k4kyv at hotmail.com
Fri Dec 31 14:21:29 EST 2004
> > >
> > > Resonance, as I understand it, occurs when the capacitive and
> > > reactances cancel.
> > not cancel .. 'are equal'.
> > R = X(sub L)=X(sub C) or when Reactance is at, or very near 0 (Zero)
>Yes, but if they are equal and opposite would they not cancel?
Yes. Inductive reactance is defined as positive, while capacitive reactance
is defined as negative. At resonance, the reactances cancel, leaving an
impedance that is purely resistive. With tank circuits, you try to achieve
as near to zero ohms as possible, so that the only resistance in the
cincuit is the load resistance. With antennas, you are looking for an
impedance that approaches purely resistive, the radiation resistance. If
your antenna presents a reactive component to the transmitter, you tune out
the reactance with the matching network or antenna tuner.
That is why riceboxes shut down if they are not working into a load near 50
ohms nonreactive. There is no tuning network (tank circuit) to match the
load and cancel the reactance. So you add an external tank circuit
(transmatch) to allow you to work beyond the vicinity of one frequency.
With older tube type transmitters the tank cincuit was part of the rig.
With newer solid state transmitters, the tank circuit comes as an outboard
option that costs additional money.
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