2.11 Transmission lines – Design and Practical Realisation
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As in the case of the coaxial line, the Electric field is inversely proportional to the dielectric
constant and hence as such a constant increases, the field and therefore the voltage and impedance
also decrease. This is shown in Figure 2.11-15.
Figure 2.11-16 Effective Dielectric Constant versus actual dielectric constant
We must remind the reader that all microstrip equations are approximate. What's more,
they do not take the thickness of the metal into account, so we would not recommend relying on
them for critical designs on thick copper boards. Having a finite thickness of metal for the conductor
strips tends to increase the capacitance of the lines, which affects the
and calculations.
Other parameters which one needs to take into account when selecting a substrate are loss
tangent and thermal conductivity. The loss tangent quantifies the substrate dissipation of
electromagnetic energy. The lower this value the lower the electromagnetic energy lost due to the
substrate material. The thermal conductivity gives an indication of how well heat may be extracted
from the substrate. The Electrical
resistivity of the metal which is used for the line also plays a role.
The resistivity is a measure of how strongly a material opposes the flow of electric current.
Microstrip lines are very widely used in modern electronic circuits because they are compact
and can be easily and cheaply fabricated and integrated on printed circuit boards. Let us now see
how Microwave Office and its TXLine tool can help us designing these useful structures.
Video 2.6 illustrates how to design and verify a 50 Ω microstrip line. It also shows how the different
parameters of the line affect its characteristic impedance.
Conquer Radio Frequency
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