What Are Planar Transmission Lines?

Planar transmission lines are guided-wave structures fabricated on flat substrates using photolithographic processes, designed to carry electromagnetic signals at radio and microwave frequencies with controlled impedance, minimal radiation loss, and dimensions compatible with printed circuit board and integrated circuit manufacturing. They are the foundational interconnect and distributed-element building blocks of modern RF and microwave circuits, including filters, couplers, amplifiers, and antenna feed networks. The family encompasses several distinct geometries, each trading off radiation loss, dielectric loss, ease of fabrication, shielding, and frequency range.

The defining characteristic shared by all planar transmission lines is their suitability for batch fabrication: conductors and dielectrics are deposited and patterned on a substrate rather than machined from bulk material, making them compatible with the same processes used to manufacture transistors, diodes, and passive components on the same substrate. This compatibility drove their adoption in microwave monolithic integrated circuits (MMICs) beginning in the 1970s and remains the primary reason they dominate RF circuit design today.

Microstrip

Microstrip is the most widely used planar transmission line geometry. It consists of a conducting strip on one face of a dielectric substrate, with a continuous ground plane on the opposite face. The signal field occupies a mixed region partly in the dielectric and partly in the air above the strip, producing an effective permittivity between that of the substrate and unity. Characteristic impedance is set by the ratio of strip width to substrate thickness, and because lateral dimensions are defined photolithographically while substrate thickness varies by a few percent, impedance control is moderate. Microstrip is straightforward to fabricate and accommodates shunt and series components at any point along the line, making it the standard choice for circuits operating from a few hundred MHz to approximately 30 GHz. The open structure allows the strip to radiate, which raises loss at millimeter-wave frequencies.

Stripline

Stripline places the signal conductor inside a sandwich of two parallel ground planes separated by a uniform dielectric. The closed structure prevents radiation and provides good electromagnetic isolation between adjacent lines, making stripline preferred for multi-layer PCBs where cross-talk between signal paths must be minimized. Because the fields are entirely within the dielectric, the effective permittivity equals the substrate permittivity and the wave velocity is lower than in microstrip. Fabricating vias to access the buried conductor adds process complexity compared with microstrip.

Coplanar Waveguide and Related Structures

Coplanar waveguide (CPW) places the signal conductor and both return-current planes on the same face of the substrate, separated by narrow slots. The lateral arrangement means that electrical characteristics depend on the ratio of strip width to slot width rather than on substrate thickness, giving tighter impedance control under the photolithographic process variations typical of high-frequency fabrication. CPW performs well above 20 GHz because the fields are more tightly confined than in microstrip, reducing both dielectric and radiation loss. Slotline, a related structure consisting of a gap in a ground plane without a center conductor, supports a non-TEM mode and is used in specialized components such as balanced mixers and certain antenna feeds.

Filters and Distributed Elements

Planar transmission lines enable distributed-element filters by exploiting resonant sections and coupled-line structures. A spurline filter, for instance, uses a stub-terminated open slot etched into a microstrip line to create a bandstop notch; spurline filters fabricated in microstrip achieve moderate rejection bandwidths of roughly 10 percent around the center frequency with a compact, single-layer layout. More elaborate coupled-line bandpass filters interleave half-wavelength resonators separated by controlled gaps, with the filter synthesis described using Wiley-IEEE microwave filter design frameworks. These distributed circuits replace bulky waveguide or coaxial cavity components at frequencies where lumped-element components become impractically small.

Applications

Planar transmission lines have applications in a range of fields, including:

• RF and microwave printed circuit board design for wireless communications equipment
• Monolithic microwave integrated circuits (MMICs) for radar and satellite systems
• Millimeter-wave 5G and 6G antenna feed networks and beamforming modules
• On-chip interconnects in RFIC design operating above 10 GHz
• Compact bandpass and bandstop filter elements in cellular base stations
• Transmission-line resonators in frequency synthesizers and oscillators