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Published on: 10 August, 2022

What is strip line?

Strip line:

Strip lines are essentially modifications of the two wire lines and coaxial line. These are basically planar transmission lines that are widely used at frequency from 100 MHz to 100 GHz.

A strip line consists of a central thin conducting strip of width w which is greater than its thickness t, place inside the low loss dielectric εr substrate of thickness b/2 between two wide ground planes. The dominant mode for the strip line is a TEM mode. The width of the ground plane is at least five times greater than the spacing between the palates there by avoiding any vertical side walls at the two transverse end.

The characteristic impedance of strip line has been analyzed by a combination of analytical and empirical techniques by H. the design equations are divided into high- impedance region and low- impedance region determined by the ratio of w to (b-t).the impedance of strip line is inversely proportional to the width w of the inner conductor to the distance b between the ground planes.

The high impedance region is defined as

$$\frac{w}{b-t}\le0.35$$ and $$\frac{t}{b}\le0.25$$

The characteristics impedance in the high impedance region is

z0=$$\frac{60}{\sqrt{\varepsilon_r}}ln\left(\frac{4b}{\pi d}\right)\ \mathrm{\Omega}$$

Where $$d=\frac{w}{2}\left[1+\frac{t\left\{1+ln\left(\frac{4\pi w}{t}\right)+0.51\pi\left(\frac{t}{w}\right)^2\right\}}{\pi w}\right]\$$

The lower impedance region is defined as

$$\frac{w}{b-t}>0.35$$

The characteristics impedance in the lower impedance region is

z0=$$\frac{94.15}{\sqrt{\varepsilon_r}\left(\frac{w}{bA}+B\right)}\mathrm{\Omega}$$

Where $$A=1-\frac{t}{b}$$ and $$B=\frac{1}{\pi}\left[\frac{2}{A}\ln{\left(1+\frac{1}{A}\right)}-(\frac{1}{A}-1)\ln (\frac{1}{A^2}-1)\right]$$

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