Abstract
Since 2002, when the Federal Communication Commission (FCC) released the bandwidth 3.1-10.6 GHz, there has been increasing interest in the use of UWB systems because of their low power consumption, low cost, precise positioning and promising candidate for short-range high-speed indoor data communications. Planar circular monopoles like designs are a good example for UWB applications due to their merits such as ease of fabrication, Acceptable radiation pattern, and large impedance bandwidth. However, some narrowband systems also operate in this frequency like WiMAX, WLAN and X-Band satellite downlink communication band etc. cause interference in UWB range. To overcome any interference with these systems it is desirable to design UWB antenna with band notches. However, most techniques of obtaining notches uses antenna design specific approaches therefore EBG structures can be used to obtain single and multi-notch antennas. The technique used for obtaining notches using EBG is antenna design independent and can be applied to most of the antennas without compromising antenna performance.
TopAdvantages And Disadvantages Of Microstrip Antennas
Microstrip antennas have several advantages over traditional microwave antennas over the whole frequency range from ~100 MHz to ~100 GHz. Some important advantages are low volume, light weight, conformability, cheap fabrication for mass production, easy to achieve different polarizations with antenna feed; multi-frequency operation can be achieved, easy to integrate with integrated circuits and easy to fabricate feed lines with antenna itself. These antennas have high performance because a huge number of matching elements, power dividers, and phase shifters can be added without any cost enhancement. Microstrip array is very reliable because traditional antennas may fail at the points of interconnections where as microstrip antenna entire array is made on continuous sheet of copper. However these antennas have some limitations like narrow bandwidth, lower gain and large ohmic loss in antenna array feed structures, mostly radiates in half space, poor end fire radiations, radiations from feed and junctions, low power handling capability, mutual coupling among individual elements in antenna arrays, higher levels of cross-polarization and excitation of surface waves in antenna substrates.
Applications of Microstrip Antennas
With tremendous being research done on microstrip antennas today the advantages of microstrip antennas exceeds far more than its disadvantages. Initially Microstrip antenna’s use was limited to systems such as rockets, aircrafts, smart weapons, missiles and satellites. Today, because of tremendous ease of fabrication process and availability of good substrates these antennas are being used in commercial applications (Huang, 1995; Lee & Chen, 1997) and are expected to replace traditional antennas for most of applications. Some of the important applications of microstrip antennas are in mobile and satellite, Global Positioning Systems (GPS), Direct to Home (DTH) applications and in medicines for treatment of hyperthermia.
Feeding Methods in MPA
The five most commonly used feeding techniques in antenna design are microstrip line, proximity coupling, aperture coupling, co-planar waveguide feed and coaxial feed shown in Figure 1(a),Figure 1(b),Figure 1(c),Figure 1(d) and Figure 1(e).
Figure 1. (a) Rectangular patch fed by Microstrip feed, (b) Proximity coupling, (c) aperture coupling, (d) Coplanar Waveguide (CPW),(e) Top view and side view of a co-axial feed microstrip antenna [Source: (Kumar & Ray, 2003), © Artech house, 2003], (f) Materials classifications (g) Electric and Magnetic fields on Yee’s lattice [Source: (Yang & Rahmat-Samii, 2009), © Cambridge University Press 2009]
Key Terms in this Chapter
Surface Waves: These waves are undesired and cause end–fire radiations in patch antennas. With the image currents in antennas being out of phase it further causes weakening of the radiation pattern.
Mushroom Electromagnetic Band Gap Structures: Mushroom EBG consists of metallic patches and shorted pins named via that connect patches into the ground planes. The operation of Mushroom EBG can be seen as a LC filter where L is due to current flow through via and C is due to gap effect between adjacent patches.
Metamaterials: These are artificial materials designed to interact with and control EM waves. When EM wave interact with these materials it induces electric and magnetic moments thereby considerably affecting the effective permittivity and permeability of that medium.
Dispersion Diagram: It is a diagram between Phase constant (ß) and Wave Number ( k ) is plotted with each propagating mode having different phase velocity, group velocity and fields vectors.
Microstrip Antennas: Consists of a radiator on one side of dielectric substrate and a conducting ground plane on the other side. Microstrip array is very reliable because traditional antennas may fail at the points of interconnections where as microstrip antenna entire array is made on continuous sheet of copper.
Photonic Band Gap (PBG) Structures: PBG structures may include 1D,2D and 3D periodic structures which behaves like a band-stop filter,stops progtion of waves at specified frequencies.There may be to combinations either of two different dielectric materials or a metal and a dielectric material.
Uniplanar Electromagnetic Band Gap Structures: Uniplanar EBG serves similar purpose to Mushroom Electromagnetic Band Gap structures. These structures have no vertical vias and are suitable for thin substrate material which makes the fabrication easier. Moreover, these are less sensitive to polarization and incident angle.
Notched UWB Antennas: In UWB range (3.1-10.6 GHz) some narrow band devices create interferences and hence UWB notch antennas have gained importance recently.
Electromagnetic Band Gap structures (EBGs): Sievenpiper highlighted the use of EBGs in suppressing surface waves in a particular band gap and zero phase reflections coefficient to the incident waves on EBG structures in this band.