COBHAM Sea Tel 2400, Руководство по установке

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Site Survey Sea Tel 2400 Dual Band C/Ku
3-4
EAR Controlled - ECCN EAR99
3.5.1. Vertical Masts
Vertical masts are a very ancient and common mast design. In essence, it is the mast derived from the
sailing mast and adapted for mounting the ever-increasing array of antennae which ships need to
communicate with the world. This drawing of a vertical
mast shows the preferred mounting of the ADE center-
line above the plane of the radar. Alternatively the ADE
is mounted below the plane of the radar signal
Vertical masts are most commonly found on cargo ships
– they are simple, inelegant and functional. They are
also fairly stiff against torsional reaction and lateral
vibrations, as long as the ADE is mounted on a stiff
pedestal near the vertical centerline of the mast. If
centerline mounting is impractical or otherwise
prohibited, the mast platform the ADE is mounted on
should be checked for torsional vibration about the
centerline of the mast and the orthogonal centerline of the platform.
If the estimated natural frequency of the mast or platform is less than 35 Hertz, the mast or platform
should be stiffened by the addition of deeper gussets under the platform or behind the mast.
3.5.2. Raked Masts
Raked masts are found on vessels where the style or appearance of the entire vessel is important. Again,
the inclined mast is a direct descendant from the
masts of sailing ships – as ship owners wanted their
vessels to look more unique and less utilitarian, they
‘raked’ the masts aft to make the vessel appear
capable of speed. This drawing shows a raked mast,
again with the preferred ADE mounting above the
radar and alternate with the ADE below the radar.
Raked masts pose special problems in both evaluating
the mast for stiffness and mounting of antennae. As
can be seen in the drawing, all antennae must be
mounted on platforms or other horizontal structures
in order to maintain the vertical orientation of the
antenna centerline. This implies a secondary member
which has a different natural frequency than the
raked mast’s natural frequency. In order to reduce the mass of these platforms, they tend to be less stiff
than the main box structure of the raked mast. Thus, they will have lower natural frequencies than the
raked mast itself. Unfortunately, the vibratory forces will act through the stiff structure of the raked mast
and excite these lighter platforms, to the detriment of the antenna.
3.5.3. Girder Masts
Girder masts are large platforms atop a pair of columns. Just like girder constructions in buildings, they
are relatively stiff athwart ship – in their primary axis –
but less stiff longitudinally and torsionally. An example
of a girder mast is shown in this drawing, with the
preferred ADE mounting outboard and above the radar
directly on one of the columns and alternate with the
ADE centered on the girder above the plane of the radar.
The greatest weakness of girder masts is in torsion –
where the girder beam twists about its vertical centerline
axis. As with all mast designs discussed so far, mounting
the antenna in line with the vertical support structure will
reduce the vibration tendencies. Mounting the antenna directly above the girder columns provides
ample support to the antenna pedestal and locates the antenna weight where it will influence the natural
frequency of the mast the least.