DESIGN and ANALYSIS of 28 GHz MICROSTRIP PATCH ANTENNA for DIFFERENT TYPE FR4 CLADDINGS

In this study, rectangular patch microstrip antennas operating at 28 GHz frequency compatible with 5G mobile technology are designed with Computer Simulation Technology (CST) program for different patch materials and the performances of the designed antennas are compared. For each of the same sized antennas designed with the selected patch materials, it is found that they are suitable for the 28 GHz band and the best return loss performance is obtained by using the tantalum conductor while the silver conductor has the best antenna efficiency.

dupont-951 dielectric materials. Yoon and Seo, 2017 proposed a 2x2 U-slotted array antenna operating at 28 GHz for the broadband communication system. In their studies, they achieved about 3.35 GHz bandwidth and about 13 dBi gain in this operating frequency. A small microstrip patch antenna was implemented for the 5G wireless standard using High-Frequency Structure Simulator (HFSS) in Verma et. al. (2016). From the measured results, it was observed that the designed antenna resonates at 10.11 GHz with a bandwidth of 380 MHz. In Khalily et. al. (2016), the modified serial-feed patch antenna array is designed and implemented for 28 GHz millimeter wave applications. The proposed designs are predicted to be applicable for future 5G. Kiran  In the light of relevant literature it is our believe that the performance of a microstrip patch antenna for different patch and ground layer materials has not been studied yet. Therefore, in this study, the performance analysis of a microstrip patch antenna with FR-4 dielectric material operating in 28 GHz band for different patch and ground materials was performed by Computer Simulation Technology (CST)-program.

CONVENTIONAL MICROSTRIP PATCH ANTENNA (MPA) DESIGN
A microstrip antenna in its simplest form is a type of antenna with a radiating patch on one side of a dielectric substrate and a ground plane on the other side, as illustrated in Figure 1. Normally, copper and gold-made patch conductors can actually be in any form. For ease of analysis, the patch can practically be in familiar shapes like square, rectangular, circular, triangular, and the like. Furthermore, a variety of substrate types have been developed with various dielectric constant and loss tangent values (Garg et. al. 2001).

Figure 1:
The structure of a microstrip rectangular patch antenna For a rectangular patch, the patch length L is generally between λ 0 /3 and λ 0 /2 where λ 0 is the wavelength of free space. The patch thickness t is chosen to be very thin such that t << λ 0 . In general, the substrate height h is 0.003λ 0 ≤ h ≤ 0.05λ 0 . The dielectric constant of the substrate ε r is ordinarily in the range of 2.2 to 12 (Balanis, 2016).
In order to get efficient radiation, a practical rectangular patch width W can be given as where f r is the resonant frequency of the antenna and c is the speed of light in vacuum. The effective dielectric constant of rectangular patch antenna ε reff is expressed as (2) Due to the fringing effects, the effective length L eff is obtained as The actual length of the patch is determined by where ΔL is extension length and given by In order to operate a microstrip antenna, there are number of feeding techniques such as microstrip feed, inset feed, coaxial feed, proximity-coupled microstrip feed, aperture-coupled microstrip feed and coplanar waveguide feed. The choice of feeding technique depends on various factors. The most important one is the efficient transfer of power between the radiating structure and feeding structure. The structure of the inset feed microstrip patch is shown in Figure 2.
The structure of the inset feed microstrip patch Here, the characteristic impedance of the microstrip feeding line is given as follows: . .  (6) Then, the following statement can be obtained (Pozar, 2012).


Here, G 1 is the conductance of the patch in its transmission line model while G 12 is the mutual conductance due to mutual effects between the slots. These quantities are given below. Lastly, the minimum dimensions of the ground plane, L g_min and W g_min can be given as (Nakar, 2004)

SIMULATION RESULTS OF DESIGNED ANTENNAS
In this study, FR-4 with a dielectric constant of 4.3 was selected as a dielectric substrate and its height was 0.1 mm. According to these values and the resonant frequency of 28 GHz, W and L dimensions of the patch obtained from theoretical formulas were approximately 3.29 mm and 2.44 mm, respectively. In each designed antennas, inset microstrip feeding was used. The structure of antennas designed by using the CST is shown in Figure 3. From the formulas through 1-14, it was obtained as the inset feeding length, d = 0.95 mm, the width of feed length, W d = 0.194 mm, notch width, g = 0.0174 mm, the minimum dimensions of the ground plane, L g_min = 6 mm and W g_min = 6 mm. The designs were realized for copper, aluminum, gold, silver, iron, platinum, tantalum and molybdenum conductors, respectively. The return loss graphs obtained for these designs are shown in Figures 4-11, respectively. Then, the results are listed in Table 1 for evaluation.

Figure 4:
The return loss graph for copper conductor      According to these results, the tantalum conductor provided the best return loss value at a resonance frequency around 28 GHz. The far-field patterns in 3 dimensional and 2 dimensional (phi=90° and theta=90°) of the antennas designed for different patch conductors are presented in Figures 12-19, respectively.                The results are listed in Table 2 for evaluation. The results showed that the silver cladding had the best radiation and total efficiency at 28 GHz while the other performance parameters are about the same for all conductors used.

CONCLUSION
In this work, the rectangular patch microstrip antennas operating at 28 GHz frequency were designed for different patch conductors with the same dimensions. According to the design results, it has been seen that each of the patch conductors utilized has the potential to work in accordance with the desired operating frequency. The best performance due to the return loss criterion was tantalum conductor and the worst performance was obtained with a silver conductor. On the other hand, antenna efficiency performance was vice versa, that is, tantalum conductive was the worst while the silver conductor was the best. It is concluded that the loss of the silver-cladding antenna is less than the loss of the other cladding antennas used. This study is important in terms of predicting the effect of different patch claddings used in microstrip antennas on performance.