Design principle and structure

The proposed radiator's configuration with labeled parameters is shown in Fig. 1. The FR4 substrate is used in the proposed unique antenna design, which has μ_r = 1, loss tangent = 0.025, and ɛ_r = 4.3. Figure 1 depicts the geometry of numerous elements; the overall dimension is 25 × 25 × 1.6 mm³. The upper side of FR4 (annealed) is copper having a conductivity of 5.8 × 107 S/m. The impedance bandwidth of the radiator is 128%, with a central frequency of 7.54 GHz (2.91–12.17 GHz). The “L” represented length and width by “W”. The thickness of the substrate is denoted by “H”. The presented radiator's patch is rotated frame type. To develop a modified frame structure, firstly, the patch antenna is converted to a square frame by trimming out a piece of square shape from inside. The side of the outer square frame is denoted by “a1”. The square frame is then rotated by 15° (degrees). Further stage of the structure is done by making a fractal geometry out of the frames. Furthermore, inside the outer frame, a square frame of side length “a2” is made and rotated such that it comes just in contact with the outer frame. The same process is completed three more times with side lengths of subsequent frames to be “a3”, “a4”, and “a5” to complete the fractal frame structure. Five fractal levels were selected as going any deeper with smaller frames inside was not improving the results anymore.

Fig. 1.

Front, side and backplane of the proposed antenna with labeled parameters.

The substrate thickness is 1.6 mm. The patch is fed through a 50 Ω strip line with a width of “ws” in mm. The thickness of the ground plane and of the patch is 0.0018 mm. All the square frames are of width 1 mm. Then there has been an addition of two parasitic elements of rectangular shapes, which helped to resonate at lower frequencies. There is a rectangular patch with length “k” and width “W” in the backplane. Two rectangular slots of length “d” and width “g” are cut into it. These rectangular slots are properly adjusted on both sides of the microstrip line of the front plane. A rhombical-shaped slot is also cut into this patch and placed between these rectangular slots. There is another fractal frame design in the backplane placed in the upper left region of the backplane. “m and n” represent the length and width of this structure. The line width of the fractal structure is represented by “z”, and spacing between these lines is denoted by “i” and “j” for the vertical and horizontal spacing, respectively. This fractal structure is connected to the main patch in the ground plane using three strips, each of width “t”. The spacing between these strips is “s”, and their length is “v”. All the other dimensions with their optimum dimensions are shown in Table 1.

Table 1.

Design parameters of the presented antenna (all values are in mm)

Development of antenna

Figure 2 shows the 06-stages development of the designed antenna. The development was done using simulation on Computer Simulation Technology (CST) software. Figures 3 and 4 show the antenna's reflection coefficient (S ₁₁) for all the developmental stages.

Fig. 2.

06-stages development of 5 G antenna.

Fig. 3.

(S ₁₁) of stage 01, stage 02, and stage 03.

Fig. 4.

(S ₁₁) of stage 04, stage 05, and stage 06.

Stage-01 shows a regular rectangular patch antenna with a partial ground plane. A square patch of side length 11 mm is fed through a 3 mm wide strip line and 8.75 mm of length. The partial ground plane length is 8 mm, and starts oscillations from 3.9 to 13.8 GHz. Stage-02 is made by turning the square patch into a square frame of width 1 mm by cutting out a square cross-section from the middle of the square patch. This frame is then rotated by 15 degrees. A further modification is done by repeating the above process for smaller square patches inside the outer one and turning them by enough to touch the outer frame. Due to this, the lower and higher frequencies get shifted, and start oscillations from 3.8 to 14.4 GHz. Stage-03 shows modifications done in the backside. A slot was cut out in the partial ground patch, of width 3 mm and length 1.1 mm, at the top middle part. Further, there is an addition of a fractal shape of 7.2 mm and a width of 10.8 mm. This fractal structure is made of rectangular frames of width 0.225 mm, due to this lower band shifted to below 3.5 GHz.

The addition strips in the backplane to shifted lower band to below 3 GHz as visible in stage-04. These strips connected the fractal shape structure to the main patch in the backplane with a length of 9.3 mm and a thickness of 1 mm. They are placed 1.5 mm apart from each other. Stage-05 helps improve results at higher frequencies by introducing parasitic elements in the front plane. An 11 mm wide rectangular patch with 3.1 mm of length is added directly below and aligned with the patch. Another parasitic element was added, which is triangular. The length of its sides is 3.1, 11, and 11.4 mm, respectively.

This was also aligned with the previous rectangular element and the patch. Stage 06 is completed by making slots in the backplane. Two rectangular slots are cut into the plane of size 6.5 mm × 9.5 mm. There was a more cut out in the shape of a rhombical space with a side length of 1.98 mm. The corners that were along the antenna's length were joined by a strip of width 0.2 mm. Because the slots are in the backside, higher bound increased to more than 12 GHz, which shows the final 5G rotated frame radiator.

Simulated parameter study

We performed a parameter study to optimize the parameters that could impact the results during the designing of the radiator. Here in this section, we discussed the variation of 04-parameters and optimized the design based on their impact on the reflection coefficient (magnitude of S ₁₁) results.

Figure 5 shows the variation in the reflection coefficient (S ₁₁) of the designed antenna by variation in “a1” parameter, which varied from 9 to 13 mm. “a1” is the side length of the front square frame fractal. When it is reduced, the antenna is not in operating performance from 4 to 8 GHz. Due to the variation of parameter “a1”, there is variation in the impedance matching, and the reflection coefficient curves show a large number of return losses. Based on this observation, we conclude that the optimized result (reflection coefficient) is obtained when the side length is fixed at 11 mm.

Fig. 5.

Variation in “a1” in terms of (S ₁₁).

Figure 6 shows the variation of reflection coefficient curves with variation in “Ls”, which varied from 7 to 10.5 mm. “Ls” is the length of the feeding line. Change in the length of feeding line causes, in this design, change in size and location of parasitic elements, resulting in a change in results, as impedance matching between the partial backplane and microstrip line is affected. When it is longer at 10.5 mm, the results are helpful only from 3 to 4 GHz. If the size is reduced by much to 7 mm, the result is better but not in range. Thus, 8.75 mm is chosen as an optimized value.

Fig. 6.

Variation in “Ls” in terms of (S ₁₁).

Figure 7 shows the variation of reflection coefficient curves with “k” parameter. “k” represents the backplane's length. “k” ranges from 6 to 10 mm. By changing the length in the backplane also changes the dimensions of the slots cut into it. The size of “k” is kept at 6 mm; the radiator is helpful for a small bandwidth from 7 to 8 GHz. As the size of “k” is increased, a bandwidth of 2.9–12.2 GHz at 8 mm was observed. As the size of “k” further increases, results deteriorate. Thus the optimized results are obtained at 8 mm.

Fig. 7.

Variation in “k” in terms of (S ₁₁).

Figure 8 shows the variation in parameter “s”, which results in variation in S-parameter results. “s” represents the distance between the three strips connecting the main patch with the fractal structure in the backplane changing from 1 to 2 mm. Generally, in this case, impedance mismatching is visible at higher frequencies. Thus the optimized results are seen at 1.5 mm.

Fig. 8.

Variation in “s” in terms of (S ₁₁).