Product specifications

COMMUNICATIONS
Sharp Electronics Corporation
While cables do have capacitance, it is very small, due mostly to the fact that the
wires are also small. Capacitance in cables is almost always measured in
picofarads-per-foot. (pF/ft.) A picofarad is a trillionth of a farad, the unit of
capacitance. So why would we have any interest in an effect that small? Because
you don’t use just one foot of cable. Most often you are using tens, hundreds, even
thousands of feet. And this capacitive effect adds up. That is, a 1,000-ft. cable will
have 1,000 times the capacitance as a one-foot piece. Then you can get up to some
serious capacitance!
The real problem with capacitance is that it is affected by the frequency of the signal
on the cable. The higher the frequency, the more the capacitance "stores" that signal
as a charge. This "reaction" to frequency creates "capacitive reactance" also
measured in Ohms, like resistance. But the effect changes with frequency, which
resistance does not. Being "Frequency-dependant", capacitance is responsible for
the "Frequency response curve" of any cable.
Inductance
The electrical signal down a wire also creates a magnetic field down that wire. This
effect is called "inductance". However, on most cables, the inductive effect is so tiny,
that it is never listed in a catalog. The effect, with a frequency running on the cable,
is called "inductive reactance".
Because the inductance is tiny on most cables, the inductive reactance is also tiny.
Inductance and capacitance are reverse effects. Therefore, they cancel each other
out. But, in almost every cable, the capacitance and capacitive reactance and so
much greater that they cancel out the inductance and inductive reactance/ But there
is still capacitance, and capacitive reactance, left. This is why capacitance is a
critical number in almost every cable type from analog audio to high-speed UTP, and
inductance is essentially ignored.
Inductance is based mainly on the size of the wire (AWG) and can be most easily
changed by changing the size of the wire.
Impedance
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