Owner manual

Effective Date: 9/98 RDR 2100 Pilot's Guide: Rev 1

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 Beam Diagram” tool is provided. This tool is a trans-

parent 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

“below freezing” part of the cell. This tool should be understood and kept

handy when trying to interpret the weather display.This tool illustrates

that at greater distances, the weather cell doesn’t fill the cone shaped

beam. Under these conditions the distinction of the weather cell from the

ground clutter is most difficult. The following figure illustrates this condi-

tion.

RDR 2100 PG 3/12/07 3:44 PM Page 9