A REVIEW ON DESIGN TECHNIQUES OF MICRO STRIP PATCH ANTENNA Jayanti Kumari
M.E. Scholar, JEC Jabalpur Prashant Kumar Jain Asst. Prof., JEC Jabalpur
Abstract - Modern communication equipment supports higher bandwidth applications.
So the antenna needed for such applications must have a wider bandwidth and be smaller in size. Since it is easy to manufacture, low volume and low cost micro-strip patch antenna. It is widely used for wireless communication applications. Micro-strip patch antenna is generally used in modern communication equipment. In recent years most of the researchers has focused on compact Micro-strip antenna design. According to the Micro-strip patch antenna, they offer low profile, low cost and low volume. Study of the literature of recent year’s shows that the leading work on MPA is focused on the design of compact broadband micro-strip antennas. But inherently MPA has a narrow bandwidth, so to increase the bandwidth, various techniques are used. Today, communication devices support various applications that require higher bandwidth;
such as cell phones today are getting thinner and smarter, but many applications supported by them require higher bandwidth, so the micro-strip antenna used for this operation should provide greater bandwidth, and their size should be compact so that it is less space while keeping the size of the device as small as possible. This article provides an overview of different techniques used for compact and broadband micro-strip patch antennas.
1 INTRODUCTION
Micro-strip patch antennas came into existence in 1953. But during this period very less work was done on these antennas. Until 1970s, these antennas became quite popular as the advantages of these antennas were known to the researchers. The idea of micro-strip patch antennas coined from printed circuit technology which was used only for the electronic circuitry and transmission lines. A micro-strip patch antenna consists of a radiating metallic patch printed on one side of the dielectric substrate and ground plane on the other side of the substrate. These antennas gained serious attention of the researchers through the ample of advantages like: conformal to planar and non- planar surfaces, low profile, easy fabrication, compactness and low cost. Micro-strip antennas find a wide variety of applications in communication, biomedical, RFID etc.
They can be used for the synthesis of pattern required that are difficult to achieve using a single element. These antennas can be used in scanning of the beam, enhance the directivity and gain. Compact size and cost effective antennas can be obtained using short- circuited patch antennas designed with fractal geometries. Furthermore, ultra
wide-band impedance matching by optimizing width of feed-line has been obtained. The gain and directivity can be enhanced by using antenna array as compared to single antenna. The radiation pattern observed in case of array antenna is quite intensified in comparison to single antenna. The only major drawback of micro-strip patch antennas was there narrow bandwidth. On energizing the micro- strip patch antennas, propagation of waves takes place in all the directions.
The surface waves travelling in the substrate do not cause the main radiation. Hence, energy is lost which limits the maximum gain between 5–9 dB. So, more and more research is being done to overcome these flaws.
The basic form of a micro-strip patch antenna consists of an arbitrary shaped copper or any other metal trace on one side of a PCB substrate with other side grounded. The metal trace can have the shape of a rectangle, square, circle, triangle, dipole, or any other geometry. Among these shapes, square, rectangle, dipole, and circle are most famous for ease in design, analysis and fabrication, and having appealing radiation characteristics especially low cross- polarization. The
dipole shape is appealing for possessing larger bandwidth and occupying less space. This property makes micro-strip dipoles desirable for the antenna arrays. The patch antenna can be fed using coaxial, stripline, aperture- coupling or proximity-coupling methods.
The basic geometry of micro- strip patch antenna suffers from three main disadvantages:1) narrow bandwidth, 2) small gain, and 3) larger size. The gain of antenna can be enhanced using antenna arrays and size can be reduced using multi-layer stacked structures, but these two issues will not be discussed here. The multi-layer structures can increase the bandwidth but their fabrication is a challenging task. For simplicity, this article is restricted only to the discussion of single layer solutions for achieving wide bandwidth. The antenna arrays also increase antenna bandwidth but they are not discussed due to space limitations.
2 WORK STUDY
In this section, the literature survey of micro-strip patch antennas is discussed based on the previous work on micro- strip antennas. The study shows that the micro-strip patch antenna exhibits many noticeable advantages like light weight, low cost, low profile, planar configuration, easy of conformal, superior portability, suitable for arrays, easy for fabrication, and easy integration with external circuitries etc. The narrow bandwidth is one of the main drawbacks of these types of antennas. The direct method to improve the bandwidth can be done by increasing substrate thickness, but the major disadvantage of increasing thickness is the reduced efficiency since the large portion of the input power is dissipated in the resistor which takes away the available Power that can be radiated by antenna.
