"Inertia" is a relative thing. Consider a one-cylinder motor with no
flywheel at all (imagining it could still run). The crankshaft rotation
would be very jerky, wouldn't it? Now put a small flywheel on it. The
jerks wouldn't be as abrupt, but the rotation would still be jerky. As
you make the flywheel bigger and bigger, the jerks smooth out, but the
flywheel has to get really big before the rotational speed becomes, for
all practical purposes, constant, without varying some during each
rotation. That's a pretty good analogy. A low-Q tank circuit is like the
little flywheel, and a high-Q tank like a big flywheel.

A tank that provides the other half of an RF cycle does interfere with
abrupt phase changes. But the circuit can usually be designed to provide
enough restoration of the carrier sine wave while retaining enough of
the modulation characteristic to be useful. Also, a single-resonator
tank circuit isn't the only trick in the engineer's bag. More complex
filters, such as multiple pole bandpass and lowpass filters, can be
designed that are much more selective in what they do than a simple
single LC tank circuit.

The larger and more abrupt the changes, the more careful and clever the
designer has to be. But the design of wideband modulation systems is
well within the capabilities of a competent RF engineer.

If you have an oscilloscope and a signal generator capable of being
frequency modulated, you can run some experiments with LC circuits and
filters that should be quite educational. And a spectrum analyzer would
enhance the educational value considerably.

Roy Lewallen, W7EL

gary wrote:
Thanks for your response. I am struggling with this. I still have
trouble visualizing how a 180 or 270 degree change can occur in a
single rf cycle and be able to overcome the "inertia" (probably a
poor choice of words) of the rf circuits , feed line and antenna
system. These are large abrupt changes, not more suttle changes like
i would expect relative to voice modulation. I have read that in some
amplifiers the tank provides the other half of a single rf cycle. Why
wouldnt the same the action interfere with a phase shift in a rf
cycle. I could understand being able to detect a phase shift after a
given period of time with respect to a previous period. The period
being relatively long compared to the rf cycle time.

Gary
W4AF