53
When these are decided, the situation of the
occurrences of the shipwreck in the near shore
waters and the accident of falling in the sea are
researched, furthermore, the relation between tempe-
rature of seawater and time that person who fall in
the sea can live, was considered. The main change
point is a use temperature and continuous operation
time. Though the height of the antenna is not
especially decided, the shoulder or the top of helmet
of man who floats on the sea wearing the life jacket
are assumed.
As the result, the use of a small general-purpose
battery became possible, and the main body became
the size of 8 mm videocassette tape. The prototype
is shown in Figure 4.
4 COMPARISON BETWEEN PRESENT SART
AND CIRCULAR POLARIZATION SART
When these had been designed, in the process of
examining the miniaturization of the antenna and the
connection of the main body of SART and the
antenna about the simplification, it has been
understood that the use of circular polarization has
advantage. The antenna of SART is provided the
horizontal polarization according to ship’s radar.
SART, however, doesn’t necessarily keep the pola-
rization, because big wave motion is generated at
sea. It is thought that there is attenuation based on
the rotation of the polarization by the reflection on
the surface of big wave. Therefore, the difference in
the effect is almost nothing between the horizontal
polarization and the circular polarization in rough
wave.
The polarization plane rotates in the circular
polarization. Therefore, because null that originates
in interference by the direct wave and the reflection
wave becomes small, it is possible to receive it with
stability. Moreover, differing height pattern becomes
an effect, and the effective range expands.
In the examination in the water tank and the sea,
circular polarization often gave strong receiving
signal for the horizontal polarization.
Figure 5 shows the one example when sea state is
calm. The lower line (SARTradar) in the graph is
theoretical value of horizontal polarization and the
block dots are received signal from circular
polarization SART. The value that is stronger than
the theoretical value of horizontal polarization is
indicated, and it is also understood that detection
distance has expanded as a experimental result.
In the use of circular polarization, the antenna can
be miniaturized. Furthermore, there is an advantage
with an easy separation of the distance of the main
body and the antenna because of connect possible by
the semi- rigid cable. It means the antenna can be
miniaturized and the design has flexibility for
installation.
Figure 6 shows the result when sea state is rough.
Fig. 5. Theoretical value of horizontal polarization and
observed value of circular polarization
Fig. 6. Signal strength that radar received from SART in wave
height 2 – 3 m
The maximum value of the SART signal of
circular polarization and that of horizontal polariza-
tion in each observation point were compared, and
the difference is -0.8 to +5.6 dB. The value
numerically increases, when the signal of circular
polarization is stronger. Therefore, the signal of
circular polarization is stronger than or equivalent to
the horizontal polarization
Because of the difference of heights is not
constant at each observation point, each observation
data cannot be compared in same height pattern,
however, it was confirmed that SART of circular
polarization was no inferior comparing with
horizontal polarization.
5 ONE MILE SART
Furthermore, the authors aimed at the development
of less expensive and smaller SART based on these
results. To reduce the size to always carry it, and to