posted 03-19-2012 08:55 PM ET (US)        

This topic came up recently, and I saw a lot of very odd advice and commentary given about it in regard to installation of VHF Marine Band transmitters and antennas on small boats. This caused me to look more closely at this topic. Let me share my findings. First, I'll estimate the radiation density that migh occur on a small boat. Then I'll compare that estimate with the recommended exposure guidelines.

In the USA, the Federal Communications Commission Office of Engineering & Technology has published some guidelines on RF Radiation Exposure. You can find their recommendations in a paper titled

Evaluating Compliance with FCC Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields

It is available for download from:

http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/ bulletins/oet65/oet65.pdf

On page 19 of the above literature a method for predicting RF radiation exposure is given for a far-field case. If this method is used for a near-field case it will be a "worst-case" estimate, that is, this method is likely to over-estimate the RF exposure that occurs when you are near an antenna. The equation used to predict RF fields is

S = P x G / 4 x pi x R^2

where:

S = power density (in appropriate units, e.g. mW/cm^2)
P = power input to the antenna (in appropriate units, e.g., mW)
G = power gain of the antenna in the direction of interest relative to an isotropic radiator
R = distance to the center of radiation of the antenna (appropriate units, e.g., cm)

We can now apply this formula for a typical VHF Marine Band radio installation on a small boat.

The power input to the antenna is estimated as follows: the VHF Marine Band radio service generally is limited to a 25-watt transmitter. Most typical units produce slightly less than 25-watts output. There is typically about 20-feet of RG-59C/U feed line to connect the transmitter to the antenna. Taking into account the anticipated power output and feed line loss, we can estimate the power reaching the antenna to be 20-watts, or 20,000-mW.

The power gain of the antenna is estimated as follows: the typical marine band antenna is usually rated for 6-dB of gain. Although this is often a fanciful number, that is, I doubt that most of these antennas really have that much gain, we can use it for this analysis. In our field prediction equation, we want the numerical gain, not the decibel gain. Therefore a 6-dB gain antenna will have gain of 4.

We can assume that the distance to the antenna will be at least one meter. On small boats there is generally room for the antenna to be at least one meter or three-feet from any human. One meter is 100-cm. We assume the person being affected by the radiation is in the precise center of the main lobe of the antenna.

Now we can predict the RF field as follows:

S = P x G / 4 x pi x R^2

where:

P = 20,000 mW
G = 4
R = 100 cm

S = 20,000 x 4 / 4 x 3.14 x 100^2

S = 0.636 mW/cm^2

This is the estimated RF field for a 25-watt transmitter with a 6-dB gain antenna at one-meter in the main lobe. Now we have to compare this density to the recommended density tolerance.

The tolerance to RF radiation varies with frequency. In the range 30-MHz to 300-MHz, recommended maximum exposure is given in the FCC literature on page 67 in Appendix B. From TABLE 1, we see that in our frequency range the limit is recommended to be a power density of 0.2 mW/cm^2 averaged over a 30-minute time.

In recreational boat operation I think it is safe to say that the transmitter of the VHF Marine Band radio will only be on for perhaps two minutes out of any 30-minute interval. We now take our field exposure estimate and apply this average duty cycle:

0.636 mW/cm^2 x 2-minutes/30-minutes = 0.042 mW/cm^2 averaged over 30-minutes

We now compare our exposure to the recommended guideline:

Our exposure = 0.042
Limit = 0.2

We see that we are under the limit by a factor of about 4.7 to 1.

posted 03-19-2012 09:30 PM ET (US)            

S = P x G / 4 x pi x R^2

where:

P = 1000 mW
G = 4
R = 100 cm

S = 1,000 x 4 / 4 x 3.14 x 100^2

S = 0.032 mW/cm^2

At this power level we are below the limit of 0.2 mW/cm^2 by a factor of 6 even we transmit continuously.

On my boat the antenna is elevated with respect to the helm position and any person standing at the helm is going to be in a portion of the antenna radiation pattern which is off of the main beam. It is reasonable to consider that the antenna has no gain in this direction. We can recalculate the field with a gain of 1:

S = P x G / 4 x pi x R^2

where:

P = 1000 mW
G = 1
R = 100 cm

S = 1,000 x 1 / 4 x 3.14 x 100^2

S = 0.008 mW/cm^2

posted 03-20-2012 12:27 PM ET (US)            

S = P x G / 4 x pi x R^2

where:

P = 5,000 mW
G = 0.5
R = 15 cm

S = 5,000 x 0.5 / 4 x 3.14 x 15^2

S = 0.88 mW/cm^2

Using the duty-cycle of 2-minutes ON each 30-minutes, we derate the exposure to an average of

0.06 mW/cm^2

posted 03-20-2012 09:08 PM ET (US)            

quote:

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The FCC guidelines incorporate two separate tiers of exposure limits that are dependent on the situation in which the exposure takes place and/or the status of the individuals who are subject to exposure. The decision as to which tier applies in a given situation should be based on the application of the following definitions.

Occupational/controlled exposure limits apply to situations in which persons are exposed as a consequence of their employment and in which those persons who are exposed have been made fully aware of the potential for exposure and can exercise control over their exposure. Occupational/controlled exposure limits also apply where exposure is of a transient nature as a result of incidental passage through a location where exposure levels may be above general population/uncontrolled limits (see below), as long as the exposed person has been made fully aware of the potential for exposure and can exercise control over his or her exposure by leaving the area or by some other appropriate means. As discussed later, the occupational/controlled exposure limits also apply to amateur radio operators and members of their immediate household.

General population/uncontrolled exposure limits apply to situations in which the general public may be exposed or in which persons who are exposed as a consequence of their employment may not be made fully aware of the potential for exposure or cannot exercise control over their exposure. Therefore, members of the general public would always be considered under this category when exposure is not employment-related, for example, in the case of a telecommunications tower that exposes persons in a nearby residential area.

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I used the general population guidelines, which are lower, in my examples above. The power density limit in the 30 to 300-MHz frequency range for occupational exposure is 1.0 mW/cm^2, or five times higher than the general population limit I used of 0.2 mW/cm^2. It may seem odd that the RF radiation exposure limit in the guideline for occupational exposure is higher than for the general public, but the FCC explains that a person who is working around radio transmitters is likely to be aware of the RF radiaton and the risk of exposure to it. That knowledge will permit a person to control his exposure to a safe level. The general public is unlikely to be aware of exposure to RF radiation that might be occurring, and the limit for them is set lower.