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The Characteristics of Planar Transmission Lines in RF and Microwave Printed Circuit Boards

Key Takeaways

  • Planar transmission lines are a type of connector commonly used in RF and microwave PCBs.

  • Planar transmission lines can be classified as striplines, microstrip lines, slotlines, coplanar striplines, or parallel-plate waveguides. 

  • In microstrip lines, coplanar striplines, and slotlines, electric and magnetic fields are distributed in the dielectric substrate and air. This kind of distribution results in a quasi-TEM mode of wave propagation. 

Planar transmission lines

Planar transmission lines

If you have some knowledge of RF and microwave circuits, then the terms microstrip line,  coplanar waveguide, slotline, stripline, and parallel-plate waveguide might be familiar to you. These are metalized planar constructions found on RF and microwave printed circuit boards. They are used to transmit signals, realize lumped elements, and for impedance matching

Innovations in the semiconductor industry, the introduction of low-loss factors ceramics, and the availability of low-cost PCBs have contributed to the development of transmission lines. Another type of transmission line, the planar transmission line, is commonly used due to its low cost. The characteristics of a planar transmission line depend on the mode of wave propagation through it.  

In this article, we will focus our discussion on the characteristics of planar transmission lines. 

The Characteristics of Planar Transmission Lines

The planar transmission line is a type of connector used in RF and microwave PCBs. The planar transmission lines in microwave and RF circuits are not only interconnection paths, but also perform as lumped elements. 

Passive elements, such as inductors and capacitors, can be realized by arranging planar transmission lines in a specific fashion. This method mitigates the crowding of components in circuits and makes the circuit more affordable. The use of planar transmission lines can also help in matching the impedance between the source and load. 

Planar Transmission Line Fabrication

Planar transmission lines are planar constructions that combine the dielectric surface and metal conductors for effective signal transmission. Thin-film technology and photolithography techniques are used to fabricate planar transmission lines. Depending on the arrangement of the conductors and dielectrics, planar transmission lines can be either metallic lines or dielectric lines. 

Classifications of Planar Transmission Lines

Planar transmission lines can be classified into the following categories:

  1. Striplines utilize planar geometry and consist of a metal strip surrounded by a dielectric material, which is sandwiched between upper and lower conducting ground planes.

  2. Microstrip lines are composed of the bottom ground plane, the dielectric layer, and the top conductor. 

  3. Slotlines are made by etching a slot out on the dielectric substrate. The dielectric substrate is in contact with the ground plane, which is made of metal. 

  4. Coplanar striplines are built by placing two adjacent conductive strips on one side of the dielectric substrate.

  5. Parallel-plate waveguides, where the signal propagation is between two perfectly conducting plates. 

Modes of Wave Propagation

Even though each classification of planar transmission line shares common planar geometry, the modes of wave propagation through them differ. To support the transverse electromagnetic mode (TEM) of wave propagation, a minimum of two conductors are required. The two conductors need to be placed in the homogenous dielectric substrate, and both the electric and magnetic field lines should be confined to the substrate. In planar transmission lines where the wave propagation takes TEM mode, there are no electric and magnetic field components in the direction of wave propagation. The stripline and parallel-plate waveguides support TEM mode. 

When the electromagnetic field spreads beyond the substrate and conductor, then it leads to non-TEM modes of wave propagation. In microstrip lines, coplanar striplines, and slotlines, the electric and magnetic fields are distributed in the dielectric substrate and air. This kind of distribution results in a quasi-TEM mode of wave propagation. Quasi-TEM mode is similar to TEM mode, but with slower variation in the characteristic impedance and propagation constant with an increase in frequency. 

The Advantages of Planar Transmission Lines

Some advantageous characteristics of planar transmission lines include: 

  • Lightweight and compact size.

  • Flash mounting on metal surfaces.

  • Increased reliability.

  • Cost-effective.

  • Easy to mount components.

  • Control over characteristic impedance by varying the physical dimensions; suitable for microwave integrated circuits.

  • Easy design of passive circuits by changing the dimensions of the line in one plane.

The Disadvantages of Planar Transmission Lines

Some disadvantages of planar transmission lines include: 

  • The small size of the planar transmission line limits its power handling capability.

  • Open structures suffer radiation losses.

  • The planar transmission line limits the quality factor of some active devices.

Each type of planar transmission line exhibits its own advantages and disadvantages, depending on the dimensions, frequency of operation, conductivity, permeability, and permittivity of the substrate. The planar structure of transmission lines is well-suited for RF and microwave integrated circuits. 

The characteristics of planar transmission lines play a significant role in maintaining signal integrity and circuit reliability. Cadence’s software offers PCB design tools for routing planar transmission lines in RF and microwave printed circuit boards. 

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