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Microwave Transmission line basics

This article describes microwave transmission line basics and mention types of transmission lines and their advantages.

Transmission lines are means of transporting RF energy between subsystems. They have many applications between exciter OUT port to the transmitter IN port and also from transmitter OUT port to the antenna INPUT port. Transmission lines are characterized by basic three electronic components R,L and C as shown in the following figure. The other way of transporting RF frequency is waveguide.

microwave transmission line Fig.1 Microwave Transmission line

Types of Transmission Lines

Following are the types of transmission lines.

•   Parallel Line
•   Coaxial line(standard)
•   Gas Filled Coaxial line
•   Coaxial Line(Air Articulated dielectric)
•   Double shielded coaxial Line
•   Parallel Line(Shielded)
•   Microstrip line, READ MORE

Transmission Line Characteristic Impedance

As shown in the figure, Transmission line is also referred as RLC network and its characteristic impedance Zo depends on per unit of length parameters R(Resistance), G(Conductance) , L(Inductance) and C(Capacitance) and their relationship equation is mentioned below.

transmission line characteristic impedance

Here Zo is the characteristic Impedance in Ohm.
w= angular frequency in radians per second (2 pi F)
R is in Ohms, G in Mhos,L in Henrys, C in Farads and all are per unit length values.

For Microwave frequency, R and G are negligible compare to L and C respectively and hence Zo depends only on L and C parameter values. The same is mentioned in the Zo equation. Hence at transmission line is composed of only L in series and C in parallel.

Example: Calculate Characteristic Impedance(Zo) for lossless microwave transmission line having unit length inductance of 4nH and unit length capacitance of 2 PF.

Zo= 44.72 Ohm

For the parallel Wire line type of transmission line characteristic impedance is determined using following equation.

Zo = 276 Log (2*S/d) Ohm
d is the diameter of the conductor used in parallel line and -S- is the spacing between the two conductors.

Transmission Line Equations

Velocity of wavefront transmitted along the transmission Line
v = 1 / (LC)0.5

Transmission Line impedance as a function of voltage and current,
ZL = Vinc + Vref / Iinc + Iref


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