Page 122 - Intro to Space Sciences Spacecraft Applications

109
Communications
netic wave to be formed which propagates from the antenna in a certain
pattern depending on the physical characteristics of the antenna.

Antenna Patterns. An antenna pattern is a geometrical plot that repre-
sents the way in which an antenna propagates electromagnetic energy. In
most cases, the pattern is the locus of points that represent the range at
which the transmitted power is decreased decreased equivalently by 3
decibels (dB) of its original value (or some normalized equivalent value).
The relationships between decimal values and decibel values are given by:

and

x dB
-
x=lO 10 (5 - 4)

The second of these equations shows that -3 d3 equals one-half in dec-
imal value. This indicates that the locus of -3 dB points of the antenna
pattern represent the half-power locus delineating where the signal has
decreased to one half of its originally transmitted power level.

Isotropic Radiator. An isotropic radiator is a (theoretical) structure from
which electromagnetic radiations would propagate in all directions at
equal power strengths. The antenna pattern for such a radiator would look
like a perfect sphere of half-power points emanating uniformly in all
directions from a central point.

Anisotropic Radiators. In many cases, it is desired to concentrate trans-
mitted radiations in a particular direction or pattern in order to reach cer-
tain areas and/or certain power levels in those directions. All practical
antennas are designed, depending on their specific purpose, to transmit
(and receive) signals in such a way. For instance, the radio antenna on
your car must be able to receive AM and €34 signals from nearby radio
stations in any horizontal direction (since you may be making turns as you
drive), but because no stations currently exist in space or underground, the
antenna does not need to pick up signals from these directions. The famil-
iar pole antenna on your car has an antenna pattern that looks like a
doughnut (when looking from above or below), indicating that the anten-