11_microstrip_transmission_lines

11_microstrip_transmission_lines - Microstrip Transmission...

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Microstrip Transmission Lines
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Transmission lines Recall: Propagating waves resemble that of a propagating plane wave Composed of conductors and dielectric Can be analyzed using circuit theory based (V and I) and field analysis based (E and H) analysis For circuit theory based analysis to be accurate, transverse electromagnetic (TEM) propagation must occur Voltages and currents can be clearly defined only when TEM is present
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Transmission Lines TEM propagation usually occurs when dielectric of the transmission line structure is uniform around the conductor/s 2 wire line Coaxial line
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Microstrip Transmission Line A 2 conductor transmission line which is only partially filled with a dielectric material Consists of a strip conductor supported by a dielectric slab on a large (infinite) conductor ground plane
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Quasi-static Fields of MS lines Quasi-static (low frequency) approximation is applied to microstrip lines Due to non-uniform dielectric of the structure, propagation mode in microstrip lines is not purely TEM Quasi-TEM mode of propagation
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Quasi-static Fields of MS lines Consider the fields in a microstrip transmission line, with only air as the dielectric All fields are in the transverse plane No field component in the direction of propagation Equivalent to 2 wire tx line (image theory)
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Quasi-static Fields of MS lines When a dielectric substrate is placed between the 2 conductors, field distribution will change For boundary conditions to hold at the air- dielectric interface, directions of the fields will change in the transition from air to dielectric
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Quasi-static Fields of MS lines In order for the boundary conditions to be satisfied completely for all regions, fields in the direction of propagation will arise Hybrid propagation mode is present Closely resembles TEM propagation Field components in direction of propagation are very small (Quasi-TEM) For relatively low frequencies (a few Ghz), TEM approximation is good enough Becomes more of an issue as frequency increases
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Quasi-static Fields of MS lines Phase velocity point of view: Part of the fields propagate in air Part of the fields propagate in the dielectric Impossible for phase match to occur Fields at the boundary are not continuous Gives rise to fields in direction of propagation (hybrid TE v p = c v p = c r
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Effective Relative Permittivity For quasi-static analysis, we assume that the fields are TEM (quasi-TEM) As earlier said, this is a good enough approximation of the fields and other parameters of the line Would it still be accurate to solve the structure in terms of the relative permittivity of the dielectric material?
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Part of the fields propagate in the dielectric and the air Effective phase velocity of quasi-TEM waves can no longer be accurately defined by the dielectric constant of the substrate, nor air
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11_microstrip_transmission_lines - Microstrip Transmission...

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