A microstrip antenna is characterized by its Length, Width, Input impedance, and Gain and radiation patterns. Various parameters of the microstrip antenna and its design considerations were discussed in the subsequent chapters. The A microstrip antenna consists of conducting patch on a ground plane separated by dielectric substrate. This concept was undeveloped until the revolution in electronic circuit miniaturization and large-scale integration in 1970. After that many authors have described the radiation from the ground plane by a dielectric substrate for different configurations. The early work of Munson on micro strip antennas for use as a low profile flush mounted antennas on rockets and missiles showed that this was a practical concept for use in many antenna system problems. Various mathematical models were developed for this antenna and its applications were extended to many other fields. The number of papers, articles published in the journals for the last ten years, on these antennas shows the importance gained by them. The micro strip antennas are the present day antenna designer’s choice. Low dielectric constant substrates are generally preferred for maximum radiation. The conducting patch can take any shape but rectangular and circular configurations are the most length of the antenna is nearly half wavelength in the dielectric; it is a very critical parameter, which governs the resonant frequency of the antenna. There are no hard and fast rules to find the width of the patch.
The rapid development of microstrip antenna technology began in the late 1970s. By the early 1980s basic microstrip antenna elements and arrays were fairly well established in terms of design and modeling, and workers were turning their attentions to improving antenna performance features (e.g. bandwidth), and to the increased application of the technology. One of these applications involved the use of microstrip antennas for integrated phased array systems, as the printed technology of microstrip antenna seemed perfectly suited to low-cost and high-density integration with active MIC or MMIC phase shifter and T/R circuitry.
At some point in the summer of 1984 they arrived at the idea of combining these two geometries, using a slot or aperture to couple a microstrip feed line to a resonant microstrip patch antenna.
After considering the application of small hole coupling theory to the fields of the microstrip line and the microstrip antenna, they designed a prototype element for testing. Their intuitive theory was very simple, but good enough to suggest that maximum coupling would occur when the feed line was centered across the aperture, with the aperture positioned below the center of the patch, and oriented to excite the magnetic field of the patch.
Waves on Micro strip Antenna:
Surface waves :
The waves transmitted slightly downward, having elevation angles θ between π/2and π – arcsin (1/√εr), meet the ground plane, which reflects them, and then meet the dielectric-to-air boundary, which also reflects them (total reflection condition). The magnitude of the field amplitudes builds up for some particular incidence angles that leads to the excitation of a discrete set of surface wave modes; which are similar to the modes in metallic waveguide. The fields remain mostly trapped within the dielectric, decaying exponentially above the interface .
The vector α, pointing upward, indicates the direction of largest attenuation. The wave propagates horizontally along β, with little absorption in good quality dielectric. With two directions of α and β orthogonal to each other, the wave is anon-uniform plane wave. Surface waves spread out in cylindrical fashion around the excitation point, with field amplitudes decreasing with distance ®, say1/r, more slowly than space waves. The same guiding mechanism provides propagation within optical fibers . Surface waves take up some part of the signal’s energy, which does not reach the intended user. The signal’s amplitude is thus reduced, contributing to an apparent attenuation or a decrease in antenna efficiency.
Leaky waves :
Waves directed more sharply downward, with θ angles between π – arc sin (1/√εr) and π, are also reflected by the ground plane but only partially by the dielectric-to-air boundary. They progressively leak from the substrate into the air , hence their name laky waves, and eventually contribute to radiation. The leaky waves are also non uniform plane waves for which the attenuation direction α points downward, which may appear to be rather odd; the amplitude of the waves increases as one moves away from the dielectric surface.
When realizing printed circuits, one locally adds a metal layer on top of thesubstrate, which modifies the geometry, introducing an additional reflecting boundary.
Waves directed into the dielectric located under the upper conductor bounce back and forth on the metal boundaries, which form a parallel plate waveguide. The waves in the metallic guide can only exist for some Particular values of the angle of incidence, forming a discrete set of waveguide modes. The guided waves provide the normal operation of all transmission lines and circuits,in which the electromagnetic fields are mostly concentrated in the volume below the upper conductor. On the other hand, this build up of electromagnetic energy is not favourable for patch antennas, which behave like resonators with a limited frequency bandwidth.