Furthermore, reducing the height of the structure may appear to be a suitable solution, but it may lead to a reduced impedance bandwidth and lower radiation efficiency. This is often a tradeoff in realizing compact antennas while maintaining performance characteristics. Thus, different other
improved techniques are adapted to provide wide-impedance bandwidths of micro-strip antennas, including using high permittivity dielectric resonators for micro-strip patch antenna but high permittivity substrate has a poor choice for antenna bandwidth, since the bandwidth of a micro-strip antenna is best For low dielectric constant substrates as a portion of the total available power for direct radiation becomes trapped along the surface of the substrate. Compared to this single- pin-shorted three-dielectric layer substrate is used to improve the radiation efficiency and bandwidth without sacrificing, the cost and operational advantages. Another use of electromagnetic band gap (EBG) structures has attracted much attention in the recent years in the microwave community for its unique properties. These structures are periodic in nature that forbids the propagation of all electromagnetic surface waves within a particular frequency band – called the band gap – thus permitting additional control of the behavior of electromagnetic waves other than conventional guiding and/or filtering structures. Adding negative capacitor/inductor high gain can be achieved using a single almost rectangular micro-strip radiator reactively loaded with active negative capacitor, and composite-resonator micro-strip antennas using meta- material resonators which provide wide bandwidth and high gain. Similarly another problem to be solved is the low gain for conventional micro-strip antenna element. Array topologies like dual patch antenna arrays; basically the term topology optimization is often used to label the most general type of design optimization methods, in which the shapes as well as the connectivity of individual parts of the device are subject to design. The most common way of carrying out topology optimization is through the material distribution approach, in which the design domain is divided into small elements, which together represent an image of the device, Cavity backing technique has been used to eliminate the bidirectional radiation pattern, which is used to provide higher gain
compared with conventional micro- strip antenna. Lens covering is an alternative way to achieve gain enhancement. The lenses with canonical profile, like hemi elliptical, elliptical, hyper-hemispherical, and extended hemispherical provide to focus the radiation beam from the radiator elements. The integrated micro-strip lens antenna can be treated as composite antenna by combining micro-strip radiator elements and dielectric lens together, which is very useful for high frequencies (mm, sub- mm, terahertz (THz), and optical waves) applications. Partial Substrate Removal in Multiple-layer Dielectric Substrate, It is also well known that antenna array is an effective means for improving the gain. Similarly the last limitation of conventional micro-strip antennas is minimization of the relatively large size, particularly at lower microwave frequencies, since the operational frequencies are related to the electrical size of antenna. In general, the size of the rectangular micro-strip antenna (RMSPA) should be of order of a half-guided wavelength.
This limitation was mathematically studied by Wheeler and Chu. There have been numerous efforts are made to minimize the antenna size and obtain the electrically small micro-strip antenna with the raised demand towards smaller and smaller wireless communication devices. Inductive or capacitive loading is one of the effective ways to reduce the size of micro-strip antennas. The size of micro-strip antenna can be miniaturized using composite metamaterial resonators.
Magneto-dielectric substrates have been widely utilized to miniaturize micro-strip antennas due to magnetic substrates can provide wider bandwidths than dielectric substrates.
Fractal geometries (topologies), which are composed by self-similar structures, have opened an alternate way for antenna miniaturization.
3 DESIGN TECHNIQUES
There are many methods of feeding a micro-strip antenna. The most popular methods are:
1. Micro-strip Line.
2. Coaxial Probe (coplanar feed).
3. Proximity Coupling.
4. Aperture Coupling.
Because of the antenna is radiating from one side of the substrate, so it is easy to feed it from the other side (the ground plane), or from the side of the element. The most important thing to be considered is the maximum transfer of power (matching of the feed line with the input impedance of the antenna), this will be discussed later in the section of Impedance Matching.
Many good designs have been discarded because of their bad feeding. The designer can build an antenna with good characteristics and good radiation parameter and high efficiency but when feeding is bad, the total efficiency could be reduced to a low level which makes the whole system to be rejected.
3.1 Micro-strip line Feeding Techniques
In this kind of feeding process (Fig.1), the edge of the micro-strip patch is connected directly to a conducting strip.
This feeding method offers the benefit that the conducting line can have the opportunity of engraved on same substrate of patch antenna providing a planar shape. The width of conducting element is smaller as compared at the patch antenna.
Fig. 1: Micro-strip line feed 3.2 Coaxial Probe Feeding Techniques The outside conductor of a coaxial connector attached at ground plane, while the inside is extends across the dielectric and is welded at the radiating element antenna. However, the disadvantage of this technique is a difficult to model and produce à narrow bandwidth. Figure 2 show this type of feed technique.
Fig. 2: Coaxial probe feed
3.3 Feeding Techniques with Proximity coupled
This feeding technique (Fig.3) utilized two dielectric substrates in order that the feed line, firstly, is between two substrates and on the other hand the radiating element is on top of the upper substrate.
Fig. 3: Proximity coupled feed 3.4 Aperture coupled feed
This type of feed technique (Fig.4), a micro-strip feed line is separated by the ground plane to the radiating patch. The feed line and the radiating element is coupled through an aperture or a slot in the ground plane.
The variations in the coupling will depend of width and length of the slot to improve the simulation result of bandwidths and return losses. The slot is usually centered under the radiating element.
Fig. 4: Aperture coupled feed 4 CONCLUSION
This article presents a review on micro- strip patch antenna with emphasis on broad band designs. The idea of micro- strip antenna was proposed in 1950 but it was implemented in the 1970s following the maturity of PCB technology. Basically, a micro-strip patch antenna is composed of a arbitrary shaped copper or any other metal trace on one side of a standard PCB substrate with other side grounded. With this basic geometry, only 2 – 5 % bandwidth is possible which is not fit for modern-day wireless communication technologies. However, through extensive studies in the last few decades on antenna performance improvement: various techniques have been developed to enhance the bandwidth. Although impressive work has been done on micro-strip antennas, still a lot more is to be done.
With rapid development of microwave and millimeter-wave technologies, and machining capabilities, systems are going towards miniaturization.
Therefore antenna size is to be reduced.
Various methods are developed for size reduction but majority of them have caused considerable degradation in patterns of the antenna. Furthermore, the strategy to deal with the effects of size of ground plane on the characteristics of antenna is not sufficiently developed. For broadband designs, methods are available but all result in increased volume of antenna diminishing its low profile advantage.
Therefore search for broadband, proficient, and low profile patch antennas still remains the topic of interest for antenna community.
Besides, modern radars and other
systems of commercial and military interests are desired to have re- configurability. Producing high power re-configurable antennas for these systems is a big challenge.
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