Tuesday, June 10, 2008

Basic Concepts and Terminology

In this post I enlist some basic concepts and terminology involved in the domain of microstrip antennas. A proper comprehension of these concepts is imperative before any further deliberation on microstrip antennas.

Stripline and Microstrip

The terms stripline and microstrip are often encountered in the literature, in connection with both transmission lines and antennas. A stripline or triplet device is a combination of three parallel conducting layers separated by two thin dielectric substrates, the center conductor of which is analogous to the center conductor of coaxial transmission line. If the center conductor couples to a resonant slot cut orthogonally in upper conductor, the device is said to be a stripline radiator.

On the other hand, microstrip consists of two parallel conducting layers separated by a single thin dielectric substrate. The lower conductor functions as a ground plane and the upper conductor may be a simple resonant circular or rectangular patch, or a resonant dipole, or a monolithically printed array of patches or dipoles and the associated feed network.

Single Microstrip Line

A basic microstrip transmission lines consists of a conductive strip and a ground lane separated by a dielectric as shown in the figure below.

Fig: Single microstrip line

The figure above shows the general structure of microstrip line. The conduction strip, which is the microstrip line, has a width W, a thickness t. It is placed on the top of dielectric substrate, which has a relative dielectric constant €r, with a thickness of h, and a ground plane below on the substrste.

The single transmission line forms the basics of microstrip and therefore a basic knowledge of its working imperative. As mention earlier, two parameters, the effective dielectric constant €r and the characteristic impedance describe transmission characteristics.

Microstrip Antenna

A microstrip antenna in its basic form consists of a metallic radiating patch on one side of dielectric substrate, which has a ground plane on other side. The radiating patch can be considered as extension of microstrip line.

Fig: Rectangular Microstrip patch antenna

Assuming a voltage input at feedline, when operating in transmission mode, current is excited on feedline to the patch of a vertical electric field between the patch and the ground plane. So therefore the patch element resonates at certain wavelength and this result in radiation.

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