Miniaturized Wideband Aperture Coupled Microstrip Patch Antenna by Using Inverted U-Slot

This paper presents a linear polarized aperture coupled inverted U-slot patch antenna with small steps at the edges. The proposed design exhibits wideband behavior, acceptable return loss, VSWR, gain, small size, and less complexity. The theoretical analysis is based on the finite element method (FEM).This design has wide bandwidth, good return loss, VSWR, and radiation characteristics by implanting the inverted U-shaped stepped slots on a single aperture coupled patch. The proposed antenna design shows the measured return loss within acceptable range throughout the band (11.08GHz–13.25GHz) and maximum return loss is achieved with proper impedance matching. In this paper, the design considerations are presented and results are validated by the calculated and measured parameters.


Introduction
The microstrip patch antenna has been popularized because it is being utilized in many modern communication systems for its tremendous characteristics such as light weight, small size, low cost and easy fabrication [1,2].Pozar [3] has proposed the aperture coupled microstrip antenna first time in 1985.The title aperture coupled is assigned to the antenna as per its different feeding mechanism in which there is no galvanic contact between the patch antenna and feedline.The aperture is usually providing the magnetic coupling between antenna and feedline and exhibits numerous advantages.For instance, the spurious feed radiation can be reduced up to a specified level or eliminated by introducing a ground plane between the patch and feedline.Recently, the aperture coupled microstrip antenna attracted much attention of the researchers [4][5][6].However, further investigations should be carried on to enhance the gain and bandwidth of this kind of antenna for the practical applications.The performance parameters of the proposed design are to be achieved by aperture couple feeding mechanism and implanting inverted U-shaped slotted structure stepped at the corners and dimensions of the aperture slot.The resonant frequencies at different return losses throughout the band can be achieved by varying the dimensions of the slots as well as stub length of the feedline.Several antenna parameters are analyzed with the full wave analysis by using transmission line model.In recent times, a lot of research has been proposed for the advancement of the aperture coupled antennas as Ku band 16 × 16 aperture coupled array was presented by Song and Bialkowski in 1998 [7], Gao et al. proposed a broadband dualpolarized microstrip patch antenna with aperture coupling in 2003 [8] and Wong and Tung [9] demonstrated an inverted U-shaped patch antenna for compact operation in 2003.In 2004, Rao and Johnston [10] proposed a modified aperture coupled microstrip antenna in which they employed a back cavity to block radiations from the slot.In 2005, Rao et al. [11] presented a new aperture coupled microstrip slot antenna to improve the radiation performance.Sim et al. [12] experimentally studied dual-feed dual-polarized aperture coupled patch antenna with low cross-polarization and high isolation structure in 2009.In 2012, Qin et al. [13] 2 International Journal of Antennas and Propagation proposed an aperture coupled microstrip antenna with corrugated ground plane in which they demonstrated the gain enhancement and radiation pattern.Our proposed antenna design is simple in construction as compared to [13,14], miniaturized (7.40 mm × 6.03 mm) in size, low cost and increase in bandwidth [6,12].Therefore, in the present work, the design considerations are presented and results are validated by numerical simulations.

Design Essentials of Aperture Coupled
Microstrip Patch Antenna

Proposed Aperture Coupled Patch Antenna Configuration
The architecture of the aperture coupled microstrip patch antenna embedded with inverted U-slot and small tuning stubs is represented in Figure 1.The dimensions of the aperture slot and feedline are clearly represented in Figure 2.
The dimensions of the radiating structures, aperture slot, and microstrip feedline are chosen according to [2].The dimensions of inverted U-slot with small tuning stubs on it are adjusted to radiate apparently around the resonant frequency and their position is selected so that the radiating structure must be capable of producing wider bandwidth and proper impedance matching.Moreover, the dimensions of the aperture slot and the size of the feedline stub are responsible for the proper impedance matching between the microstrip feedline and the radiating patch.It is important to discuss here that if the coupling between the feedline and radiating structure is not achieved, most of the fed signals will be reflected which results in the performance deterioration.
The length and width of the radiating patch are finite, and consequently the radiated fields at the radiating edges of the antenna experience fringing.Fringing affects the dimensions  of patch height and dielectric constant of substrate and hence the resonant frequency.The constructional detail of the proposed antenna is shown in Figure 1; dimensions of the patch, slots, and small stubs are mentioned in Table 1.
All the design parameters of the proposed antenna tabulated in Table 1 are calculated as per the transmission line model and iterative trials.As listed in Table 2, first of all, design 1 is analyzed and necessary modifications have been done in a progressive manner to achieve the final optimum designs that is design 6.

