Maritime Mobile Amateur Radio - Technical Considerations - Safety and EMC

Safety and EMC

See also: EMC problem (excessive field strength)

A modern sailing boat, or any other modern sea-going craft, is a much more complex electronic environment than ever in the past, and even more so than a normal home-based amateur radio "shack". The vessel will probably have electronic navigation instruments, one or more GPS receivers, electronic automatic steering, domestic radio and perhaps television receivers as well as probably radar and VHF transmitters and receivers too. It may have various GMDSS devices too, such as a Navtex receiver and an AIS system. Many of these items are computerised and many of them are networked together with data, RF and power connections. All of this is crammed into a very compact, three-dimensional space and that space is shared by not only the radio operator, but perhaps several other crew or family members. Safety and EMC issues have never been so important.

One important fact to bear in mind is that the antenna lead, from the point where it leaves the relevant terminal on the HF ATU is a transmitting antenna. There is nothing that can or should be done to prevent RF radiation from this lead. It should be a specialist, high-voltage insulated, single-core wire and it should be rigidly mounted, well away from any other wiring, outside of any conducting faraday cage, in the shortest, straightest possible route to the point where it will connect to the external antenna, be that a whip or an insulated part of the standing rigging. If it has to cross other wiring, crossing should be at right-angles and with the maximum possible spacing or separation, preferably with earthed copper tape between the two cables and perhaps with ferrite chokes added to the other lead both sides of the crossing point. The purpose of any cable or lead so crossed should be noted and consideration given to de-powering whatever equipment it serves before any transmissions are ever made. Whatever it is will probably misbehave due to the RF interference it will receive, and it may permanently be damaged by the RF currents that are induced in it by a 100 - 200 W transmission coupled in this way.

With regard to the transmitting antenna above decks, it is essential that all aboard are aware when the transceiver is in use and transmissions are likely to be made. Anybody holding on to the relevant stay or touching the antenna feed wire could suffer severe HF burns if the transmitter is keyed.

As discussed above, good earthing is essential in the installation of transmitting equipment, and good RF management will also pay dividends in terms of the ability to use other electronic equipment while transmitting, without damage to the other gear, or debilitating interference in the other gear during transmissions. It may be possible to disengage electronic steering and switch off or do without navigation instruments while the radio operator aboard makes a scheduled call, but others aboard will be much more grateful if this is not necessary and life aboard can continue as normal while people gather around the transceiver to hear news from ashore and from other boats, as well as weather or other important routing information.

Other equipment aboard may also interfere with the reception of low-power SSB or digital signals. Usual candidates include anything with a badly suppressed electric motor, such as a fridge, fans, or an electronic self-steering system, as well as many depth sounders and some radar systems. Better suppression of DC motors may be retrofitted and RF chokes can be made by winding some turns of the power and transducer leads through a suitable ferrite bead. Failing this, it may be necessary to shut off these items during important radio reception times. One radio receiver that may be overlooked is the Navtex, if fitted. By tuning the HF rig down to its relevant frequencies (e.g. 518 kHz) it may be possible to identify sources of interference that are limiting its range, and switch them off or improve their EMC characteristics. Once this is done and these frequencies are quiet, far from land it is surprising what range Navtex can achieve, especially at night.