Practical_Antenna_Handbook_0071639586

712 A p p e n d i x A : U s e f u l M a t h
Since decibels are logarithms of ratios, decibels are dimensionless; that is, they’re not
volts or watts or meters or anything else. Sometimes, however, power, signal level, and
field strength are stated as being relative to a fixed value of some sort. When this happens,
it is standard practice to add a reference identifier to the abbreviation “dB”. Here
are a few that are often found in conjunction with antenna specifications or testing:
dBm
dBµv/m
dBi
dBd
power level relative to a reference level of 1 milliwatt (1 mW)
E-field strength relative to a reference level of 1 microvolt/
meter (1 mV/m)
antenna gain in dB relative to the same specification for an
isotropic radiator (see Chap. 3)
antenna gain in dB relative to the same specification for a
half-wave dipole (dBd is always 2.15 dB less than dBi)
A.3 Sinusoidal Curves
Radio waves of a single frequency are sine waves; if we use an oscilloscope at some single
point along a wire to monitor the effect of a signal of frequency f C applied to one end of
the wire, we might see the voltage there follow the curve of Fig. A.3.1 as time passes. If
we measure from one positive peak to the next, we find that this waveform repeats f C
times each second. We define T—the time interval between the same position on two
consecutive cycles of the waveform (such as two consecutive positive peaks)—as the
period of the waveform, and it turns out that
1
T = (A.3.1)
f
But why is a sine wave the shape it is? And what physical significance does it have?
If you’ve ever ridden a Ferris wheel at a carnival, you’ve created a sine wave yourself—you
just didn’t realize it at the time! Let’s see how you did that.
C
Amplitude
T
time
T = 1 fc
Figure A.3.1 Sine wave.