Understanding Stripline in RF and Microwave Engineering

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’s 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 an 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 an extended version of a microstrip line.

As shown in Fig-1, it looks like a sandwich structure. Here, ground planes exist on both sides of the substrate while the metal strip of design lies in the middle.

Fig-1: Stripline field configuration

Stripline provides a homogeneous medium for EM waves compared to an 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 be exposed outside. Hence, a TEM mode can be obtained in this type of configuration. Stripline is formed by etching one side of a grounded substrate and later covered with another grounded substrate of the same height. Specific bond films are used to attach both with utmost care.

Effective Dielectric Constant

It is the same as the relative dielectric constant of the substrate dielectric material.

ε_eff = ε_r

Phase Velocity and Guide Wavelength

Phase velocity (V_p) and guide wavelength (λ) for stripline are mentioned below:

V_p = c / √(ε_r)

λ = λ₀ / √(ε_r)

Here, λ₀ is the free space wavelength, and c is the velocity of light in a vacuum.

Characteristic Impedance

The stripline characteristic impedance for a small trace with thickness (t), width (W), and substrate height (h) can be given as follows:

Z₀ = ( 30 *Pi / √(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 the sum of conductor loss and dielectric loss in stripline. Conductor loss can be calculated as per the 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 a few of these strip lines.

Double Conductor Stripline

It is a transmission line configuration that consists of two parallel conductors separated by a dielectric substrate and surrounded by two ground planes on each side. It is also known as twin lead stripline.

As shown in Fig-2, a double conductor stripline is a combination of two equal or two unequal trace width microstrip lines.

Fig-2: Double conductor stripline

The key features of the 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 reduce crosstalk between two close conductors. This makes it suitable for high-speed data transmission.
• The parallel conductors help maintain a 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 a conductor embedded within a dielectric substrate positioned between two ground planes.

The key feature of the offset stripline is that the conductor is located slightly off-center within the dielectric substrate.

Fig: Offset stripline

Offset stripline can be formed by joining two substrates of unequal heights with glue.

As the name suggests, the center conductor does not lie in the middle and is slightly offset.

Following are the benefits or advantages of this strip line type:

• Its 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 maintain signal integrity and minimizes reflections in high-frequency circuits.
• The asymmetry of the conductor’s position helps to reduce radiation from the 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 a conductor running between two ground planes with a dielectric material supporting it.

It is the most popular among all stripline variants.

Fig: suspended stripline

The key features of the suspended stripline are as follows:

• It can achieve low dielectric losses compared to other configurations like microstrip. This is due to the 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 sides as h << H and hence ‘h’ can be neglected. This configuration supports pure TEM mode propagation.

Assembly and housing procedures are complex in suspended stripline.

But it has the 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 compared to microstrip line of same impedance and height.
• Tuning or troubleshooting is complex.

Application

Stripline-based Directional coupler

Striplines are commonly used in RF and microwave circuits. They are found in applications in wireless communication systems, radar systems, and other high-frequency electronic devices. The following are typical specifications of a stripline 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.