Types of Transmission Lines - Microwave Engineering

What is Types of Transmission Lines?

The orthodox open-wire transmission lines are not appropriate for microwave transmission, as the energy losses would be high. At Microwave frequencies, the transmission lines employed can be broadly classified into three types. They are −

  • Multi conductor lines
    • Co-axial lines
    • Strip lines
    • Micro strip lines
    • Slot lines
    • Coplanar lines, etc.
  • Single conductor lines (Waveguides)
    • Rectangular waveguides
    • Circular waveguides
    • Elliptical waveguides
    • Single-ridged waveguides
    • Double-ridged waveguides, etc.
  • Open boundary structures
    • Di-electric rods
    • Open waveguides, etc.

Multi-conductor Lines

The transmission lines which have more than one conductor are called as Multi-conductor lines.

Co-axial Lines

This one is typically used for great frequency applications.

A coaxial line contains of an inner electrode with inner diameter d, and then a concentric cylindrical insulating material, round it. This is enclosed by an outer conductor, which is a concentric cylinder with an inner diameter D. This structure is well agreed by taking a look at the resulting figure.

The fundamental and leading mode in co-axial cables is TEM mode. There is no cutoff frequency in the co-axial cable. It passes all frequencies. Though, for higher incidences, some higher order non-TEM mode starts spreading, producing a lot of weakening.

Strip Lines

Frequencies from 100MHz to 100GHz are the planar transmission lines, used at A Strip line contains of a central thin conducting strip of width ω which is greater than its thickness t. It is located inside the low loss dielectric (εr) substrate of thickness b/2 among two wide ground plates. The width of the ground plates is five times better than the arrangement between the plates.

The thickness of metallic central conductor and the thickness of metallic ground planes are the same. The resulting figure shows the cross-sectional view of the strip line structure.

The important and leading mode in Strip lines is TEM mode. For b<λ/2, there will be no propagation in the transverse direction. The impedance of a strip line is inversely proportional to the ratio of the width ω of the inner conductor to the distance b between the ground planes.

Micro Strip Lines

The strip line has a difficulty that it is not reachable for adjustment and tuning. This is evaded in micro strip lines, which permits mounting of active or passive devices, and also allows making minor adjustments after the circuit has been fabricated.

A micro strip line is an unsymmetrical parallel plate transmission line, having di-electric substrate which has a metallized ground on the bottom and a thin conducting strip on top with thickness 't' and width 'ω'. This can be understood by taking a look at the following figure, which shows a micro strip line.

The characteristic impedance of a micro strip is a function of the strip line width (ω), thickness (t) and the distance between the line and the ground plane (h). Micro strip lines are of many types such as embedded micro strip, inverted micro strip, suspended micro strip and slotted micro strip transmission lines.

In addition to these, some other TEM lines such as parallel strip lines and coplanar strip lines also have been used for microwave integrated circuits.

Other Lines

A Parallel Strip line is similar to a two conductor transmission line. It can support quasi TEM mode. The following figure explains this.


A Coplanar strip line is shaped by two leading strips with one strip grounded, both being placed on the same substrate surface, for convenient connections. The following figure explains this.

A Slot line transmission line, consists of a slot or gap in a conducting coating on a dielectric substrate and this fabrication process is identical to the micro strip lines. Following is its diagrammatical representation.

A coplanar waveguide contains of a strip of thin metallic film which is put on the surface of a dielectric slab. This slab has two conductors running head-to-head and parallel to the strip on to the same surface. The resulting figure explains this.


All of these micro strip lines are used in microwave applications where the use of large and lavish to manufacture transmission lines will be a disadvantage.

Open Boundary Structures

These can also be specified as Open Electromagnetic Waveguides. A waveguide that is not completely surrounded in a metal shielding can be measured as an open waveguide. Free space is also measured as a kind of open waveguide.

An open waveguide may be defined as any physical device with longitudinal axial symmetry and unbounded cross-section, capable of guiding electromagnetic waves. They own a spectrum which is no longer separate. Micro strip lines and optical fibers are also instances of open waveguides.

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