> Now we add the output load C and it becomes a Pi-net. Traditionally,
> this circuit is viewed at two L networks back to back, primarily
> because that view simplifies the computation of values (in the days
> before computers). But, I personally think its reasonable to think of
We view it that way because that is how the network behaves. If
you moved along the inductor measuring voltage to ground with a
RF voltmeter, you would see voltage at some point in the network
is LOWER than either the input or output voltage.
We have two L networks, BOTH stepping impedance down when
looking from each end towards some point in the network. Phase
shift is more than 90 degrees, and system Q is more than the
square root of the ratio of impedances.
If the Pi-network has inadequate Q, it behaves like a single L
network and one of the capacitors behaves "backwards" or has
very little effect when adjusted. Some "pi-network" antenna tuners
are actually L networks, because the second capacitor is far too
small and Q is far to low to make them behave like Pi-
networks....even though they have three components.
> what's going on differently. The Tune C and L create a tuned circuit
> with some Q (higher than what's needed for the L network)...which
> creates circulating currents. The load C shunts some of the
> circulating current back to the tank but lets some of the current flow
> in the output. The less load C, the higher its impedance and the more
> circulating current flows in the output load
Only when the network has enough Q to really be a Pi.
Also, the input and output areas each have a Q...and that Q is
usually different. The network Q is higher than the Q at one end, so
when we are using Q's of 12 in simple tank circuit calculations the
Q is really higher.
73, Tom W8JI
W8JI@contesting.com
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