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4009-9 Installation Manual
2.5.
Mast Configurations
Sea Tel recommends mounting the ADE in a location that has both a clear line-of-sight to the target satellites in
all potential azimuth/elevation ranges and sufficient support against vibration excitement. If possible, mounting
the ADE pedestal directly to ship deckhouse structures or other box stiffened structures is preferred. However, in
many cases, this imposes limits on the antenna system's clear line-of-sight.
Often the solution for providing the full azimuth/elevation range the antenna needs is to mount the ADE on the
ship's mast. Unfortunately, masts do not consider equipment masses in design and often have harmonic
frequencies of their own.
There are many designs of masts used on ships – masts are nearly as unique in design as the ship is – but the
designs often fall into a few categories. These categories can be addressed in terms of typical responses and
problems with regards to vibration and mounting of ADE. The most common categories of masts are:
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
2-4
Site Survey
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