Planar transmission lines are transmission lines with conductors, or in some cases dielectric (insulating) strips, that are flat, ribbon-shaped lines. They are used to interconnect components on printed circuits and integrated circuits working at microwave frequencies because the planar type fits in well with the manufacturing methods for these components. Transmission lines are more than simply interconnections. With simple interconnections, the propagation of the electromagnetic wave along the wire is fast enough to be considered instantaneous, and the voltages at each end of the wire can be considered identical. If the wire is longer than a large fraction of a wavelength (one tenth is often used as a rule of thumb), these assumptions are no longer true and transmission line theory must be used instead. With transmission lines, the geometry of the line is precisely controlled (in most cases, the cross-section is kept constant along the length) so that its electrical behaviour is highly predictable. At lower frequencies, these considerations are only necessary for the cables connecting different pieces of equipment, but at microwave frequencies the distance at which transmission line theory becomes necessary is measured in millimetres. Hence, transmission lines are needed within circuits.
The earliest type of planar transmission line was conceived during World War II by Robert M. Barrett. It is known as stripline, and is one of the four main types in modern use, along with microstrip, suspended stripline, and coplanar waveguide. All four of these types consist of a pair of conductors (although in three of them, one of these conductors is the ground plane). Consequently, they have a dominant mode of transmission (the mode is the field pattern of the electromagnetic wave) that is identical, or near-identical, to the mode found in a pair of wires. Other planar types of transmission line, such as slotline, finline, and imageline, transmit along a strip of dielectric, and substrate-integrated waveguide forms a dielectric waveguide within the substrate with rows of posts. These types cannot support the same mode as a pair of wires, and consequently they have different transmission properties. Many of these types have a narrower bandwidth and in general produce more signal distortion than pairs of conductors. Their advantages depend on the exact types being compared, but can include low loss and a better range of characteristic impedance.
Planar transmission lines can be used for constructing components as well as interconnecting them. At microwave frequencies it is often the case that individual components in a circuit are themselves larger than a significant fraction of a wavelength. This means they can no longer be treated as lumped components, that is, treated as if they existed at a single point. Lumped passive components are often impractical at microwave frequencies, either for this reason, or because the values required are impractically small to manufacture. A pattern of transmission lines can be used for the same function as these components. Whole circuits, called distributed-element circuits, can be built this way. The method is often used for filters. This method is particularly appealing for use with printed and integrated circuits because these structures can be manufactured with the same processes as the rest of the assembly simply by applying patterns to the existing substrate. This gives the planar technologies a big economic advantage over other types, such as coaxial line.
Some authors make a distinction between transmission line, a line that uses a pair of conductors, and waveguide, a line that either does not use conductors at all, or just uses one conductor to constrain the wave in the dielectric. Others use the terms synonymously. This article includes both kinds, so long as they are in a planar form. Names used are the common ones and do not necessarily indicate the number of conductors. The term waveguide when used unadorned, means the hollow, or dielectric filled, metal kind of waveguide, which is not a planar form.
General properties
Planar transmission lines are those transmission lines in which the conductors are essentially flat. The conductors consist of flat strips, and there are usually one or more ground planes parallel to the flat surface of the conductors. The conductors are separated from the ground planes, sometimes with air between them but more often with a solid dielectric material. Transmission lines can also be constructed in non-planar formats such as wires or coaxial line. As well as interconnections, there are a wide range of circuits that can be implemented in transmission lines. These include filters, power dividers, directional couplers, impedance matching networks, and choke circuits to deliver biasing to active components. The principal advantage of the planar types is that they can be manufactured using the same processes used to make printed circuits and integrated circuits, particularly through the photolithography process. The planar technologies are thus particularly well suited to mass production of such components.
Making circuit elements out of transmission lines is most useful at microwave frequencies. At lower frequencies the longer wavelength makes these components too bulky. At the highest microwave frequencies planar transmission line types are generally too lossy and waveguide is used instead. Waveguide, however, is bulkier and more expensive to manufacture. At still higher frequencies dielectric waveguide (such as optical fibre) becomes the technology of choice, but there are planar types of dielectric waveguide available.Yeh & Shimabukuro, p. 99 The most widely used planar transmission lines (of any kind) are stripline, microstrip, suspended stripline, and coplanar waveguide.Jarry & Beneat, p. 19
Modes
An important parameter for transmission lines is the mode of transmission employed. The mode describes the electromagnetic field patterns caused by the geometry of the transmission structure.Flaviis, p. 539 It is possible for more than one mode to exist simultaneously on the same line. Usually, steps are taken to suppress all modes except the desired one.Connor, p. 67 But some devices, such as the dual-mode filter, rely on the transmission of more than one mode.Hunter, pp. 255–260
TEM mode The mode found on ordinary conductive wires and cables is the transverse electromagnetic mode (TEM mode). This is also the dominant mode on some planar transmission lines. In the TEM mode, the field strength vectors for the electric and magnetic field are both transverse to the direction of travel of the wave and orthogonal to each other. An important property of the TEM mode is that it can be used at low frequencies, all the way down to zero (i.e. DC).
