Furuno TZT9 Operator's Manual page 140

Shadow sector
Funnels, stacks, masts, or derricks in the path of the antenna stop the radar beam. If
the angle opposite the antenna is more than one or two degrees, a sector or shadow
sector appears on the screen. Targets are not displayed within the sector.
Radar position
Large ship
Sidelobe echoes
Each time the radar transmits, some radiation leaves on each side of the beam. This
lost energy has the name "sidelobes." If a target is detected by both the sidelobes and
the mainlobe, the sidelobe echoes can appear at both sides of the true echo at the
same range. Sidelobes normally show only on short ranges and from strong targets.
You can reduce sidelobes if you reduce the gain, or correctly adjust the sea clutter.
Wharf and its echo
Shadow sector
occurs because
wharf is hidden
behind ship.
Mainlobe (beam)
Sidelobe Sidelobe
Antenna
Radar position
Size of blind sector depends on
size of obstruction and range.
False echoes by sidelobes
6. RADAR
Wharf and its echo
Shadow sector occurs
because obstruction
(like mast) is in path of
radar beam.
True target
6-21