power beamwidth (HPBW) of 11.9/11.7 degrees in the E-plane/Hplane for the NF-focused transmitarray.A wider beam is obtained in the far-field for the NF-focused transmitarray due to the concentration of power in the near-field.The normalized NF power density distribution along the focal plane at Ro = 310 mm for the FF-focused transmitarray and the NF-focused transmitarray in x-y plane are indicated in Fig. 6.The contours of the power density are closer to each other for the NF-focused array with -10 dB spot area 54 mm × 49 mm compared to the larger separation between the contours for the FF array with -10 dB spot size of 99 mm × 96 mm. Figure 7 shows the distribution of the normalized power density along the x-axis and y-axis at the distance Ro for the FF-focused transmitarray and the NF-focused transmitarray.The contour curves show that the field amplitude of the power density decays more slowly in the case of FF-focused transmitarray than that for the NFfocused transmitarray.

Design of dual-focused (FF/NF) transmitarray design
In order to have a single structure transmitarray with dualfocused capability the chess-board unit-cell element arrangement is employed.The new single structure works in the FF-and NF-focused at the same time with high efficiency.The new design idea depends on rearranging the unit-cell elements successively using the pervious FF-and NF-focused transmitarrays to get the single structure with dual-focused.Principle of chess-board design for dualfocused (FF/NF) transmitarray design is presented in Fig. 8. Table 2 shows with SLL of -13.4 dB/-13.9dB and HPBW of 8.3/8.9 degrees in the E-plane/H-plane.The gain variations versus frequency for 13×13 FF-focused transmitarray, NF-focused transmitarray, and dual-focused transmitarray antennas are shown in Fig. 11. Figure 12 shows the distribution of the normalized power density for FF-, NF-focused, and dualfocus transmitarrays for in x-y plane.The dual-focused transmitarray introduces an intermediate performance between the FF-focused transmitarray and the NF-focused transmitarray using the single structure.

Design of NF/ NF and FF/FF dual-focused transmitarray
The previous concept is applied to obtain dual-focused in the near field region or dual-direction beams in the far field region.The design depends on the chess-board unit-cell element arrangement.The arrangement of the cells depends on the arrangement of the cells at different two distances in the Fresnel zone at Ro = 310 mm and Ro = 347 mm from the antenna aperture.Table 3 shows the phase distributions and corresponding hole radii for first quadrant of the NF/NF-Dual-focused transmitarray.The contour plot of the normalized power density in the x-y plane for the NF/NFdual-focused transmitarray is shown in

Conclusion
The paper introduces a single transmitarray antenna structure with dual-focus using the chess-board arrangement.The dual-focus of the transmitarray in near-field and a dual-beam directions in the far-field are designed.The transmitarray consists of 13×13 unit-cell elements made of a perforated dielectric sheet.Different transmitarrays with dual-focus in the NF/NF, FF/FF, NF/FF and FF/NF regions are designed using the chess-board arrangement.The FF/NF-focused transmitarray introduces a maximum gains of 23.45 dB/17.26dB with a side lobe level of -15.7 dB/-8 dB in the E-/H-plane.In the same structure the array produced power density closer to each other with -10 dB spot area of 63×60 mm 2 at a distance Ro = 310 mm.The NF/NF transmitarray with power focused in two near field distances at Ro1 =310 mm and Ro2 =347 mm is designed.Finally, dualbeam transmitarray is proposed to obtain two beams in different directions with angles 20 o and -20 o in the same structure using the chess-board arrangement.The maximum gains for beam at θ = 20 o is 15 dB and for the beam at θ = -20 o is 17 dB.

Figure 1 :
(a) Transmitarray unit-cell element design at 10 GHz.(b) The variations of the transmitted phase and magnitude with hole radius at 10 GHz.

Figure 2 :
Figure 2: The NF focused at a distance z = Ro from the transmitarray aperture.

Figure 5 :
Figure 5: The E-plane and H-plane radiation patterns variations versus the elevation angle for configuration 13×13 FF focused transmitarray and 13×13 NF focused transmitarray at F/D = 0.9 and frequency 10 GHz.
the phase distributions and corresponding hole radii for first quadrant of the FF/NF-Dual-focused transmitarray.The 3-D view of the normalized power density in the transverse plane (x-y plane) at a distance Ro = 310 mm is shown in Fig.9a.The contour plot of the normalized power density in the x-y plane for the dual-focus transmitarray at a distance Ro = 310 mm is shown in Fig.9b.The contours of the power density are closer to each other with -10 dB spot area of 63 mm × 60 mm.The E-plane and H-plane radiation patterns for 13×13 dual-focused transmitarray antennas at frequency 10 GHz compared with others transmitarrays planes are shown in Fig. 10.The dualfocused transmitarray introduces a maximum gain of 21 dB

Fig. 13 .
Figure 13: A contour plot of the simulated normalized power density of for the dual-focused transmitarray in a plane at (a) Ro1 = 310 mm and (b) Ro2 = 347 mm.

Figure 14 :
Figure 14: The 3D power pattern of dual beam transmitarray in the same structure.

Table 1 :
The phase shift and the corresponding hole radius for FF and NF transmitarray (first quadrant).

Table 2 :
The phase shift and the corresponding hole radius for FF/NF transmitarray (first quadrant).

Table 3 :
The phase shift and the corresponding hole radius for NF/NF transmitarray (first quadrant).

Table 4 :
The phase shift and the corresponding hole radius for dual beam transmitarray (first half).