The rapid development of wireless communication systems such as cellular communication systems, wireless sensor networks and IoT devices, requires a smaller size of device. Likewise, the required antenna on the communication system device becomes smaller . It is necessary to miniaturize the antenna size to provide compatible antenna size with the development of wireless communication systems.
Microstrip antennas is one type of antenna that is widely engineered to produce smaller sizes and compatible with wireless communication system devices. Microstrip antennas have been the subject to study in many years because physical size, low profile and easy to fabricate, even though it has disadvantages such as narrow bandwidth and relatively smaller gain .
One solution to reduce the size of microstrip antennas is by using split-ring resonator (SSR) and complementary split-ring resonator (CSRR) metamaterial structures [3,4]. CSRR is a structure that can be printed on the ground or patch antenna. The CSRR material composition is the opposite of SRR which consists of two slot rings (circle or rectangular) where the inner and outer slots are separated by a metal/copper strip [4,5].
The equivalent circuit of CSRR is quite simple with an inductance and a parallel installed capacitance (parallel LC). Inductance and capacitance values are determined by the width of the CSRR ring and slot [5,6]. By using this concept, the addition of the CSRR structure to a microstrip antenna can reduce the antenna size of the ultra-wideband (UWB) antenna size significantly . The additional of the CSRR structure design is also reducing the size of the MIMO antenna and the multiband antenna [8,9].
In this paper, design and modified metamaterial CSRR structure to reduce antenna size is proposed. The design will develop by using microstrip array antenna at 2300 MHz LTE frequency band. This design was developed to reduce antenna size significantly with control antenna performances degradation.
2. Design of Antenna and CSRR Structure
The proposed microstrip antenna is designed numerically by using CST microwave studio software to operate at 2300 MHz LTE band frequency. Design of microstrip antennas using FR4-Epoxy substrate with a thickness of 1.6 mm. The Overall microstrip antenna dimensions resulting from design optimization are 44.1 mm x 40.1 mm for substrate material and 35.25 mm x 29.4 mm for patch antennas as shown in Figure 1.
In this study to observe the effect of CSRR structure, the sample array antenna was designed with 2 elements as shown in Figure 2. Microstrip array antennas are designed using feeding system for two elements with λ/2 spacing between patch elements.
CSRR structure design
The addition of the CSRR structure begins with calculating the geometry of one cell, than will continue to adding the number of orientation cells 4 4. The initial stage of designing a CSRR cell to find the λ /2 lengths for the outer ring resonator, and λ/4 for the inner ring resonator. The next step to determine the design of the outer and inner ring through approximation as shown in Figure 3.
The proposed outer ring approach for CSRR design, ; ; and put approximation for mm. Using the same approach to inner ring, ; ; and put approximation for mm. For the distance between rings and the width of the ring is equal, so that the values of H and L can be calculated by . In this design, we proposed and . All calculations for the dimensions of the CSRR structure are proposed as shown in table 1.
|Outer Geometry Elements of CSRR|
|Inner Geometry Elements of CSRR|
The proposed CSRR structure design as shown in Figure 4.
The next step is the iteration step to create a 4 4 cell orientation, where the size of one cell is reduced because will be impact to the matching level, especially for gain will be negative value. The reduction of CSRR dimensions and 4x4 orientation design in patch array antenna as shown in Figure 5.
The simulation result of CSRR structure on microstrip patch array antenna as shown in Table 2 and Figure 6. The additional CSRR effect is obtained by comparing the dimension and antenna performances parameter using CSRR structure and without CSRR structure.
|Dimensions (mm)||Antenna Parameter|
|Antenna||Patch||<Substrate and Ground||Slot Inset||(MHz)||(dB)||(dB)|
To validate the simulation results, we measured the 2 element array antenna fabrication with the addition 4x4 CSRR structure as shown in Figure 7. There were no significant differences between the simulation and measurement results as shown in Table 3.
Simulation and measurements result of the array antenna with the addition of CSRR structures show that there is a significant reduction of antenna dimension compared to conventional array antenna (31.05%). Antenna parameters also shows improved bandwidth and return loss, but there is a slightly decrease in gain. This is a consequence of the addition of CSRR structure, but compared to previous research this result shows an improvement of gain degradation. Future studies will be developed methods and techniques for adding CSRR structures to at least maintain gain performance.
A proposed design of metamaterial CSRR to reduce antenna dimension was described. The simulation and measurement result shown significant reduction of antenna dimension and improved bandwidth and return loss parameter with slightly decrease in gain. This proves that addition of CSRR structure is one of solution to miniaturization of microstrip antenna.
This work was supported by the FT Research Grant Universitas Negeri Jakarta, the Ministry of Research, Technology and Higher Education the Republic of Indonesia under Grant No. [461.a/SP/2018].
The authors would like to thank their colleagues for their contribution and support to the research. They are also thankful to all the reviewers who gave their valuable inputs to the manuscript and helped in completing the paper.