Slotline Basics: Types, Advantages, and Disadvantages

This article describes slotline basics and mentions slotline types viz. antipodal slotline and bilateral slotline.

Fig-1: Slotline

As shown in Fig-1, a slotline is a planar transmission line. Here, a small narrow slot is etched out on one side of the dielectric substrate from the ground plane metal surface. The other side will not have any metallic material. This width ‘W’ determines the slotline’s characteristic impedance. The impedance increases with an increase in ‘W’ and is less sensitive to the height of the substrate.

It is a two-conductor transmission line with a major electric field component oriented across the slot. Hence, it does not support the TEM mode. The electromagnetic wave travels along the slotline, encountering the substrate and air as the medium for transmission. Hence, the effective dielectric constant is equal to the average of both, i.e., εeff = (εr + 1) / 2.

There are various methods to determine the characteristic impedance and losses in the slotline. The slot wavelength is as follows:

λs = λ0 / SQRT(εeff) (SQRT stands for Square Root of)

Slotline Types

Antipodal Slotline

As shown in the figure, in an antipodal slotline, two conductors are printed on opposite sides of the dielectric substrate. The width (W) defines the characteristics of this slotline. Tapered antipodal slotlines are widely employed in tapered slot antennas.

Bilateral Slotline

As shown in the figure, in a bilateral slotline, two identical slotlines are printed on both sides of the substrate. The characteristic impedance of this line reduces with substrate height and increases with the dielectric constant. It is used for broadband applications due to its low impedance and matching characteristics with stripline. It requires precise alignment during the fabrication process. Performance degrades if the top and bottom slotlines do not overlap exactly.

Slotline Advantages

• Higher-order modes are possible to realize.
• Easy to fabricate and troubleshoot.
• Can be etched along with a microstrip line on the same PCB.

Slotline Disadvantages

• A slotline is relatively lossy compared to other lines.
• It cannot be used for broadband circuit designs as it supports only the TE mode. This mode is dispersive in nature.