The RYA has published guidance on new radio licence requirements for limiting exposure to electromagnetic fields EMF.
One of the ways to ensure compliance is to use the OFCOM EMF Calculator.
Here are some top tips to make effective use of the OFCOM EMF calculator and to interpret the technical terms associated with its use. Firstly, we need to understand what it is trying to achieve. All marine equipment that transmits a signal also emits Electromagnetic Field (EMF) emissions. Any equipment that transmits above a power of 6.1 watts ERP (see definition below) must comply.
The purpose of these new requirements is to ensure that members of the public, are not exposed to EMF emissions above the specified general public limits. This is achieved by calculating the safe separation distance between the equipment emitting the EMF and members of the public. For the purposes of these regulations, OFCOM includes family and friends in their definition of general public.
Using the EMF calculator
The calculator is very basic and only requires two pieces of information; your transmitter power and transmitting frequency (the input parameters). However, whilst it is simple to use, it does use some technical jargon that goes way beyond the knowledge of the average marine radio operator.
The calculator refers to ERP and EIRP.
ERP: Effective Radiated Power is the total power radiated by an actual antenna. ERP, for our purposes, is the transmitting power of your equipment at the antenna.
EIRP: Effective Isotropic Radiated Power is the total power radiated by an isotropic antenna in a single direction. (An isotropic antenna is a theoretical antenna that radiates equally in all directions with the same power, but you do not need to worry about that). EIRP, for our purposes, is calculated by multiplying the equipment’s ERP by a factor of 1.64. EIRP is the input parameter used to calculate your safe separation distance.
Operating frequency: This is the frequency that the equipment is transmitting, for example a VHF marine band radio transmits on 156MHz, X band radar operates between 8000MHz 12000MHz.
By inputting the transmitter power and the transmitting frequency the calculator will determine the safe separation distance to be maintained between the antenna and members of the public. However, it should be noted that this does not take account of other mitigating factors, including the duty cycle (period of time actually transmitting in a set period) and any loss of power due to the length of coaxial cable. Both of these factors will further reduce the safe separation distance.
Here are some examples:
A VHF radio transmitting on 156MHz at 25watts ERP (25W x 1.64 = 41watts EIRP) would require a separation distance of 2.04m (generated by the OFCOM EMF calculator).
This is the compliance distance that should be maintained if the radio is being used 100% of the time. However, if you know that the radio will not transmit continuously, you can take this into account in the calculator. However, if you know that the radio will not be used for more than 10% of the time, you can calculate the average power and enter this into the calculator instead:
10% of 41 Watts = 0.1 x 41 = 4.1 Watts EIRP Using input parameters of 4.1 Watts EIRP and a transmit frequency of 156 MHz, the Ofcom calculator produces a compliance distance of 0.65 metres.
As most VHF transmissions are transmitted at 1 Watt, the safe
separation distance from the antenna is easily achieved. Calculations are only
required for transmitting at 6.1watts ERP and above.
The separation distance could be further reduced by factoring signal loss due to the length of coaxial cable and connectors. However, it should be noted that this calculation is dependent upon many factors, including length of cable, type of cable and quality of cable. In the case of most ‘common’ installations, it is unlikely that factoring in line loss will result in any significant reduction in safe separation distance.
Part of the regulations requires you to keep a record of how you have achieved compliance. If you have used the OFCOM EMF calculator, it is recommended that you keep a note of the safe separation distance for transmissions both at 1 watt and 25 watts.
Similar calculations should be conducted for radar. Most marine installations will be operating on X band or 12GHz frequency.
To use the OFCOM EMF calculator for radar, you need to convert GHz into MHz (1GHz = 1000MHz)12GHz = 12000MHz
A marine radar transmitting on 12000MHz at 17 watts ERP (27.88 EIRP) would require a separation distance of 0.75m (generated by the OFCOM EMF calculator).
As can be seen from the examples above, compliance is easily achievable.
Other transmitting equipment, used in an emergency, such as EPIRBs do not have to comply.
To assist maritime radio users comply with the new regulations, OFCOM has produced a guidance note 'What you need to know as a maritime radio user.'
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