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Radiocommunications for Tower Cranes |
Tower cranes often require radio communication as a means of signalling to ensure safe operation. This information sheet provides advice on the use of radio in this situation.
Radio installation and operation in this environment calls for the observance of special requirements. Any radio system mounted high up on a structure is likely to receive and cause interference to other users over a wide radius unless it is carefully engineered. Since there are only a limited number of radio channels available, it is likely that a crane will be sharing a radio frequency within the same conurbation, and in worst cases on the same construction site. It is necessary, therefore, to engineer a "clear" channel.
A number of approved techniques have been developed to provide clear channels. The consequence of not adopting one or more of these is that safety of life could be put at risk both on the construction site and elsewhere for other services using radio.
When a radio is used at ground level surrounding buildings and ground clutter soak up some of the signals and reduce the distance over which the signals can be received. Any radio located high up on a crane is able to transmit into free space and will be receivable many miles away.
At first sight this does seem too much of a problem as most of the talking is done by the banksman and the crane driver is mainly listening. However, not every banksman is located at ground level and often he will obtain just as good propagation as the crane driver.
Radio communications between banksman and controller pose different propagation problems as construction progresses. On the construction site, the building itself may act as an obstacle to proper radio communication and it may not therefore be possible to reduce the output power to prevent interference to others. Often reducing output power does little to contain transmissions into free space but prevents communication into parts of the building such as a lift shaft.
Although, as the building goes up, the banksman will increasingly be liable to transmit from a raised position he could still have transmissions blocked by the building in a particular direction. The crane is likely to be above the construction and able to freely transmit in all directions throughout the duration of the work. The area where it will be hardest to penetrate is the opposite side of the building should it be necessary to operate in that area.
1.
Signalling systems
The use of CTCSS (Continuous Tone Controlled Signalling System) or
another form of signalling will help lessen the effects of sharing. They can
not, however, solve the problem. As a crane will receive signals from a large
catchment area, tones will only reduce the transmissions which the crane driver
has to listen to.
2. Special
techniques
There are several engineering techniques available which will alleviate the
problems. These range from directional antenna or other means to restrict range which
confine the radiation vertically between operator and banksman to different types of
communication on the up and down paths.
It is possible to superimpose the radio traffic over the mains voltage. A leaky feeder from the rear jib will limit the radiation pattern. Whilst these are excellent solutions it should be noted that both systems have been patented. The use of a magmount aerial, because it is omni-directional, will freely radiate. This causes problems in that communications will be transmitted and received over a wide area. It is, therefore, essential to reduce as far as possible the radius for both transmission and reception.
3.
Callsigns
One very simple way to improve the safety aspects of radio communication is
to use callsigns at the start of the message. When there is a danger that a
radio frequency could be shared with another crane, and messages will be of
a similar nature, a unique callsign is a useful tool to ensure that a message
is acted upon only by its intended recipient. Another reason why they should
be used is that when an interference complaint is received, we can more easily
identify the users involved when callsigns are used. There is much that contractors
could do to help by ensuring their crane operators use callsigns and explaining
the benefits to them. It is also a licensing requirement.
The Agency can allocate either a single or dual frequency channel. If a crane is going to be at a site for a considerable amount of time the user will require a licence and it will be necessary for the Agency to assign a channel. However, if the duration of the contract is less than twelve months, equipment can be hired from a radio equipment supplier holding a current PBR Suppliers Licence.
The use of all radio must be licensed but users are free to choose either a single or dual frequency and the system employed. The Radiocommunications Agency is not concerned about which particular system is chosen provided that interference to other users is kept to a minimum. Engineering trials have shown that it is possible to share frequencies much closer than had previously been thought possible providing the systems are carefully engineered.
In defining what does or does not cause unacceptable interference, it has been decided to define a maximum signal strength at a distance from a crane no matter which techniques are employed. This shall be a maximum of 48 dB/µv per metre measured at 2 kilometres from the crane at a height of 3 metres above ground level. It is not intended that every crane radio should be required to conform with this figure. It would not, for example, be necessary in areas of the UK where use of radio communications is relatively light. However, if an interference complaint is received, and the offending radio system does not conform to these parameters then it will be necessary for that system to be re-engineered so that it does conform.
For further information about radio use contact the RA Enquiry Point on 020 7211 0211
Radiocommunications
Agency
Wyndham House
189 Marsh Wall
London
E14 9SX
| RA 195 (Rev 5) July 2000 |
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