Enhancement of Feed Source through Three Dimensional Printing

The three-dimensional printed wideband prototype (WBP) was proposed, which is able to enhance the horn feed source by generating a more uniform phase distribution that is obtained after correcting aperture phase values. The noted phase variation obtained without the WBP was 163.65∘ for the horn source only, which was decreased to 19.68∘, obtained after the placement of the WBP at a λ/2 distance above the feed horn aperture. The corrected phase value was observed at 6.25 mm (0.25λ) above the top face of the WBP. The use of a five-layer cubic structure is able to generate the proposed WBP with dimensions of 105 mm × 105 mm × 37.5 mm (4.2λ× 4.2λ× 1.5λ), which can improve directivity and gain by 2.5 dB throughout the operating frequency range with a lower side lobe level. The overall dimension of the 3D printed horn was 98.5 mm × 75.6 mm × 192.6 mm (3.94λ× 3.02λ× 7.71λ), where the 100 % infill value was maintained. The horn was painted with a double layer of copper throughout its surface. In a design frequency of 12 GHz, the computed directivity, gain, side lobe level in H- and E- planes were 20.5 dB, 20.5 dB, −26.5 dB, and −12.4 dB with only a 3D printed horn case and, with the proposed prototype placed above this feed source, these values improved to 22.1 dB, 21.9 dB, −15.5 dB, and −17.5 dB, respectively. The realized WBP was 294 g and the overall system was 448 g in weight, which signifies a light weight condition. The measured return loss values were less than 2, which supports that the WBP has matching behavior over the operating frequency range.


Introduction
At microwave frequency, a horn antenna has wide applications, and is used as a feed element in large antennas, radar communication, and mobile and satellite communication, as a standard antenna to compare it with other antennas [1]. The modern wireless system requires greater bandwidth with efficient data transmission. This requires the system to maintain a greater frequency band with improved channel capacity and wider bandwidth [2,3]. The gain level of a horn antenna is usually maintained by aperture size and its length. However, higher aperture size results in increased length of the horn. Several studies have been conducted to address this issue, such as structure design across the interior E-plane wall of a pyramidal horn antenna, which is able to decrease side lobe levels and increase the gain of the horn antenna [4]. The growing use of the three-dimensional printing process is used to realize 3D printed horn prototype either through a metal 3D printing process [5,6] or by using a spray of metal on 3D printed parts [7,8]. This has resulted in an increase of costs during fabrication. Additionally, studies on meta surfaces are summarized in [9], which shows the flexible capability of microwave metamaterials in the manipulation of electromagnetic waves. It has been widely used in antenna design in recent years due to its planar structure and its potential to enhance antenna performance in terms of directivity, gain, and side lobe levels. The printed patches are generally used to realize the subwavelength meta surfaces as studied in [10], maintaining a proper thickness of dielectric plate and gap between nearby patches [11], a series of subwavelength metallic split ring resonators [12], use of multilayer non uniform meta surfaces that are formed by squares and ring metal patches [13], and an ultrathin metasurfaces lens developed to provide transmission phase compensation for a wide flare angle conical horn [14]. However, these metasurfaces have limited adjusting capacity to electromagnetic waves so difficulties arise in the further application of antenna design. In addition, a metal wideband phase correcting structure was proposed in [15] but it shows a higher value in aperture phase distribution and the required defined distance between two metal layers showing a complex geometrical structure. Interestingly, the application of 3D printing is widely used in the microwave field [16]. A dielectric loaded profile conical horn antenna was designed in [17], which uses more dielectric materials to be filled. In order to have a low profile, a light weight and the use of fewer 3D printed materials with a lower side lobe level, a cube structure was considered, as in [18], to show a reduced side lobe level, whereas in [19] the considered structure was able to maintain phase correction for about 40 • . The advancement in additive manufacturing technologies is growing in significance in the field of electromagnetic wave propagation; these were studied in [20][21][22][23][24]. Considering this aspect, the novelty of this manuscript was in proposing a simple, low profile, easily realizable geometrical structure that will be able to have a more uniform phase distribution, which can be further analyzed to study the deviation of the radiated beam direction. Hence, we designed a low profile, light weight, 3D printed horn antenna through Fused Deposition Modeling (FDM) with a double layer of copper throughout its surface, which is fed by a WR-75 waveguide through the bottom part. The five layers of the cube structure generated by the Multijet Printing technique was able to generate a more uniform phase distribution that shall ultimately improve the performance of the feed source. The proposed work shows efficiency in the utilization of the 3D printing technique in realizing the meta surface. The proposed structure, which was realized by a defined cube pattern, and size was helpful for understanding phase uniformity analysis against other literature studies performed in [13][14][15]17]. This article is organized as follows: Section 1 describes the requirements for the WBP structure. Similarly, Section 2 depicts the generation of a five-layer cube structure and its proper placement in the defined aperture position. Section 3 presents the corrected phase values observed in the operating frequency band. Section 4 highlights the obtained simulated and experimental results along with the uniqueness of the proposed WBP. Section 5 presents the conclusions.

