COBHAM SEA TEL 9711IMA-70, Руководство по установке

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Site Survey 9711IMA-70
2-4
2.5.1. 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.
2.5.2. 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.
2.5.3. 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.
EAR Controlled - ECCN EAR99