Seiwa SWRx series - Manual (Page 47)

49 User Manual
A. What is Radar?
A.1 GENERAL
The word “radar” is an acronym for “RAdio Detecting And Ranging.” In very sim-
ple terms, this is how it works. A radio transmitter sends a quick microwave
pulse, and then a receiver listens for that signal’s echo when it is bounced back
from something in its path. The returning signal is processed by a computer to
determine its relative distance, position and bearing. This information is graphi-
cally displayed on a screen for you to see. Other boats or ships, navigational
markers, landmasses and such are referred to as targets.
By knowing how long it takes for a signal to return, the distance to a target can be
determined. As the Radar antenna scans through a 360-degree rotation, it can
show where the target is relative to your position. By repeated scans, you can see
which direction another vessel is moving.
Buoy
Other ship
Radar
Antenna
Radar
Fig. A.1 - Radar
A.1.1 Antenna
How Radar will perform is largely determined by its antenna or scanner. Increas-
ing the size of the antenna improves long-range performance and target discrimi-
nation, or the ability to distinguish two separate targets at a distance. The critical
factors are the antenna’s beam width and side lobe level. Typically, a Radar an-
tenna will radiate a tightly focused beam from the front of the array. The longer
the antenna array is, the narrower the beam width will be.
Additionally, it will also emit smaller amounts of energy to each side. The lower
the side lobe level, the less the effect of a false echo.
A.1.2 Side Lobe
The beam in which the strongest radio signal is radiated from the antenna is
called the “main lobe”. Those beams that are radiated in other directions are
referred to as the “side lobes”. The side lobe level refers to the difference in level
(signal strength) between the largest side lobe and the main lobe.