When dealing with boat antennas, whether VHF or radar, one often overlooked safety issue is that of electromagnetic radiation coming from the antenna. Pick up any installation manual for a VHF or radar transmitter and you will see warnings to this effect.
My goal here is to provide some insight into this topic. I am far from an expert, but research that I have done has revealed some interesting information. This is a summary of the reference material provided at the bottom of this page.
Just what is radiation anyway?
In nature, radiation is the emission of energy that takes the form of waves or particles. Two types of radiation exist, particle radiation and electromagnetic radiation. Generally, particle radiation is the release of fast-moving particles, while electromagnetic radiation consists of energy waves having both electric and magnetic properties.
Alpha and Beta rays are examples of particle radiation, while the radio transmission we are familiar with is an example of electromagnetic radiation.
Depending on the frequency and energy levels, radiation is either ionizing or non-ionizing. High energy radiation ionizes atoms by forcing an electron from its orbit. Non-ionizing radiation lacks the energy to ionize atoms. As might then be expected of the two, ionizing radiation is far more dangerous. Ionizing radiation can damage biological tissue.
The good news is that the electromagnetic radiation we experience in the recreational boating environment is of the non-ionizing type. Damage from non-ionizing electromagnetic radiation is generally limited to heating of the cells that receive exposure. Burns and cataracts are the typical risk factors.
While sufficient exposure to either ionizing or non-ionizing radiation can cause injury or death, ionizing radiation alters biological material, while non-ionization energy is thermal in nature, such as a microwave oven.
While there are many standards bodies in the US - both government and commercial that publish radiation standards - the Federal Communications Commission (FCC) is concerned only with the radio-frequency spectrum. Therefore, we can rely on the FCC to ensure that manufacturers provide required safety information for their products.
Depending on the frequency and other factors of the electromagnetic radiation in question, there are preferred methods of measuring radiation exposure limits. Exposure limits are generally classified as short-term (6 minutes or less), or long-term (continuous exposure).
One way to measure exposure limits, especially for radar frequencies, is by a power-density factor. One such measurement is stated as Milliwatts per Square Centimeter (mWCm^2). If you can vision a 1Cm square patch of space, the energy intensity within that cross-section would be measured, hopefully in milliwatts. As electromagnetic radiation travels from the antenna, its intensity decreases at an exponential rate, so the energy density lessens with distance. The rate of which this occurs is a factor of wavelength, distance, antenna gain, and the antenna's radiation pattern.
For example, the typical VHF 8ft whip antenna produces a donut-like radiation pattern, while a radar antenna has its energy more concentrated in a beam, not unlike a flashlight. Therefore, the dispersion rate for each type of system will be different. But in all cases, the farther distance from the antenna, the less intense the electromagnetic radiation is.
Therefore, the two important safety factors in limiting electromagnetic radiation exposure is distance from the antenna, and the amount of time one is exposed to the radiation.
An early RF safety standard, created in the 1950s, used an exposure intensity of 10mW/Cm^2 (ten milliWatts per square Centimeter), as this was the radiant energy need to heat 1 gram of water 1 degree C. This was considered the maximum exposure level that could be tolerated for constant exposure.
Since that time, several organizations have developed their own standards. However, 10mW/Cm^2 can still be found in many standard references.
Ever since I first saw that 10mW/Cm^2 reference as a microwave radio technician in the US Air Force in the 1970s, I really was curious about its background, and really never undersood it until now.
It is interesting to note that the same mW/Cm^2 reference can be applied to other forms of radiation, such as solar radiation. During the peak summertime season, the maximum solar radiation from the sun is regarded as 100mW/Cm^2. Since this is 10 times the accepted limit of 10mW/Cm^2, it should be expected that long term exposure can cause thermal heating of the skin; which of course, is why we become sunburned.
Cataracts are also believed to develop from the thermal effects of prolonged over-exposure to electromagnetic radiation, primarily due to the inability for the body to dissipate heat from the eyes.
The following chart shows the different levels of radiation and their potential affect on the body:
This is important, so it will be stated again. You must rely on the manufacturer’s information and warnings contained within the owner’s manual. Failing to do so may expose you or your boat’s occupants to unsafe levels of electromagnetic radiation.
A device called a "Field Strength Meter" can be used to determine exactly what power intensity exists within a specific location, however, at radar frequencies, they can be expensive, and the use of these devices may be beyond the capability of a non-professional. For most, simply reading the installation manual should suffice.
Some practical examples.
I recently installed a 2Kw radome on my boat. In the manual, it provides the following information:
Distance to 100mW/Cm^2 = nil
Distance to 10mW/Cm^2 = 1 meter.
This is somewhat of an odd specification for most boaters, and not all manufacturers publish this data. But it is important for you to understand this specification.
I garner from this information that:
Distance to 100mW/Cm^2 = nil means that the unit is not capable of producing enough power to get a burn equivalent to over-exposure to summer time sun, even if I were to place my hand directly over the operating radome.
Distance to 10mW/Cm^2 = 1 meter means that I still must be 1 meter from the radome surface to meet the long-term exposure standard. This can be achieved by having the radome mounted 1 meter behind the seating area. Fortunately, since microwave energy is highly directional, if we are also below the radome, this is an additional safety measure, and there should be little chance of over-exposure.
The left image is an example of the correct installation of the radome. Notice that the radar arch on this particular boat is very low, so much so that if the radome were to be placed directly on the arch, its energy would irradiate the flybridge's occupants from behind. Fortunately this boater used a pedestal extension to get the radome above the occupants. The right photo shows the same model boat with a radome attached directly to the arch. Due to the low arch height, the flybridge's occupants would be exposed to radiation when the radar was in operation.
In this example, a radome is attached directly to the crown of the flybridge. Unfortunately this position will also result in any occupant on the flybridge becoming irradiated if they were to be at that location with the radar in operation. However, many setups such as this have a lower helm as well - and the radar should only be used when the occupants are not on the flybridge.
My VHF radio indicates that ideally 3 meters, and a minimum of 1 meter must separate the closest human and the base of the antenna. It also cautions that an antenna with a gain of more than 9dB not be used.
Unfortunately, this is harder to achieve, due to the omni directional nature of the antenna. However, when mounted on an extension on top of the radar arch, my installation meets the minimum requirement.
One other factor is that electromagnetic exposure to the VHF radio tends to be short-term, at least in comparison to the radar. If the setup was marginal, radiation exposure can be minimized by limiting the transmit time, which is probably not a bad idea anyway.
There is a factor called "Effective Radiated Power" (ERP);, that basically provides an efficiency rating of how efficient the antenna "couples" to the electromagnetic wave it transmits. This is expressed in the unit of a decibel, and typically are 3dB, 6dB, 9dB and so on.
Since this is an exponential ratio, one can make the conclusion that for a VHF radio, that an antenna having higher gain will "effectively" produce a higher radiation density output Therefore, the higher gain antenna will potentially require more distance from the operator. If your antenna location is marginal, one possible mitigation is to use a lower gain antenna.
In conclusion, do not let this discussion scare you. Simply be aware of the phenomenon, know the risks, and heed the recommendations by the manufacturer of each transmitting device you install, and you will not have to worry about electromagnetic radiation exposure.
Federal Communications Commission: Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields.
Microwaves 101; Tuscon Az: Biological effects of electromagnetic radiation.