GUIDANCE MARINE Artemis Mk6, Installer'S Manual

Page: 19 / 54

Installing a Fixed Station
Site Requirements
When choosing a location for the Fixed Station:
•
Avoid objects like poles or structures in the direct vicinity or in front of the antenna. Artemis
requires line of sight between the antenna of the Fixed Station and the antenna of the Mobile
Station.
•
Avoid a relatively large land path in front of the Fixed Station.
Do not mount the antenna unit too close to any obstacles. The antenna must preferably be able
to turn around freely without striking obstacles, which may damage the antenna. However, if
the antenna cannot turn 360° around but can make a large enough sector to cover the work
area, physical limits can be set to avoid contact with obstacles. These limits can be set with the
operating panel. See
Setting Station Parameters with the Operating Panel on page 21 for
details.
!
The standard Artemis MK6 Fixed Station may not be installed in an explosion
hazardous environment. For operation in a zone 2 hazardous environment, an
optional pressurisation unit is available.
Influence of Antenna Heights
When choosing the height at which the Fixed Station is installed, the following aspects have to be
considered:
•
The influence of the curvature of the earth when operating the system at long ranges. This is
called the radio horizon. This influence tends not to be important for the Artemis Mk6 system
because the maximum range of the system is generally less than the radio horizon, even with
low antenna positions.
•
Reflection of the microwaves off the surface of the water may cause interference with the
direct signal. This can cause weakening or extinguishing of the signal. The positions at which
this phenomenon is experienced are generally referred to as ’range holes’, ’dip zones’ or
’interference zones’
The position and width of these interference zones are dependent on:
•
The antenna heights.
•
The wavelength of the transmitted signal.
The strength of the interference depends on the distance between the transmitting and receiving
antenna and the nature of the water surface. A smooth surface reflects the signal better than a
choppy surface. The signal at the receiving antenna in the diagram below is extinguished when
D1 + D2 – D = n
λ
, where:
Reflection of microwaves against the water surface
D1 + D2 = indirect path length.
D = direct path length.
n = 1, 2, 3, …..
λ
= wavelength.
Computer calculations show that the major interference zones are out of range of the Artemis
system. They fall at a distance over 5 kilometres. Nevertheless, a general rule is that lower Fixed
and Mobile Station positions are in favour, because:
•
The interference zones fall closer and hence are easier to overcome because the signal
strength is still relatively high.
•
The interference zones are smaller than for higher antenna positions.
19