Generation of Wide Band Prototype
The realized WBP was placed at a defined h1 = 12.5 mm (λ/2) distance from the base antenna as shown in Figure 1. We considered an aperture dimension of the horn of 4λ × 4λ (λ = 25 mm at 12 GHz). The variation in the phase of 163.65 • was noted above the feed horn source, which was decreased to 19.68 • after the placement of the WBP at h2 = 6.25 mm (λ/4) above the top face of the WBP.
The proposed WBP prototype, along with the unit cell arrangement, are shown in Figure 2. The cubes arranged in five layers are shown in Figure 2a, where the variation in cube dimension in each respective layer generates a corresponding change in the transmission magnitude and phase values. We set the cubes' dimensions as x1, x2, x3, x2, x1, and arranged them from Port 2 to Port 1. The variation in cube size was from a minimum of 0.5 mm (0.02λ) to a maximum of 7.5 mm (0.3λ). The proposed WBP perspective view is depicted in Figure 2b. The various cube dimensions, as arranged in five layers, are shown in Figure 3. The respective rounds were maintained from a central position of WBP across the end aperture dimension, where Round 1 lies in a central position, whereas Round 7 appears towards the end portion. The higher transmission magnitude (|S21| > 0.8) was maintained to provide an enhanced gain improvement. Round   The relative dimensions of the cubes were noted from normalized phase values that were calculated in the h1 position. Synthesis algorithms prepared for generation of proposed prototype are highlighted in the below steps.
Step 1: Calculate the required phase value at a 0.5λ distance above the defined aperture of the horn feed source. Phase values obtained in aperture positions of 3.75 mm, 11.25 mm, 18   The WBP is suitable to manufacture using 3D printing techniques that utilize Vero CMYK ( r = 2.8 and tanδ 0.124). The respective size of the cylindrical rods and cube dimension variation are detailed in [19]. The length of perpendicular cylinders is maintained at 7.5 mm (0.3λ), which holds the cubes of defined dimensions. The cubes' dimension is increased in a step size of 0.5 mm (0.02λ) from minimum dimensions of 0.5 mm 3 until the maximum dimension of 7.5 mm 3 . The optimal value of the cube size is 7.5 mm 3 and analytical calculations are performed to obtain the respective values of transmission magnitude and phase variations. The generated transmission magnitude and phase are arranged for specific layers of cubes in a five-layer structure. The phase values that correspond to the transmission magnitude of unity are considered and arranged in tabular form. The generated transmission magnitude and phase values for the arrangement of five layers of cubes are detailed in Tables 1 and 2.

Phase Correction as Observed above WBP
The generated uniform distribution of phase patterns were observed as shown in Moreover, uniformity in the conversion of spherical to planer wave-fronts were observed as highlighted in Figure 6.

