Practical_Antenna_Handbook_0071639586

C h a p t e r 3 : A n t e n n a B a s i c s 83
area.) Similarly, the application of a constant voltage across the resistor terminals
will result in a steady current flow through the resistor until the voltage is
removed. The power dissipated in a resistor is given by
2
P V I P V 2
= × or, alternatively, = or P = I × R
(3.2)
R
Note that all three forms of Eq. (3.2) are equivalent and lead to the same answer.
The resistance of an ideal resistor (i.e., one with no capacitance or inductance)
does not depend on frequency.
• Capacitors. A capacitor is a two-terminal component capable of storing electrical
energy in the form of stored electric charges. The fundamental unit of capacitance
is the farad, and the Capacitor total energy stored in a capacitor is ½ CV 2 , where C is the
capacitance of the specific capacitor in question, and V is the voltage (in volts)
across its terminals. Another useful relationship for capacitors is
= × ∆ V
I C
∆t
(3.3)
where I (in amperes) is the current through a capacitor of value C when the
voltage across the capacitor terminals changes by some amount DV (in volts)
over a short time interval Dt (in seconds). A simple capacitor you can make at
home consists of two identical sheets of aluminum foil insulated from each
other by a sheet of waxed paper of the same size.
Note The only time current flows on the leads of a capacitor is when the voltage across its
terminals is changing: I = C DV/Dt.
A capacitor is said to exhibit capacitive reactance. Reactance works much like
resistance to impede the flow of current in a circuit but the two are very different:
44
The reactance of an ideal capacitor does not dissipate any power.
44
The reactance of a capacitor is frequency dependent.
The magnitude of capacitive reactance is defined in Eq. (3.4):
1
X = − (3.4)
C
2 π fC
where f is the frequency in hertz and C is the capacitance in farads. Alternatively,
f can be in megahertz (MHz) and C in microfarads (µF). The minus sign is
important in distinguishing the effect of capacitors from that of inductors.
• Inductors. An inductor is a two-terminal component capable of storing magnetic
energy in the form of current, or moving electric charges. The fundamental unit
of inductance is Inductor the henry (abbreviated H), and the total energy stored in an
inductor is ½ LI 2 , where L is the inductance of the device in question and I is
the current (in amperes) going through it. Another useful relationship for
inductors is