Design and simulation of rectangular patch microstrip antenna with inset feed for S-band application

Microstrip antenna is one of the most applied antennas for various radio applications as its low profile and high performances. This paper discusses design and simulation of the rectangular patch microstrip antenna (RPMA) for S-band radio by using inset feeding. Initially, theoretical calculation was performed, followed by performance evaluation by using AWR simulator. Inset deep iterations were conducted through repeated simulation adjustment to obtain optimal design. Based on the concluded simulation adjustment, the minimum achieved VSWR is 1.056 to get bandwidth of 104.5 MHz (3.1636 – 3.2681 GHz). The minimum return loss that can be achieved is -31.33 dB with gain of 5.777 dB.

Inset feed is a simple and general feeding for patch antennas. Inset feed uses substrate efficiently as it requires small patch intervention. Many antenna applications use this feeding techniques such as for wireless LAN application (Shankar and Chaurasiya 2015), Bluetooth (Barman et al. 2017), GPS and Radar (Putranto et al. 2017). Inset feed performances are not homogenous for all applications depending upon the working frequency, patch shape, patch materials and others.
This paper reports RPMA design by using inset feed. The FR4 substrate for the designed antenna has parameter εr of 4.4 with thickness of 1.6 mm and work at S-Band frequency of 3.2 GHz.

Antenna design
A rectangular patch micro strip antenna (RPMA) with inset feed is shown in Figure 1. Its fed dimension slightly changes the RPMA shape. The dimension is width (Wi) and length (Li). Even the RPMA shape is adjusted; the approximated equation can use the original RPMA model, which is expressed in the following equations. ( ) Constant c is free-space velocity of electromagnetic wave (3x10 8 m/s), εr and h are dielectric constant and thickness of the substrate, and fr is resonant frequency. In this paper, εr  The inset deep or length, denoted by Li, is determined by the available feed line and the dimension of the designed ground plane. Meanwhile, the inset width (Wi) should be carefully chosen based on the results of AWR simulations.

Simulation results
The theoretical calculation is actually an initial step in order to achieve the intended RPMA performances. Approximation in design should be performed as irregularity of realized dimension, simplification of material constant and variables as well as non-ideal components always exist. Figure 2 shows the characteristics of the theoretical designed RPMA with inset feed. This graph is an initial pattern of return loss and should be considered as an initial characteristic.  Adjustment should be performed so that lower return loss as expected can be achieved. Inset length adjustment can be performed by using the AWR simulations as depicted in Figure 3.

Figure 3. Return loss pattern as result inset length adjustment
By considering the return loss pattern variations, the optimal characteristic for the expected design can be observed. The best position is figured up when inset length is 4 mm (Figure 3 with Li 4 mm). The optimal return loss value is -31.33 dB on frequency of 3.22 GHz. At this condition, the achieved VSWR pattern is as plotted in Figure 4.  Figure 5 shows the optimal directional radiation pattern with gain achieving 5.777 dB. The geometry of the designed RPMA with inset feed is illustrated in Figure 6.

Conclusion
This paper has presented design and simulation of the RPMA with inset feed. The expected design has been found that works in S-Band frequency of 3.2 GHz. The optimal design is achieved when inset length is 4 mm. This optimal design occurs in frequency 0f 3.22 GHz, producing minimum return loss of -31.33 dB and minimum VSWR of 1.056. The antenna has bandwidth of 104.5 MHz with gain achieves 5.777 dB.