Optimization of an inset-fed calculations for rectangular microstrip antenna

ABSTRACT


INTRODUCTION
Since Desachamps introduced microstrip antennas in 1953 [1], research and development of microstrip antennas for various applications has increased, with the number of scientific articles published in IEEE journals totaling 1360 papers [2]. The first attempt to manufacture microstrip antennas was carried out by Howel [3] and reported in 1972. The scientific article on microstrip antennas was first published by Munson [4] in the IEEE Journal Transactions on Antennas and Propagation in January 1974.
Analysis with transmission line method (TLM) modeling assumes that "the radiating part of the antenna occurs at both ends of the antenna which resembles a slot. Electromagnetic fields are emitted from each slot on either end of the patch antenna. The two fields emitted from the two slots add up to each other in a superposition and form a complete antenna radiation pattern" [5]. Several methods of feeding techniques have been proposed. Rectangular microstrip patch antennas are known to feed rectangular microstrip patch antennas, both contact and non-contact [6]- [11]. Contact feeding includes: (a) patch antenna edge feeding (final feeders), (b) coaxial bait feeding, (c) inset feeding.] Research by Samaras [12], Basilio [13] and Hu [14] reported that the value of the input resistance of a rectangular microstrip antenna will decrease slowly when the feed point is away from the edge of the antenna patch, and will be zero if the feed point is in the middle of the patch antenna. Thus, at some distance from the edge of the patch, the input resistance of the antenna will be 50 Ohms. If the distance from the edge of the antenna patch to the 50 Ohm input resistance point is expressed by , a formula is needed to determine the distance of .
Matin and Sayeed [15], Carver [8], [16], [17] proposed a method of calculating the input resistance of a microstrip rectangular patch antenna as a function, where L is the length of the microstrip patch antenna and is the dielectric constant of the substrate. Figure 1, shows a typical rectangular patch antenna TLM topology, consisting of a long L ideal, lossless transmission line with a characteristic impedance Zo, terminated at both ends by a network of identical passive ports. In Figure 2, shows the inset-fed terminal with the feed point 0 from the edge of the patch.

METHOD
In this paper, we use the experimental method to find the approximation formula in order to obtain more accurate calculation results by referring to the Ramesh formula to determine the Inset-fed distance from the patch antenna edge as written in (2). Working frequency was set at 1 GHz, 1.5 GHz, 1. The first step is to calculate using the Ramesh formula. The second step, do the simulation. The third step is changing the distance gradually so that the input resistance is 50 Ohm and the SWR of the antenna is close to 1.0, as well as the lowest return loss ( 11 ). Then a comparison is made between the distance ( ′) according to the experiment to the distance according to the Ramesh formula. The comparison between the distance according to the experimental results and the distance according to the Ramesh formula produces a number of correction factors "s" for the Ramesh equation. In this experiment, the transmission line spacing ( ) to the antenna is set at 1 mm and 1.5 mm at each working frequency. Figure 3 shows geometry construction of a rectangular patch antenna using the inset-fed method.

Figure 3. Geometry of microstrip antenna using inset-fed method
The calculation of the patch antenna dimensions follows as [19] and [20] − Calculates the ∆ patch length, due to the fringing field effect, − Calculate the actual length of the patch antenna using the equation, The material of the antenna used FR4 epoxy printed circuit board (PCB) board with a substrate thickness of ℎ = 1.6 mm and = 4.4. The results of calculating the dimensions of the antenna are shown in Table 1. The next step is to calculate the inset-fed length and transmission line gap ( ) to the antenna patch. In this experiment, the gap ( ) between the microstrip line and the patch antenna was set from 1 mm, 1.5 mm. As a comparison, we will calculate the inset-fed distance from the edge of the patch ( ) with the Ramesh equation [17], and Vinayak [21], The next step is to do an experimental test to obtain a correction factor "s" in order to obtain the optimal distance in order to obtain an antenna impedance of 50 ohms. The number of correction factor "s" is tested is: (1) three rectangular microstrip patch antennas are designed that work at frequencies of 1 GHz, 1.5 GHz, 1.8 GHz, 2.0 GHz, 2.1 GHz and 2.3. GHz, 2.5 GHz, 3.0 GHz, and 3.5 GHz, with a distance of following Ramesh's equation, then (2) also designed three antennas with a frequency the same, but the distance using the developed Ramesh equation. Furthermore, the simulation is carried out and the results are compared.
In this experiment, the gap ( ) between the microstrip line and the patch antenna was set from 1 mm and 1.5 mm. The antenna material is PCB Epoxy FR4 with a thickness of h = 1.6 mm and = . .

RESULTS AND DISCUSSION
At the initial stage, several experiments were carried out to determine the ideal distance to produce the smallest return loss (RL). The best experimental results for the ideal distance are then compared to the results of Ramesh's calculations. Comparisons were made based on the same working frequency, and a number was obtained which became the correction factor for Ramesh's formula of s = 0.83477. The next stage is to design a rectangular patch microsrostrip antenna by applying the calculation of the distance according to Ramesh and according to the results of the development of the Ramesh formula.
From (8), the inset-fed distance calculated to be applied to the rectangular patch microstrip antenna design with operating frequencies: 1 GHz, 1.5 GHz, 1.8 GHz, 2.0 GHz, 2.1 GHz and 2.3. GHz, 2.5 GHz, 3.0 GHz, and 3.5 GHz. The results of the simulation calculations are shown in Table 2. Then by using the Ramesh formula development equation in (10), the inset-fed distance is calculated which is given the notation ( ′) to be applied to the rectangular patch microstrip antenna design with working frequency: 1 GHz, 1.5 GHz, 1.8 GHz, 2.0 GHz, 2.1 GHz and 2.3. GHz, 2.5 GHz, 3.0 GHz, and 3.5 GHz. The results of calculations and simulations are as in Table 2.
In general, it can be seen that applying the correction factor to the Ramesh equation to calculate the inset-fed distance on a rectangular patch microstrip antenna, can significantly improve the performance of  Figure 4 and Figure 5.  The results of the proposed calculations produce a lower VSWR value at the resonant frequency than the results obtained by Panda [22] and Chemkha [23]. In terms of antenna efficiency, Samarthay et al. [21] experiment obtained greater efficiency.

CONCLUSION
Based on the experimental results it can be concluded that calculation of distance using the Ramesh formula with a correction factor s=0.83477 results in a lower Return Loss (S11) and VSWR compared to using the original Ramesh formula.