on 11/9/99 07:11, Bob Duckworth at wb4mnf@atl.org wrote:
> What are these mysterious circulating currents?
> I'm having trouble finding a satisfactory explanation.
Well, Bob, no one has offered an explanation for this yet, so I'll take a
shot at it!
Basically, it stems from the fact that capacitors and inductors story
energy. Capacitors store electrical field energy and inductors store
magnetic field energy.
In a lossless resonant circuit, this energy is continuously transferred
between the capacitive and inductive elements. Q is a measure of how well
an inductor or resonant circuit store this energy. A lossless resonant
circuit would have infinite Q.
Anyhow, when an RF signal is applied to these elements, they get charged up
with their respective field components - an inductor becomes magnetically
charged and a cap electrically charged. Another way to look at it is that a
cap is charged with voltage while an inductor is charged with current.
Since energy has been applied to the circuit, by nature it is stored. Now,
this RF energy is taken away. Since the circuit is resonant or somewhat so,
there is a transfer of energy taking place between the components. In a
tank circuit some of this energy gets out through the antenna port. The
rest (based on Q factor) circulates among the components.
This is the same concept as that of a flywheel on an engine. A flywheel can
be used as a device to store and transfer energy mechanically. Same thing
with a tank circuit (hence the name "tank"). This explains why a Class B
amplifier, which by definition conducts 50% of the time, can operate in any
kind of a linear fashion. During that portion of time when the tube is not
conducting energy that is stored in the tank circuit is still being applied
to the load.
In an amplifier design, we don't want the Q of the tank circuit to be too
high. Why? Because then not enough of the RF energy would be able to
"escape" the tank and be delivered to the load. Instead that RF would just
circulate around the tank between the components. And since no component is
lossless, excess heat would be generated. So why do we want Q in a tank
circuit at all then? Well, without Q, our tank circuit would not act as a
filter to filter out harmonics. So a compromise value has to be reached
between harmonic suppression and tank efficiency. Typically the good
compromise value is around 10 or so.
Hopefully, this sufficiently answers your question. The ARRL Handbook does
a great job of discussing Q and circulating current, etc. Try taking a read
on that. Even if you have to go to your library and read an old edition,
it's still valid.
Disclaimer: I am sure and know that some "experts" on this reflector will
pick apart my comments and point out errors that I have made. Some just
love to twist words and needle people to stir the pot. The point is that
what I have said while maybe not perfectly and and exactly correct,
generally covers the subject accurately enough and sufficient enough for a
general understanding of the subject.
73,
Jon
KE9NA
--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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