Microstrip is one of the most popular microwave transmission-line technologies. Ideally, it would operate without loss, and without adding noise or degrading signal integrity. In the real world, however, microstrip transmission lines suffer from three types of losses: conductor, dielectric, and radiation losses. The first two are fairly well understood, with conductor loss dependent on the type of metal used for the transmission lines and the dimensions of those lines (with loss increasing with length) and dielectric loss a function of the type of printed-circuit-board (PCB) substrate material. Radiation losses can stem from numerous factors, including the type of dielectric material, its thickness, and the shapes of transmission-line structures in a microstrip circuit. Radiation loss also depends very much on frequency, increasing with increasing frequency.
Radiation loss is not exclusive to microstrip, although its effects are more noticeable in microstrip than in stripline or coplanar-waveguide circuits. Stripline transmission lines, because they are enclosed by dielectric material, exhibit almost no radiation loss. Coplanar-waveguide circuits suffer minimal radiation losses, except at transmission-line discontinuities, such as a junction or step. But in certain microstrip circuits, such as filters and couplers, radiation loss can significantly impact performance, resulting in increased insertion loss, changes in signal phase, and shifts in resonant frequency. Understanding more about microstrip radiation losses and why they occur can help to control those losses.
That ideal microstrip transmission line mentioned earlier was without junctions that could increase radiation losses. Real microstrip circuits have transmission lines with their share of discontinuities, such as steps or changes in width, bends, open ends, gaps, and junctions. These discontinuities are typically an integral part of a high-frequency circuit design. For example, microstrip meander lines are slow-wave structures that are typically used to realize a shift in phase. Unfortunately, every deviation from a straight, continuous microstrip transmission line invites radiation of energy away from the metal conductor and dielectric material, effectively dissipating energy from the microstrip circuit into free space. In addition, the close proximity of conductive lines in a meander line configuration can result in the coupling of energy from adjacent lines, also causing losses.
Many circuit designers have learned to limit the number of discontinuities in a microstrip circuit in order to minimize radiation losses. And when discontinuities are necessary, creative designers seek out forms that will not invite radiation effects. The shape of a transmission-line discontinuity can influence the amount of radiation loss, with sharper edges on junctions causing higher radiation losses than rounded edges or more gradual transitions in step junctions, as used for impedance matching in low-noise amplifiers.
Microstrip transmission lines with excessive radiation loss can be thought of as antennas, albeit unintended ones. Even in a microstrip antenna or antenna array, unwanted radiation losses can occur at junctions and coupling points, resulting in degraded antenna efficiency, especially at higher frequencies. Microstrip radiation losses can be controlled, but it requires careful choices in dielectric substrate, the thickness of that substrate, and the geometry of the microstrip circuitry.
One of those choices is in the relative dielectric constant for the PCB material. Dielectric substrates with low dielectric constants, such as less than 5.0, have often been used in microwave microstrip circuits because of their low cost and versatility: they yield larger circuit geometries for a given frequency compared to circuit materials with higher dielectric constants, helping to simplify fabrication at higher frequencies. With the lower dielectric constant, however, less of the conducted electromagnetic (EM) energy is concentrated in the substrate and the microstrip metal conductor, leading to losses as a result of radiation effects.
Switching to a higher-dielectric-constant circuit material will reduce radiation losses because a greater part of the EM field is concentrated in the dielectric material between the ground plane and the microstrip metal conductor. Of course, the increase in substrate dielectric constant will also miniaturize the dimensions of the microstrip circuitry, roughly decreasing in size by a factor equal to the square root of the dielectric constant. If making such a switch in choice of dielectric material, attention should also be paid to other material parameters, so as to not trade lower radiation loss for higher dielectric loss.
The thickness of a PCB material also impacts the amount of radiation loss exhibited by a microstrip circuit at higher frequencies. For lower-dielectric-constant circuit materials, radiation losses can be reduced by using a thinner dielectric substrate. Radiation losses can also be reduced with a thicker substrate material having a higher dielectric constant, although a further reduction in radiation losses can also be achieved by using a thinner material at that higher dielectric constant. The shrinking of circuit dimensions that comes from using a substrate with higher dielectric constant can be a concern at higher frequencies, such as for millimeter-wave circuits, where the circuit dimensions and tolerances may become critical for some fabrication processes.
An example of a commercial PCB laminate available at higher dielectric-constant value is RO3010™ laminate from Rogers Corp. (www.rogerscorp.com). It is a ceramic-filled PTFE composite that can be specified with dielectric-constant values of 3.00, 6.15, and 10.2, each measured in the z direction at 10 GHz. Although the substrates in the product family exhibit different dielectric constants, they offer consistent mechanical properties, allowing designers to mix dielectric-constant values in multilayer circuit-board assemblies. The high-dielectric-constant material is available in different thicknesses, from 0.005 in. (0.13 mm) to 0.050 in. (1.28 mm), so that designers can also use that circuit-material parameter when combating the effects of radiation loss. And the material sacrifices nothing in terms of dielectric loss, with low dissipation factor of 0.0022 at 10 GHz, which is on a par with lower-dielectric-constant circuit materials.
Similarly, the TMM® circuit laminates from Rogers Corp. (www.rogerscorp.com) are available with high dielectric constant values such as 9.2 and 9.8, both measured in the z direction at 10 GHz. These are ceramic thermoset polymer composite materials with excellent mechanical and electrical stability, (evidenced by its exceptionally low thermal coefficient of dielectric constant)but also with low dielectric loss, as evidenced by dissipation factor of 0.0023 or better at 10 GHz for all TMM materials.
Radiation loss can be a concern in microstrip circuits at higher microwave frequencies. Fortunately, its effect can be minimized by reducing the number of discontinuities in a circuit design, and by carefully weighing the choice of circuit substrate material in terms of thickness and dielectric constant, where thinner materials and higher dielectric constants can both contribute to lower radiation losses.
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