The L networks are resonant. All three of them in a Pi-L network. Two
step
down and one steps impedance)
Looking at a single L network ( impedance step down) with a resistive
load
you will see that it is a capacitor to ground and a series inductor and
resistance across it. This forms a parallel resonant circuit.
The Q is easily calculated and so it it's effective impedance as seen
at the "input" of the L network. The HI-z side is across the capacitor and
low-z side is at the end of the inductor to ground. The impedance
transformation ratio is the square-root of the Q.
The L-network is the easiest to solve because it is a basic parallel tuned
circuit with a resistor in the inductive leg.
If the L-network has a Q of 16 and the inductor is connected to
50 ohms the input impedance, looking into the capacitor end, is 4x50 or 200
ohms. And resonance is the standard formula where XL =XC
The problem with the PI-L network is that it will not match as large of
an
impedance range as a PI network alone due to the fixed output inductance.
But it does attenuate harmonics just as well while at a lower Q allowing
for less expensive less expensive band switches and coils. But the down
side of it is that the intermediate impedance is higher than the output
impedance so the "loading" capacitor has to be of higher voltage rating and
larger capacitance as well. Some old tube type AM broadcast transmitters
often had 3 ,4 or 5 sections to assure harmonic suppression. This was
particularly hard to tune correctly.
73
Bill wa4lav
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