17-04-2009, 06:44 PM
Compact Microstrip Antenna¢ The microstrip antenna is a very thin metallic strip, placed a small fraction of a wavelength above a ground plane, separated by a dielectric.¢ Where size, weight, cost, performance, ease of installation, are of primary concern, microstrip antennas are the best solutions. Construction and Geometry ¢ 2.2 < ?r < 12. Low dielectric=>better efficiency, larger B/W.¢ A microstrip patch antenna is a radiating patch on one side of a dielectric substrate, which has a ground plane on the underside. The EM waves fringe off the top patch into the substrate, reflecting off the ground plane and radiates out into the air. Types of Microstrip antennas ) Single radiating patchesSquare, rectangular, dipole, circular, elliptical, triangular, etc.b) Single slot radiator Narrow slot, circular ring slot, wide slot, circular patch slot.c) Microstrip traveling wave antennasComb, meander line type, rectangular loop type, Franklin “ type MTWA.d) Microstrip antenna arraysConsists of series of antennas connected with each other to give better performance. Feeding methods a) Microstrip feed Easy to fabricate, simple to match by controlling the inset position and relatively simple to model. b) Coaxial probe feed Easy to fabricate, low spurious radiation; difficult to model accurately; narrow bandwidth of impedance matching.c) Aperture coupling (no contact), microstrip feed line and radiating patch are on both sides of the ground plane, the coupling aperture is in ground plane Low spurious radiation, easy to model; difficult to match, narrow bandwidth.d) Proximity coupling (no contact), microstrip feed line and radiating patch are on the same side of the ground plane Largest bandwidth (up to 13%). Criteria for substrate selection a) Possibility for surface-wave excitationb) Effects of dispersion on the dielectric constant and low tangent of the substratec) Magnitude of copper loss and dielectric lossd) Anisotropy in the substratee) Effects of temperature, humidity, and agingf) Mechanical requirements: conformability, machinability, solderability, weight, elasticity, etc.g) Cost Polarization types Figure 1:Linear PolarizationFigure 2:Circular Polarization¢ Disadvantages:¢ Relatively low efficiency (due to dielectric and conductor losses) ¢ Low power ¢ Spurious feed radiation (surface waves, strips, etc)¢ Narrow frequency bandwidth (at most a couple of percent)¢ Relatively high level of cross polarization radiation ¢ Applications:Radio and wireless communications, aircrafts, spacecraft and missile