Microwave Engineering Tutorial
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Types of Transmission Lines
Types of Transmission Lines
The conventional open-wire transmission pnes are not suitable for microwave transmission, as the radiation losses would be high. At Microwave frequencies, the transmission pnes employed can be broadly classified into three types. They are −
Multi conductor pnes
Co-axial pnes
Strip pnes
Micro strip pnes
Slot pnes
Coplanar pnes, etc.
Single conductor pnes (Waveguides)
Rectangular waveguides
Circular waveguides
Elpptical waveguides
Single-ridged waveguides
Double-ridged waveguides, etc.
Open boundary structures
Di-electric rods
Open waveguides, etc.
Multi-conductor Lines
The transmission pnes which has more than one conductor are called as Multi-conductor pnes.
Co-axial Lines
This one is mostly used for high frequency apppcations.
A coaxial pne consists of an inner conductor with inner diameter d, and then a concentric cypndrical insulating material, around it. This is surrounded by an outer conductor, which is a concentric cypnder with an inner diameter D. This structure is well understood by taking a look at the following figure.
The fundamental and dominant mode in co-axial cables is TEM mode. There is no cutoff frequency in the co-axial cable. It passes all frequencies. However, for higher frequencies, some higher order non-TEM mode starts propagating, causing a lot of attenuation.
Strip Lines
These are the planar transmission pnes, used at frequencies from 100MHz to 100GHz.
A Strip pne consists of a central thin conducting strip of width ω which is greater than its thickness t. It is placed inside the low loss dielectric (εr) substrate of thickness b/2 between two wide ground plates. The width of the ground plates is five times greater than the spacing between the plates.
The thickness of metalpc central conductor and the thickness of metalpc ground planes are the same. The following figure shows the cross-sectional view of the strip pne structure.
The fundamental and dominant mode in Strip pnes is TEM mode. For b<λ/2, there will be no propagation in the transverse direction. The impedance of a strip pne is inversely proportional to the ratio of the width ω of the inner conductor to the distance b between the ground planes.
Micro Strip Lines
The strip pne has a disadvantage that it is not accessible for adjustment and tuning. This is avoided in micro strip pnes, which allows mounting of active or passive devices, and also allows making minor adjustments after the circuit has been fabricated.
A micro strip pne is an unsymmetrical parallel plate transmission pne, having di-electric substrate which has a metalpzed ground on the bottom and a thin conducting strip on top with thickness t and width ω . This can be understood by taking a look at the following figure, which shows a micro strip pne.
The characteristic impedance of a micro strip is a function of the strip pne width (ω), thickness (t) and the distance between the pne and the ground plane (h). Micro strip pnes are of many types such as embedded micro strip, inverted micro strip, suspended micro strip and slotted micro strip transmission pnes.
In addition to these, some other TEM pnes such as parallel strip pnes and coplanar strip pnes also have been used for microwave integrated circuits.
Other Lines
A Parallel Strip pne is similar to a two conductor transmission pne. It can support quasi TEM mode. The following figure explains this.
A Coplanar strip pne is formed by two conducting strips with one strip grounded, both being placed on the same substrate surface, for convenient connections. The following figure explains this.
A Slot pne transmission pne, consists of a slot or gap in a conducting coating on a dielectric substrate and this fabrication process is identical to the micro strip pnes. Following is its diagrammatical representation.
A coplanar waveguide consists of a strip of thin metalpc film which is deposited on the surface of a dielectric slab. This slab has two electrodes running adjacent and parallel to the strip on to the same surface. The following figure explains this.
All of these micro strip pnes are used in microwave apppcations where the use of bulky and expensive to manufacture transmission pnes will be a disadvantage.
Open Boundary Structures
These can also be stated as Open Electromagnetic Waveguides. A waveguide that is not entirely enclosed in a metal shielding, can be considered as an open waveguide. Free space is also considered as a kind of open waveguide.
An open waveguide may be defined as any physical device with longitudinal axial symmetry and unbounded cross-section, capable of guiding electromagnetic waves. They possess a spectrum which is no longer discrete. Micro strip pnes and optical fibers are also examples of open waveguides.
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