10-11-2012, 04:14 PM
Stripline
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•The earliest form of planar transmission line was stripline.
• Stripline consists of a strip conductor centered
between two parallel ground planes with two
equal slabs of a dielectric, ferrite, or semiconductor
medium separating the center conductor from the
ground planes.
•Usually, the medium is a solid material, but in some applications air is the actual
dielectric used.
•The advantages of striplines are good electromagnetic shielding and low
attenuation losses, which make them suitable for high-Q and low-interference
applications.
•Transverse electric and magnetic (TEM) waves propagate within the stripline.
•Such waves have electric and magnetic components in a plane transverse to the
direction of propagation.
Effects of Finite Strip Thickness in Microstrip Lines
• Our treatment of microstrip lines started with a TEM nondispersive
approximation, which was expanded by taking into account
dispersive effects.
• These effects were due to the fact that the electromagnetic field
configuration is not really TEM.
• This was further expanded upon by considering that a practical
structure cannot be completely open to the air, but must be
enclosed.
• It has been implicit that the strip conductor was of negligible
thickness.
• Now we consider the further refinement that real strip conductors
have finite thickness, and it is sometimes necessary to correct the
physical strip width for it.
• In Figure 1(a–d) in Table 2.1, the finite thickness, t, leads to an
increase of fringing fields that can be taken into account in all the
preceding expressions involving the strip width W, by an effective
width, recommended byWheeler.
Radiation Losses
• An idealized microstrip line, being open to a semi-infinite air space, acts
similarly to an antenna and tends to radiate energy.
• Radiation losses are a major problem for open microstrip lines with low
dielectric constant.
• Low dielectric constant substrates are used when cost reduction is a priority.
• Similar materials are also used with millimeter waves to avoid excessively
tight mechanical tolerances.
• However, the lower the dielectric constant, the less the concentration of
energy in the substrate region, and, hence, the greater the radiation losses.
• Radiation losses depend on the dielectric constant, the substrate thickness,
and the circuit geometry.
Conductor Losses
• In most conventional microstrip designs with high substrate
dielectric constant, conductor losses in the strip conductor and the
ground plane dominate over dielectric and radiation losses.
• Conductor losses are a result of several factors related to the
metallic material composing the ground plane and walls, among
which are conductivity, skin effect, and surface roughness.
• In idealized lines, the conductivity is taken as infinite and the
current distribution in the metal becomes a surface, sheet current.
• With finite conductivity, there is a nonuniform current density
starting at the surface and exponentially decaying into the bulk of
the conductivemetal.
• This is the alleged skin effect, and its effects can be visualized by an
approximation consisting of a uniform current density flowing in a
layer near the surface of the metallic elements to a uniform skin
depth.