Another feature of the TEM mode is that on an ideal transmission line (one that meets the Heaviside condition) there is no change of line transmission parameters (characteristic impedance and signal group velocity) with the frequency of transmission. Because of this, ideal TEM transmission lines do not suffer from dispersion, a form of distortion in which different frequency components travel at different velocities. Dispersion "smears out" the wave shape (which may represent the transmitted information) in the direction of the line length. All other modes suffer from dispersion, which puts a limit on the bandwidth achievable.
Quasi-TEM modes Some planar types, notably microstrip, do not have a homogeneous dielectric; it is different above and below the line. Such geometries cannot support a true TEM mode; there is some component of the electromagnetic field parallel to the direction of the line, although the transmission can be nearly TEM. Such a mode is referred to as quasi-TEM. In a TEM line, discontinuities such as gaps and posts (used to construct filters and other devices) have an impedance that is purely reactive: they can store energy, but do not dissipate it. In most quasi-TEM lines, these structures additionally have a resistive component to the impedance. This resistance is a result of radiation from the structure and causes the circuit to be lossy. The same problem occurs at bends and corners of the line. These problems can be mitigated by using a high permittivity material as the substrate, which causes a higher proportion of the wave to be contained in the dielectric, making for a more homogeneous transmission medium and a mode closer to TEM.
Transverse modes
In hollow metal waveguides and optical waveguides there are an unlimited number of other transverse modes that can occur. However, the TEM mode cannot be supported since it requires two or more separate conductors to propagate. The transverse modes are classified as either transverse electric (TE, or H modes) or transverse magnetic (TM, or E modes) according to whether, respectively, all of the electric field, or all of the magnetic field is transverse. There is always a longitudinal component of one field or the other. The exact mode is identified by a pair of indices counting the number of wavelengths or half-wavelengths along specified transverse dimensions. These indices are usually written without a separator: for instance, TE10. The exact definition depends on whether the waveguide is rectangular, circular, or elliptical. For waveguide resonators a third index is introduced to the mode for half-wavelengths in the longitudinal direction.Connor, pp. 52–53, 100–101
A feature of TE and TM modes is that there is a definite cutoff frequency below which transmission will not take place. The cutoff frequency depends on mode and the mode with the lowest cutoff frequency is called the dominant mode. Multi-mode propagation is generally undesirable. Because of this, circuits are often designed to operate in the dominant mode at frequencies below the cutoff of the next highest mode. Only one mode, the dominant mode, can exist in this band.Flaviis, pp. 539–542
Some planar types that are designed to operate as TEM devices can also support TE and TM modes unless steps are taken to suppress them. The ground planes or shielding enclosures can behave as hollow waveguides and propagate these modes. Suppression can take the form of shorting screws between the ground planes or designing the enclosure to be too small to support frequencies as low as the operational frequencies of the circuit. Similarly, coaxial cable can support circular TE and TM modes that do not require the centre conductor to propagate, and these modes can be suppressed by reducing the diameter of the cable.
Longitudinal-section modes
Some transmission line structures are unable to support a pure TE or TM mode, but can support modes that are a linear superposition of TE and TM modes. In other words, they have a longitudinal component of both electric and magnetic field. Such modes are called hybrid electromagnetic (HEM) modes. A subset of the HEM modes is the longitudinal-section modes. These come in two varieties; longitudinal-section electric (LSE) modes and longitudinal-section magnetic (LSM) modes. LSE modes have an electric field that is zero in one transverse direction, and LSM modes have a magnetic field that is zero in one transverse direction. LSE and LSM modes can occur in planar transmission line types with non-homogeneous transmission media. Structures that are unable to support a pure TE or TM mode, if they are able to support transmissions at all, must necessarily do so with a hybrid mode.Zhang & Li, pp. 188, 294, 332
Other important parameters