[Antennas] Q

2002年10月12日 15:11:20 -0400

de WB2CPN South Central Pennsylvania 2002年10月12日
A simple explanation without all the math: (Don't fault math; although
there's a place in the Bible where there's a tub that measures
three measuring sticks across, and measures ten measuring sticks
around. Decimals hadn't been invented, so there's no Pi.)
The problem with understanding "Q" is that it's usually expressed
in mathematical terms which were empirically derived when it was
discovered that something was happening in inductances and capacitors
that was not understood. That was a long time ago. "Q" can be
interpreted to mean "Quality", or "how much bang for a buck". When
the early radio experimenters wound their own coils they found that
they could predict the time required for the magnetic field to build up
when they ran DC current through the coil. Also predict the time it
took for the field to collapse when the current was shut off. The thing
they couldn't easily get a handle on was how much of a field they
were going to get from various amounts of DC current, and it appeared
to depend on the resistance of the wire in the coil, leaky insulation of
the wire, nearby metal which could have an effect on magnetic fields,
and anything that prevented the coil's collapsing field from inducing as
much current back into the coil as it took for the field to be build up.
A perfect coil would lose no energy at all, while coils which were not
made very well would loose a lot. To put a number on the value of "Q"
someone invented a formula which used the amount of Inductance "L"
and the amount of Resistance (R). There's a message on this thread that
covers that very nicely.
Now the "Q" of capacitors can be understood much the same way as
coils except we expect the two plates of a capacitor to be free of leakage,
contain little if any intrinsic inductance in their connectivity or wiring, and
have a material between the plates to separate them and prevent electrical
contact, as well act as a medium through one plate's charge can influence
the other plate's charge, (Specific Inductive Capacity on the old FCC tests,
Dielectric Constant now-a-days).
We hear more about "Q" when a coil and capacitor are connected in series
with each other, (and form a series resonant circuit), or when they're
connected in parallel with each other, (and form parallel resonant circuit). In
the parallel arrangement, for instance, if the coil and the capacitor both had
a perfect "Q", a current could be temporarily induced into either component,
and it would go round and round for ever. But such an arrangement in the
real world must have losses, we've never seen perpetual motion yet.
If there are losses in the circuit, either by internal loss or by radiation, the
current will decrease at a predictable rate until it's all gone. (Radiation is
when the magnetic field built up by the coil does not all collapse on the coil,
but goes zooming off into space.) In the early days of real low frequency
radio the rate at which the oscillations in a coil/capacitor combination died
down to some minuscule value was named "Decrement". If it was too long
the spark gap "dots" would run together. Now, in modern times, the "Q"
of a resonant circuit will determine how well it reacts to frequencies which
are not exactly its own resonant frequency (Fo). How far off to one side
or the other can the frequency go and have the resonant circuit act like one.
Filters usually express their bandpass capabilities in Hz or KHz, while other
tuned circuits, particularly transmitters, make a note of the "Q" of the
resonant circuit. As you've seen from the math I mentioned, that's the way
the transmitter people express bandpass.
 73 Clete