Malik Praveen Kumar - Smart Antennas: Recent Trends in Design and Applications

Several researchers and experts are working to improve the various quality parameters like bandwidth, directivity, and gain of microstrip antenna. Some other existing solutions, such as defected ground structures (DGS), electromagnetic bandgap (EBG) structures, and composite resonator structures, create the issues of spurious radiation and very high complexity. The new approach provides the solution for this problem using metamaterial. In 1968, Russian Physicist Prof. Vaselago was the first who theoretically proposed the concept of the metamaterial. Attractive and interesting properties of metamaterials play a very important and authentic role in antenna designing. Therefore, the metamaterial can be used for the performance enhancement of microstrip patch antennas, as shown in Fig. ().

The main objective of the feedline is to provide an input signal to the antenna for excitation. Nowadays, several feeding techniques are available for microstrip patch antennas. These feeding techniques or methods are categorized into two groups (a) contacting and (b) non-contacting technique, as shown in Fig. (. The general feeding techniques are discussed briefly as follows.

i). Microstrip Feedline: A metallic strip is directly linked with a patch, as shown in Fig. (]. Its impedance matching and fabrication are easy. It produces the highest spurious feed radiation compared to other feeding techniques and provides a narrow bandwidth (2% to 5%).

ii). Coaxial Feed: In literature, the coaxial feed is also called by the name probe feed. In this feeding method, the inner conductor of the coaxial is connected with the radiating patch, and the outer conductor is attached to the ground plane []. It produces lower spurious radiations and requires simple and easy impedance matching. The coaxial feed arrangement is made with the help of soldering and drilling. It also provides narrow bandwidth of nearly 2 to 5%.

iii) Aperture Coupled Feed: In aperture coupled feed, two different layers or substrates are sandwiched to obtain the electromagnetic coupling from feed to radiating patch of microstrip antenna []. It has moderate spurious radiation and easy impedance matching. The aperture coupled feed arrangement is very difficult due to alignment requirements. This technique provides narrow bandwidth of nearly 2 to 5%.

iv) Proximity Coupled Feed: The feed line are sandwiched between the two dielectric layers, and the radiating patch is kept on the top of the upper dielectric substrate []. This method of feeding removes the spurious feed radiation. This feeding technique provides wider bandwidth (nearly 13%).

Table 2 List of Advantages and Disadvantages of different feeding methods.

Feeding Techniques	Advantages	Disadvantages
Coaxial Feed	Simple and easy impedance matching It has lower spurious radiations	It has large inductance for thick substrate For it, soldering is needed
Microstrip Line	It is monolithic It is simple and easier to fabricate Simple and easy to impedance matching	For thick substrate, it produces spurious radiation
Aperture Coupled	It does not have a deleterious effect on the bandwidth and efficiency due to the use of two dielectric layers. It does not require direct contact between feed and patch It does not produce radiation from feed and other active devices	It required a multi-layer design