Seiwa MDS-18910R - Manual (Page 10)

MDS-1R/8R/9R/10R Installation
C. False echoes caused by side lobe
An antenna's side lobe emissions are low power, and will not register distant targets.
However, if there is a strong reflecting target near your boat, it sometimes may appear as a
circular-arc false echo on the screen.
When near large targets or land, your boat's mast
may sometimes appear as a circular-arc shaped
false echo.
CAUTION
!
7
3
1
HU
Real echo
False sidelobe
echoes
Main beam
Side lobes
Antenna
Fig.2-7 False echoes of radar (Caused by side lobe)
D. Distant false echoes caused by duct phenomenon
The duct phenomenon sometimes occurs when meteorological conditions create a
temperature inversion between layers of air. When this happens, radar waves propagate
erratically and can reach a location considerably farther away from your boat than the radar's
maximum distance range. What appears onscreen is a false echo that looks to be nearer than
the actual target. Since the true echo from the distant target is outside the measurement
capabilities of the radar, its apparent distance will change when you change ranges, and you
can conclude that it's a false echo.