Sea Tel ST88-21 C/Ku-Band TVRO Installation And Operation Manual Download Page 18

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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.