Sea Tel ST88-21 C/Ku-Band TVRO, Инструкция по установке и эксплуатации

Страница: 18 / 145

Site Survey ST88-21 C/Ku-Band TVRO
2-4
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.
For these large systems, the mast for the ADE should be designed specifically for the ADE. Other equipment may be
mounted alongside, but the mast should be configured to accept the mass, loads and resonance of the ADE primarily.
The following sections describe various mast configurations and some considerations for mast design.
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.