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ODU Mounting Considerations
ODU must be properly mounted and secured. Failure to do so could result in detachment of the
unit, causing disruption in operation of the unit, or danger from a falling unit, which could result
in serious injury or death.
A suitable mounting bracket or pole may need to be procured or fabricated to support the ODU. Factors to
consider in selecting a mounting method include:
• The weight of the unit (13kg ,28 lbs) - weight includes provided mounting bracket
• The physical size of the unit (406mm high by 585mm in diameter) - height includes provided mounting
bracket
• Mechanical resonance of the system excited by:
- Mechanical vibrations induced by nearby operating machinery
- Wind induced vibration
These factors will be discussed in the following paragraphs. The mounting bracket and pole must be capable
of supporting the weight of the unit. Consideration should also be given to accessibility for installation and
service work, and the task of lifting and supporting the size and weight of the equipment safely whilst all the
connections are being made. The mounting bracket and pole must be capable of supporting the lateral loads
caused by the wind. For example at 60 mph the lateral force on the ODU alone (ie. excluding the pole) will be
approximately 3.5-4.5 kgf. This load will generate shearing and bending forces in the mounting bracket and
pole which must be distributed safely into the mounting structure. Since this loading will vary continuously
with time, the structure must not only withstand the static loading but also the fatigue loading over the
lifetime of installation.
When selecting a suitable mounting pole it is important to consider resonance as well as strength. The ODU
will be subject to vibrations caused by oscillating aerodynamic forces due to vortex shedding from the ODU
itself (in the range 0 – 10 Hz for wind speeds up to 60 mph, or storm force 10), and any upstream objects even
if they are outside the zone shown in Figure 7.
The ODU will also be subject to vibrations transmitted through the mounting structure (from engines,
motorized equipment, cable stays or guy ropes). Careful consideration must therefore be given to minimize the
potential for vibration causing damage to the ODU, by careful choice of the mounting position, pole diameter,
pole length, and any necessary supporting structure. If necessary modify the design of the mounting pole to
move its natural frequency (with ODU fitted) away from the frequency of the source of vibration. For example,
increasing the stiffness of the mounting pole will increase its natural frequency – this might be achieved by
using a shorter or thicker pole, but note that adding mass will have the contrary effect of reducing the natural
frequency. If the pole cannot be shortened, then consider adding rigid support struts or tensioned guy ropes
to reduce the effective length. The following guidelines are recommended:
• Minimum pole diameter: D = 60.3 mm (2.38 in.)
• Minimum wall thickness: t = 3.2 mm (.125 in.)
• Maximum unsupported pole length, L = 2.0 m (6.6 ft)
For 100 mph winds (hurricane force 12) the unsupported length should be reduced to 1.0 m (3.3 ft.). Longer
poles are possible if additional support is provided such as guy ropes. More sophisticated mounting designs
may be necessary to deal with vibrations in some installations. The design of vibration reducing or absorbent
mountings is critically dependent on many installation specific factors that are well outside the scope of
this manual. Hence for any such an installation, consult specialist suppliers of antenna mountings, to obtain
professional advice.
WARNING
Iridium Pilot Land Station Installation Guide
15
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