Discussion of Results
Our attention is focused on the discussion and presentation of calculated and measured results for the return loss, VSWR, gain, and radiation pattern of the proposed antenna design in this section.The frequency range of 10 GHz to 14 GHz is utilized for the analysis purpose of the proposed design.Figure 3 is representing the return loss plots as per design 1 to design 6 mentioned in Table 1.We observed that simple International Journal of Antennas and Propagation aperture coupled patch antenna (black dotted plot) exhibits −30 dB return loss at 10.84 GHz resonant frequency and 790 MHz (10.43 GHz to 11.22 GHz) absolute bandwidth.Further improvements have been achieved in the design after applying different patterns and positions of slots as represented in Figure 1.At the very first step, design 1 of Table 1 is modified to design 2 by employing one vertical slot of length   = 6 mm and   = 0.8 mm on the radiating patch and we observed slight decrease in resonant frequency from 10.84 GHz to 10.8 GHz and bandwidth increases by 10 MHz and return loss is −28 shown as green line   5 and denoted as dash-dot line structure.The reason for this degradation is that the radiation from two vertical slots on either side of the patch is not coupled mutually because these slots are departed by the conducting patch; therefore, it decreases in bandwidth.By taking into consideration this degradation, the antenna is further improved as design 4 in which a squared truncated inverted L-slot is implanted on the patch having dimensions listed in Table 1.The result of design 4 is shown in Figure 3 and represented with the dashed line; it exhibits a drastic change in return loss which is −38 dB at resonant frequency 11.38 GHz and the bandwidth remains unaltered that is 800 MHz.With these modifications in the design, we have achieved the appropriate return loss and VSWR, but the goal of bandwidth enhancement has not been achieved

Elevation pattern E(𝜃)
Azimuthal pattern E() Azimuthal pattern E() Azimuthal pattern E() Azimuthal pattern E()   4 and 5, respectively.The incident power used for these measurements was 10 mW and it is observed from the measured return loss and VSWR that 92% to 99.2%.Power is Accepted by the proposed antenna in the frequency range of 11 GHz-13.2GHz.A radiation efficiency of 91% to 97% is achieved throughout the specified frequency range.Finally, it is observed from Figures 4 and 5 that a good agreement has been achieved between the calculated and measured return loss and VSWR values.
Figure 6 represents gain versus frequency plot through which it is undoubtedly observed that the proposed antenna design exhibits acceptable gain (5.5 dB to 6.97 dB) within the frequency range of 11 GHz-13.2GHz.
A good agreement has also been achieved between the calculated and measured value of gain throughout the required frequency range.All the important parameters which we have discussed in the paper are tabulated in Table 3 so that they can easily be recognized and compared with each other.The VSWR values for all the designs have not been shown because all of these antenna designs exhibited a good value of VSWR which should range between 1 and 2.
It is observed from Table 3 that with antenna design 6, a good agreement amongst all the important parameters has been achieved.
The measured 2D radiation patterns at different resonant frequencies are presented in Figure 7.The radiation characteristics are also studied in azimuthal plane (- planes) and elevation plane (- planes) and it is observed that the proposed antenna is radiating more in the theta (- planes) direction and less in the phi (- planes) direction.Normally, the radiation characteristics are determined in the far field region as a function of coordinates and it can also be classified in terms of power pattern or field pattern.Moreover, from the total patterns of all resonant frequencies, it is clear that major lobe is stronger than the back lobe and side lobes and it is analysed that this proposed antenna is radiating normally to the antenna surface.

Conclusions
The design of inverted U-slot aperture coupled wideband microstrip patch antenna is analyzed and discussed in this work.The performance parameters of the antenna as detailed in Table 3 are achieved in a good agreement after applying slight modifications to the base design to reach up to the final optimum wideband design.Moreover, the design resulted in a smaller size antenna with a wide bandwidth operation.A good agreement between calculated and measured return loss, VSWR, and gain is achieved.All the results of the proposed design are presented and discussed and it is concluded that this antenna will give the satisfactory performance in the band of operating frequencies (11 GHz-13.5 GHz).

Figure 1 :
Figure 1: Geometry of the proposed aperture coupled patch antenna with inverted U-slot.

Figure 2 :
Figure 2: Dimensions of the aperture slot and feedline.

Figure 3 :
Figure 3: Return loss plot of all the designs in which solid black line plot represents the return loss of proposed antenna.

Figure 4 :Figure 5 :
Figure 4: Return loss versus frequency plot of calculated and measured results of proposed antenna.

Figure 6 :
Figure 6: Gain versus frequency plot of calculated and measured results of proposed antenna.

Figure 7 :
Figure 7: 2D radiation patterns of aperture coupled microstrip patch antenna by using inverted U-slot.

Table 1 :
Geometrical parameters of analyzed antenna (all dimensions in mm).

Table 2 :
Description of antenna designs mentioned in Table1.1 = 3.8 mm and  1 = 0.8 mm and observed that the resonant frequency decreases from 10.8 GHz to 10.79 GHz and bandwidth decreases to 740 MHz and return loss decreases to −25 dB shown in Figure

Table 3 :
Return loss, gain, and bandwidth of all the design analysed in this work.

Table 1 .
It is found that the bandwidth has increased with this design as 2000 MHz and the return loss is below the threshold value in the frequency range of 11.11 GHz to 13.13 GHz.Utmost value of return loss is attained, −35 dB at 11.45 GHz frequency and −30 dB at 12.65 GHz frequency.Furthermore, some small stub slots are added in the inverted U-slot structure and dimensions of these slots are detailed under design 6 of Table1.In this absolute design, we have attained 2170 MHz bandwidth (11.08 GHz-13.25 GHz) with the maximum return loss of −43 dB as undoubtedly represented in Figure3with solid black line plot which are the tremendous values of bandwidth and return loss so far.Further, the calculated and measured results of return loss and VSWR are presented in Figures