Theoretical Background and Microwave Transmission lines Theories

 Introduction:

Recently EBGSs have occupied significant portions of microwave engineering literature to enhance the performance of many microwave devices and components. The nomenclature EBG is actually used in the optical communication. Recently they are scaled down to RF engineering. They are very old concepts in terms of periodic structures. EBG structures are periodic in nature.

They originate from stopband and passband like periodic structures. To understand the stopband and passband phenomena in EBG structures, it is better to study the conventional periodic structure. Waveguides and transmission lines loaded at periodic intervals with identical reactive elements are referred to as periodic structures. This type of period structures yield two distinct properties, namely Passband – stopband characteristics; and Stemming waves with phase velocity lower than the velocity of light in free space.

In the passband the EM wave is unattenuated along the structure. There may be some incidental conductor loss only. On the other hand, in the stopband the EM wave is totally attenuated so that it cannot propagate throughout the structure. This stopband – passband characteristics are very important to suppress the surface waves, a crucial issue in microwave engineering. EM wave having the velocity lower than the velocity of light in free-space is called slow wave. Periodic perturbation in the ground plane provides periodic discontinuity. Thus the slow wave property of the EM wave is achieved. Slow wave structures (SWSs) are promising candidates for compact design.

Microwave Transmission lines Theories:

Transmission lines that are use to convey microwave frequency signals called microwave transmission lines. In an electronic system, the delivery of power requires the connection of two wires between the source and the load. At low frequencies, power is considered to be delivered to the load through the wire. In the microwave frequency region, power is considered to be in electric and magnetic fields that are guided from lace to place by some physical structure. Any physical structure that will guide an electromagnetic wave place to place is called a Transmission Line.

The basic difference between conventional circuit theory and transmission line theory is the electrical size.

·         Circuit analysis assumes that the physical dimensions of a network are much

smaller than the electrical wavelength.

·         while transmission lines may be a considerable fraction of

a wavelength or many wavelengths, in size. Thus a transmission line is a distributed-parameter

network, where voltage and currents can vary in magnitude and phase over its length.

Figure 2.1: (a) Two ports T-line, (b) Voltage and current definitions