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Stripline basics

This article describes Stripline basics and mention Stripline types viz. double conductor stripline, offset stripline and suspended stripline. Stripline directional coupler is also described below.

Stripline is considered as extended version of microstrip line. As shown in fig-1, it looks like a sandwidth structure. Here ground planes exist on both the sides of the substrate while metal strip of design lies at the middle. Stripline provides homogeneous medium for EM waves compare to uncovered microstrip line structure. Fig-1 depicts the field configurations in the stripline configuration. As shown field configurations will remain within the stripline and not exposed outside. Hence TEM mode can be obtained in this type of configuation.

Stripline is formed by etching one one side of grounded substrate and later covered with another grounded substrate of same height. Specific bond films are used to attach both with atmost care.

Effective Dielectric Constant:
It is same as relative dielectric constant of substrate dielectric material. εeff = εr

Phase velocity and guide wavelength:
Phase velocity (Vp) and guide wavelength(λ) for stripline is mentioned below.
Vp = c / SQRT(εr)
λ = λ0 / SQRT(εr)
Here, λ0 is the free space wavelength and c is the velocity of light in vaccum
SQRT stands for square root of 2.

Characteristic Impedance:
The stripline characteristic impedance for small trace with thickness(t) , width(W) and substrate height (h) can be given as follows:
Z0 = ( 30*Pi/SQRT(εr) ) * ( 1/ ((We /h) + 0.441) )

Where, We/h = W/h , if W/h > 0.35
We/h = ( W/h ) - { 0.35 - (W/h)2 } , if W/h < 0.35
Pi is equal to 22/7 or 3.14

Attenuation:
Total attenuation is sum of conductor loss and dielectric loss in stripline. Conductor loss can be calculated as per perturbation model. Dielectric loss can be expressed as below.
αd = k* tan(δ)/2 = (Pi/λ)* tan(δ)
Where, tan(δ) is the loss tangent of the dielectric and
k is the wave number equal to 2*Pi/λ

Stripline types As shown in fig-2, double conductor stripline is combination of two equal or two unequal trace width microstrip lines. Offset stripline can be formed by joining two substrates of unequal heights with a glue. As the name suggests, centre conductor does not lie in the middle and is slightly offset. The most popular among all stripline variants is suspended stripline. In this configuration, stripline is etched out on a thin substrate and the entire structure is enclosed. As the name suggests, stripline is suspended in the metallic structure.

The suspended stripline encounters air as dielectric on both the sides as h << H and hence 'h' can be neglected. This configuration supports pure TEM mode propagation. Assembly and housing procedure is complex in suspended stripline. But is has following advantages.
•  No spurious radiation
•  Wider bandwidth of operation
•  Low losses
•  High Q factor

•  Good EM shielding can be achieved with this structure
•  Low attenuation loss
•  Wider bandwidth
•  Better isolation

•  Complex and expensive in fabricating it.
•  Stripline Trace width is smaller compare to microstrip line of same impedance and height.
•  Tuning or troubleshooting is complex.

Stripline Directional coupler

Following are typical specifications of stripline directional coupler.
•  Frequency band of operation
•  VSWR
•  Mean Coupling in dB
•  Amplitude flatness(dB)
•  Directivity(dB)

The popular stripline directional coupler manufacturers include marki microwave, ARRA Inc., TRM Microwave and so on.

stripline basics and types
Microstrip line variants
Slotline basics and types
Finline basics
CPW(Coplanar Waveguide) basics
Types and basics of Microstrip line
Types and basics of coaxial line
Types and basics of transmission line
RF Terms-Useful for RF design and testing.