Result and Discussion
We used CST-Microwave Studio to calculate the various performance criteria of the WBP structure. The noted VSWR is less than 2 from 10 GHz to 15 GHz of the operating frequency band that signifies wideband operation.
The experimental setup was carried out in an NSI-700S-50 spherical near field measurement system at the Australian Antenna Measurement Facility, which is shown in Figure 7. The figures attached show the fabricated WBP structure and the overall assembled system with the feed source. Further sections detail radiation patterns and characteristic plots.  Figure 8a signifies the performance matrices of the overall system with and without WBP structure. The overall improvement of 2.5 dBi in broadside directivity and gain were noted throughout the operating frequency range. This highlights the fact that the performance of the horn can be enhanced by compromising its aperture dimension and with the use of the proposed 3D printed WBP structure. The voltage standing wave ratio (VSWR) is shown in Figure 8b, depicting the wideband performance of the overall system. VSWR values are less than 2.2 over the operating frequency range. Measured VSWR depicts the matching condition of the proposed WBP structure. Similarly, the S11 plot, as shown in Figure 8c, shows the wideband characteristics of the proposed system where S11 values are less than −10 dB over the operating frequency band from 10 to 15 GHz. The simulated and measured values with and without the proposed prototype show better S11 values in the Ku-band of the operating frequency range. Table 3 shows an improvement in measured directivity and gain values after the placement of the proposed WBP structure above the feed source. Those values are compared against the simulated results. As noticed, in a design frequency of 12 GHz, simulated directivity, gain, and measured directivity, gain for the horn-only case are 20.489, 20.463, 20.310, 20.335 dBi, which are improved by around 2.5 dBi resulting in a simulated directivity, gain and measured directivity, and gain with the proposed prototype of 22.1, 21.9, 23.372, and 23.007 dBi, respectively. Throughout the operating frequency band, the overall performances in the directivity and gain margins are improved.  The observed radiation patterns of the overall system are depicted in Figure 9, highlighting the narrow beamwidth and lower side lobe levels in 10,11,12,13,14 [25] with an overall height of around 225 mm was considered to realize the analysis of aperture phase uniformity, which is supportive of understanding the requirement of the full phase of 360 • . This will be helpful for understanding the metasurface required for the beam steering and beam deviation phenomena. The use of the proposed superstrate is applicable for standard gain horn antenna with a defined length. The shorter horn with a lower gain was unable to depict the beam steering or deviation phenomena, even though the superstrate enhances the gain. This is due to the aperture phase correction requirement where a standard gain horn antenna with a defined aperture dimension of 98.5 mm × 75.6 mm is able to generate more uniformity in phase distribution with a lower side lobe level of around −17.5 dB. Table 4 shows the preference of the proposed WBP structure as compared with other design aspects. It shows wide band operation in the Ku-band, ranging from 10 GHz to 15 GHz. The aperture dimension is relatively smaller and has higher directivity and gain values with an improved 3dB bandwidth. Additionally, side lobe levels are comparatively lower, which has generated better radiation patterns.
(e) (f) Figure 9. Simulated and measured radiation patterns of overall system with WBP structure above the feed source are shown in Figure 9 (a-f) for 10 GHz, 11, GHz, 12 GHz, 13 GHz, 14 GHz, and 15 GHz frequencies.

Conclusions
This manuscript proposed a 3D printable WBP structure which improves the broadside directivity and gain of the feed source. The proposed five-layer surface was numerically studied through CST-MWS and is able to generate a more uniform phase distribution for the 10 to 15 GHz frequency range. In design frequency, lower side lobe levels of −17.5 dB and −15.5 dB, respectively, in E-and H-planes were obtained. Moreover, an improvement of 2.5 dBi in broadside directivity and gain was observed with better radiation patterns. The overall system was 448 g in weight and the fabricated WBP was 294 g. This signifies the light weight condition of the overall system. The measured return loss was less than 2.2, which shows matching behavior over the operational frequency band. Possible future work could include approaches that will be implemented to realize a 3D-printable beam-steerable surface. This work shows the full phase requirement of 360 • , which can be further extended to be implemented in the beam deviation phenomenon. There is no use of dielectrics, which reduces the cost and weight of the overall system. The next possible applications and challenges could be observed in high power systems and radiation patterns synthesis in near field regions. This research has great potential for radar and wireless communication systems due to its capability for greater bandwidth and improved electromagnetic waves transmission.