What is Stripline in RF and Microwave

A stripline is a type of transmission line used in RF and microwave engineering to guide electromagnetic (EM) waves from one point to another. It is a structure designed to maintain controlled impedance and minimize signal loss as high-frequency signals propagate through it. There are three main types of striplines including offset, double conductor and suspended stripline.

The basic construction of a stripline consists of a flat conductor (usually made of metal) sandwiched between two layers of dielectric material that acts as insulator. The conductor can be in the form of a trace on a printed circuit board (PCB) or a thin metal strip.

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. ε_eff = ε_r

Phase velocity and guide wavelength:
Phase velocity (V_p) and guide wavelength(λ) for stripline is mentioned below.
V_p = c / SQRT(ε_r)
λ = λ₀ / SQRT(ε_r)
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(ε_r) ) * ( 1/ ((W_e /h) + 0.441) )

Where, W_e/h = W/h , if W/h > 0.35
W_e/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.
α_d = 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

There are several types each designed to meet certain RF and Microwave circuit requirements including Shielded, suspended, edge-coupled, offset and double conductor. These striplines vary in terms of configurations and constructions. Let us understand few of these strip lines.

Double conductor stripline

It is a transmission line configuration that consists of two parallel conductors separated by dielectric substrate and surrounded by two ground planes on each side. It is also known as twin lead stripline. As shown in fig-2, double conductor stripline is combination of two equal or two unequal trace width microstrip lines.

The key features of double conductor stripline are as follows.
• It is used for differential signal transmission which helps to improve noise immunity and common mode rejection.
• It helps to reduce crosstalk between two close conductors. This makes it suitable for high speed data transmission.
• The parallel conductors helps to maintain balanced characteristic impedance which minimizes signal reflections.

Offset stripline

It is a type of transmission line that combines characteristics of microstrip and stripline. It consists of conductor embedded within a dielectric substrate positioned between two ground planes. The key feature of offset stripline is that conductor is located slightly off center within the dielectric substrate.

Offset stripline can be formed by joining two substrates of unequal heights with a glue. As the name suggests, center conductor does not lie in the middle and is slightly offset. Following are the benefits or advantages of this strip line type.
• It's off-center positioning offers asymmetrical coupling that can lead to better isolation between adjacent traces. This reduces crosstalk and interference.
• It allows precise control of characteristic impedance which helps to maintain signal integrity and minimizes reflections in high frequency circuits.
• The asymmetry of conductor's position helps to reduce radiation from transmission line.

Suspended stripline

In this type of transmission line, the signal conductor is suspended in free space (e.g. air) within a grounded enclosure. It consists of conductor running between two ground planes with a dielectric material supporting it. It is the most popular among all stripline variants.

The key features of suspended stripline are as follows.
• It can achieve low dielectric losses compared to other configurations like microstrip. This is due to elimination of dielectric material directly above and below the signal conductor in this structure.
• The reduced dielectric losses allow it to operate at higher frequencies without significant signal degradation.
• The ground planes surrounding the signal conductor offer excellent isolation and shielding. This makes it suitable for applications where EM interference needs to be minimized.

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

Advantages of Stripline

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

Disadvantages of Stripline

➨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.

Application : Strip line based Directional coupler

Striplines are commonly used in RF and microwave circuits. They found applications in wireless communication systems, radar systems and other high-frequency electronic devices.

Following are typical specifications of strip line directional coupler. The popular stripline directional coupler manufacturers include marki microwave, ARRA Inc., TRM Microwave and so on.
• Frequency band of operation
• VSWR
• Mean Coupling in dB
• Amplitude flatness(dB)
• Directivity(dB)

Conclusion : The different types of striplines offer flexibility in designing RF and microwave circuits to meet specific requirements for impedance, signal integrity and manufacturing ease. The choice of stripline depends on factors such as frequency range, isolation needs, insertion loss and fabrication capabilities.

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