Page 48 - Fundamentals of Radar Signal Processing

FIGURE 1.6 One-way radiation pattern of a uniformly illuminated aperture. The
3-dB beamwidth and peak sidelobe definitions are illustrated.

(1.9)

Thus, the 3-dB beamwidth is 0.89 divided by the aperture size in wavelengths.
Note that a smaller beamwidth requires a larger aperture or a shorter
wavelength. Typical beamwidths range from as little as a few tenths of a degree
to several degrees for a pencil beam antenna where the beam is made as
narrow as possible in both azimuth and elevation. Some antennas are
deliberately designed to have broad vertical beamwidths of several tens of
degrees for convenience in wide area search; these designs are called fan beam
antennas.

The peak sidelobe of the pattern affects how echoes from one scatterer
affect the detection of neighboring scatterers. For the uniform illumination
pattern, the peak sidelobe is 13.2 dB below the mainlobe peak. This is often
considered too high in radar systems. Antenna sidelobes can be reduced by use
of a nonuniform aperture distribution (Skolnik, 2001), sometimes referred to as

tapering or shading the antenna. In fact, this is no different from the window or
weighting functions used for sidelobe control in other areas of signal processing
such as digital filter design, and peak sidelobes can easily be reduced to around
25 to 40 dB at the expense of an increase in mainlobe width. Lower sidelobes
are possible, but are difficult to achieve due to manufacturing imperfections and
inherent design limitations.
The factor of 0.89 in Eq. (1.9) is often dropped, thus roughly estimating the

3-dB beamwidth of the uniformly illuminated aperture as just λ/D radians. In
y
fact, this is the 4-dB beamwidth, but since aperture weighting spreads the
mainlobe it is a good rule of thumb.
The antenna power gain G is the ratio of peak radiation intensity from the
antenna to the radiation that would be observed from a lossless, isotropic
(omnidirectional) antenna if both have the same input power. Power gain is

determined by both the antenna pattern and by losses in the antenna. A useful
rule of thumb for a typical antenna is (Stutzman, 1998)