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Slotline basics

This article describes Slotline basics and mention Slotline types viz. antipodal slotline and bilateral slotline.


As shown in the fig-1, slotline is a planar transmission line. Here small narrow slot is etched out on one side of the dielectric substrate from ground plane metal surface. The other side will not have any metallic stuff.

This width 'W' determines the slotline characteristic impedance. The impedance increases with increase in W and is less sensitive to height of the substrate. It is a two conductor transmission line with major electric field component oriented across the slot. Hence it does not support TEM mode.

The Electromagnetic wave travels along the slotline which encounters substrate and air as medium for the transmission. Hence effective dielectric constant is equal to average of both, i.e. εeff = (εr + 1) /2. There are various methods to determine characteristic impedance and losses in the slotline.

The slot wavelength is as follows.
λs = λ0/SQRT (εeff)
SQRT stands for Square root of 2.

Slotline types

antipodal slotline

As shown in the figure, two conductors are printed on opposite sides of the dielectric substrate in antipodal slotline. Width (W) defined characteristics of this slotline. Tapered antipodal slotline is widely employed in tapered slot antennas.

bilateral slotline

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

Slotline Advantages

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

Slotline Disadvantages

•  Slotline is relatively lossy compare to other lines.
•  It can not be used for broadband circuit designs as it supports only TE mode. This mode is dispersive in nature.


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Slotline calculator

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