Micro-stripline: Its advantages and disadvantages

Microstrip line (also known as microstrip or microstrip transmission line) is a type of electrical transmission line that is widely used in microwave applications. It is a planar structure consisting of a thin conductor strip, usually made of copper, placed on a dielectric substrate.

Micro-stripline is a flat strip of conducting  material separated from a single grounded plane by a dielectric material as shown in fig below:

 

Fig: Micro-stripline 

The microstrip line is typically used for microwave frequency signals, and is commonly found in microwave circuits such as amplifiers, filters, and antennas. It can be used to transfer signals from one part of a circuit to another or to couple power into and out of a circuit.

Advantages:

• less expensive than traditional waveguide
• far lighter and smaller and more compact
• easy to fabricate
• easy to troubleshoot

Disadvantages:

• lower power handling capacity and higher losses comparing with waveguide
• poor isolation among adjacent lines
• unwanted radiation in uncovered configuration 
• higher losses  

Despite these drawbacks, microstrip line is still widely used in microwave circuits due to its low cost, low loss, and ability to be easily integrated into PCBs. Advances in technology have also led to the development of new materials and techniques that can mitigate some of the issues associated with microstrip line, making it a versatile and reliable option for many microwave applications.

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Stripline: Its advantages and disadvantages

Stripline consists of a flat strip of conducting material separated from two ground planes by a dielectric material as shown in figure below:

      

Fig: Stripline

Properties of Stripline:

• used over the frequency range from 100MHz to 30GHz.
• the central conductor needn't be equally spaced between the ground planes.
• the dielectric material may be different above and below the central conductor.
• when the separation of ground planes is less than ƛ/2 , the field can't propagate in the transverse direction and decrease exponentially.
• the energy is confined inside the line cross-section provided the width of the ground plane is at least five times greater than the spacing between the plates.

Advantages of stripline:

• smaller than waveguide or co-axial line
• has a greater frequency bandwidth
• has a lower radiation loss
• better isolation
• good EM shielding can be achieved

Disadvantages of stripline:

• complex and expensive in fabricating it
• tuning or troubleshooting is complex
• stripline trace width is smaller compare to microstrip line of same impedance and height.

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Planar transmission lines: Its advantages and disadvantages

Planar transmission lines are flat two or multi-conductor transmission lines having low profile and light weight. This geometry allows control of the characteristic impedance of the line by defining the line dimensions in a single plane and is suitable for microwave integrated circuits. The complete transmission line circuit can be fabricated in one step by thin film technology and photolithography techniques.

There are four basic forms of planar transmission lines-

• striplines
• micro-striplines
• slot lines
• co-planar striplines

The basic advantages of planar transmission lines are-

• small in size and light in weight
• can be flash mounted on a metallic body
• increased reliability
• low cost
• easy access for component mounting
• characteristic impedance can be controlled by defining the dimensions in a single plane.
• passive circuit design is possible easily by changing the dimensions of the line in one plane only.

The basic disadvantages of planar transmission lines are-

• low power handling capability due to small size.
• radiation losses from the open structures. (This loss is reduced by confining the field lines more in the dielectric substrate of a high dielectric constant.)
• circuit design using these configurations should be very accurate since matching screws and short circuit plungers can not be used in planar transmission line circuits. The design of some active devices are also limited because of low Q obtainable with ordinary microstrip configurations.

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