BendixKing IN-182A System RDR-2000 - Manual (Page 13)

Effective Date: 5/98 RDR 2000 Pilot's Guide: Rev 3
9
Theory of Operation
Note that the antenna gain versus angle characteristic is a continuous
function at all angles. This means that there is a gain value associated
with all forward angles relative to the selected tilt angle. In this figure the
tilt angle is shown as zero degrees. This means the beam center is
along the same angle as the aircraft flight angle. Next, the points on
either side of the beam where the antenna gain is down 3 dB relative to
the maximum gain defines the 3 dB beamwidth. The remainder of the
manual uses the cone shaped 3 dB beamwidth extensively to illustrate
how the beam spreads with distance, much like a flashlight beam. Also
important is the understanding that this angle is wider for smaller
antennas (10”) and narrower for larger antennas. It’s also important to
realize that the antenna gain does not go to zero outside the 3 dB
beamwidth, it just continues to reduce with increasing angles. This is
what it meant by a continuous gain function. This understanding is
important when we discuss ground clutter reflections later.
Also note that there are small lobes of the gain characteristic at fairly
large angles. These are called sidelobes. Generally these are not
important since the gain value for these lobes is down 25 or more dB
from the peak. However a bad radome can increase these sidelobes to
a point that they cause a constant radar reflection from the ground. This
is commonly referred to as an “altitude ring” because the display will
show a concentric ring at a distance equal to the slant range of the side-
lobe to the ground.
The cone formed by the 3 dB beamwidth is where most of the radar
energy is concentrated, so it is important to realize that at any given time
whatever is within this cone (and sometimes other strong targets like
clutter outside the cone) is what is being painted on the display. The
pilot should be aware of how wide this cone is as a function of range.
The primary target of interest is obviously weather cells of significance.
The typical cell is considered to be 3 nm in diameter. It is mandatory that
the beam be pointed at the wet part of the weather cell to record the
proper rainfall intensity (color level). To aid the pilot at accomplishing this
task, the “Radar Tilt Angle Calculator” tool is provided (P/N 006-18124-
0000). This tool is a transparent 3 dB beamwidth overlay for each
antenna size and range scales of 40, 80, and 160 nm in length, each of
which has multiple weather cells shown to scale at different distances. A
user can position the overlay on a given target and read the tilt angle that
will position the beam at the “above freezing” part of the cell.