CHAPTER 2 - Conveying Power at Radio Frequency
100
Let us now explore another extremely useful application of quarter-wave lines, the quarter-
wave impedance transformer. Consider the circuit show in Figure 2.9-11
Figure 2.9-11 Impedance matching with quarter wave transformer
may be calculated by means of equation (2.9-2)
In
order to get maximum power transfer to the load must
be equal to . By applying this
condition the equation above we obtain
Hence
if we are in a position to fabricate our quarter wave line in such way that its characteristic
impedance
satisfies the equation below
√
we
can then ensure maximum power transfer to the load.
In practice, realising a transmission line with a specific characteristic impedance is not a difficult task
to achieve. In fact we have already seen how the impedance of parallel-wire and coaxial lines is
related to their geometry in section 2.3 (page 41). The easiest way to create a line with a specific
characteristic impedance is to use a microstrip line (Figure 2.9-9), which is ideal for printed circuits.
Again this is a narrow-band approach i.e. it will only work at the frequency corresponding to the
wavelength or thereabouts. We will be looking at microstrip lines in section 2.11.
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Conquer Radio Frequency
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