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.
