Splendours Of Planar Transmission Lines
Beyond certain
frequencies, conducting wires are of no use due to skin depth. Waveguides
(circular, ridged and rectangular) come into the usage. As the frequency
increases beyond GHz. Microwave Integrated Circuits play dominant role. The
back bone of Microwave Integrated Integrated
Circuits (MICs) are planar transmission line. Planar transmission lines include
Stripline, Microstrip, Slot line, coplanar waveguide, coplanar strips, Fin
line, Substrate Integrated Waveguide and the their variants. Suspended
Microstrip, Inverted Microstrip, Trapped Microstrip, Image guides, Dielectric
Integrated Guides, suspended stripline are few of the variants of planar
transmission lines.
The cross sections, the field lines and the modes of each
transmission line are to be understood to make the design procedure easier.
Stripline supports Pure TEM mode, Microstrip supports Quasi TEM mode, Slot line
supports Non TEM mode and that of Fin line is Hybrid mode.
The design of the
MIC components are in terms of guide wave length (from ‘zero to one’ wave length
there are infinite wavelengths and few of them have become standard
wavelengths) and hence the design formulae remain same; what is changing is the
effective dielectric constant.
For a homogenous transmission line like
stripline, the relative dielectric constant is to be used and for all other
transmission lines, effective dielectric constant is to taken into
consideration. This is due to ‘dispersion’. If it is mm wave application CPW
family and Fin line are the ideal choice. In RF MEMS.
BioMEMS also CPW and
Microstrip play dominant roles. MICROWAVE INTEGRATED CIRCUIT Design if and only
if the simple BASICS of planar transmission lines are understood.
DR S RAGHAVAN
SENIOR PROFESSOR
ECE DEPARTMENT, NIT Trichy
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