Rich wrote:
>>agreed. However, Newtonian physics works fairly ok
below 99% of the speed of light.<<
Well said, Rich. The electrons in a wire travel at a
drift velocity measured in millimeters per second,
clearly amenable to study via Newtonian physics.
The electromagnetic fields created by the movement of
those electrons travel at 100% of the speed of light.
We've all encountered the effects of this, and we use
this knowledge regularly. For instance, when we
calculate the approximate length for a half wave
dipole, we use the speed of the electromagnetic waves,
not the speed of the physical electrons in the wire.
Inside an amplifier, we must look carefully to see if
there are structures whose physical dimensions suggest
there might be some effect that is not obvious from a
study of the schematic. For instance, in my SB-200
amplifier, the path from anode, through output tuning
network, through chassis and grid wiring, back to
anode, is about 15 inches. That's a significant
length, particularly at VHF. If I only consider
Newtonian physics, I would ignore this length, however
since I know that electromagnetic waves travel at the
speed of light, I realize that I must determine the
effects that this large loop may have, in order to
build an effective circuit model for the output
network. The presence of this loop suggests that we
make some measurements to identify what other
structures it might couple to, in addition to what
effects it would have in the plate circuit itself. It
is possible that those effeccts can be reasonably
modelled with circuit simulators, but only after
careful two-port measurements to determine what the
parameters are. Such a loop certainly suggests the
presence of both capacitive and mutual inductive
coupling factors.
An interesting time-domain treatment of these coupling
factors and loop inductance, including reasonable
rule-of-thumb equations for estimating them, can be
found in Chapter One, and Appendix C of "High-Speed
Digital Design" by Johnson and Graham.
73,
Dave W8NF
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