Page 49 - Fundamentals of Radar Signal Processing

(1.10)

Though both higher and lower values are possible, typical radar antennas have

gains from about 10 dB for a broad fan-beam search antenna to approximately
40 dB for a pencil beam that might be used for both search and track.
Effective aperture A is an important characteristic in describing the
e
behavior of an antenna being used for reception. If a wave with power density
W W/m is incident on the antenna, and the power delivered to the antenna load
2
is P, the effective aperture is defined as the ratio (Balanis, 2005)

(1.11)

Thus, the effective aperture is the area A such that, if all of the power incident
e
on the area was collected and delivered to the load with no loss, it would

account for all of the observed power output of the actual antenna. (Note,
however, that A is not the actual physical area of the antenna. It is a fictional
e
area that accounts for the amount of incident power density captured by the
receiving antenna.) Effective aperture is directly related to antenna directivity,
which in turn is related to antenna gain and efficiency. For most antennas, the
efficiency is near unity and the effective aperture and gain are related by
(Balanis, 2005)

(1.12)

Two more useful antenna concepts are the antenna phase front (or wave front)
and phase center (Balanis, 2005; Sherman, 1984). A phase front of a radiating
antenna is any surface on which the phase of the field is a constant. In the far-
field, the phase fronts are usually approximately spherical, at least over

localized regions. The phase center of the antenna is the center of curvature of
the phase fronts. Put another way, the phase center is the point at which an
isotropic radiator should be located so that the resulting phase fronts best match
those of the actual antenna. The phase center concept is useful because it defines
an effective location of the antenna, which can in turn be used for analyzing
effective path lengths, Doppler shifts, and so forth. For symmetrically

illuminated aperture antennas, the phase center will be centered in the aperture
plane, but may be displaced forward or backward from the actual aperture.
Referring to Fig. 1.5, the phase center would occur at y = 0, but possibly x ≠ 0,
depending on the detailed antenna shape.
Another important type of antenna is the array antenna. An array antenna
is one composed of a collection of individual antennas called array elements.