
ContinuousWave Whaler Moderated Discussion Areas ContinuousWave: Small Boat Electrical RF Radiation Exposure Guidelines

Author  Topic: RF Radiation Exposure Guidelines 
jimh 
posted 03192012 08:55 PM ET (US)
RF Radiation Exposure Guidelines 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 farfield case. If this method is used for a nearfield case it will be a "worstcase" estimate, that is, this method is likely to overestimate 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) 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 25watt transmitter. Most typical units produce slightly less than 25watts output. There is typically about 20feet of RG59C/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 20watts, or 20,000mW. The power gain of the antenna is estimated as follows: the typical marine band antenna is usually rated for 6dB 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 6dB 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 threefeet from any human. One meter is 100cm. 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 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 25watt transmitter with a 6dB gain antenna at onemeter 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 30MHz to 300MHz, 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 30minute 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 30minute interval. We now take our field exposure estimate and apply this average duty cycle: 0.636 mW/cm^2 x 2minutes/30minutes = 0.042 mW/cm^2 averaged over 30minutes We now compare our exposure to the recommended guideline: Our exposure = 0.042 We see that we are under the limit by a factor of about 4.7 to 1. By this method, we should be quite safe operating a 25watt VHF Marine Band transmitter into a 6dB gain antenna while standing 1meter away from the antenna in its main lobe. I believe there is plenty of safety margin in this analysis. First, we estimated the power density with a worst case approximation. Next, we assume that we are standing in the center of the main lobe of the antenna's radiation. Finally, we assume we talk on the radio a lot of the time, four minutes every hour. (A more reasonable assumption might be that we talk four minutes per day on the radio in most cases.) Even with the estimate weighted to produce a worst case outcome, it looks like there is a 4.7:1 safety factor in our exposure. We'd have to be transmitting on the radio about ten minutes out of every 30minutes to reach the recommended limit of exposure. 
djahncke 
posted 03192012 09:24 PM ET (US)
Very interesting analysis Jim. Based on your equations your exposure would be much higher when using an handheld. In that case the power is lower (typically a 5 watt maximum) but the antenna is much closer to the user's head. 
jimh 
posted 03192012 09:30 PM ET (US)
Much of the time I spend transmitting on my VHF Marine Band radio is at the 1watt power level. Let us estimate the field from a 1watt transmitter: S = P x G / 4 x pi x R^2 where: P = 1000 mW 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 S = 1,000 x 1 / 4 x 3.14 x 100^2 S = 0.008 mW/cm^2 At 1watt and off the main lobe we are below the limit of 0.2 mW/cm^2 by a factor of 25:1, again even assuming continuous transmission. 
jimh 
posted 03192012 09:32 PM ET (US)
DonI think you would have to consider the gain of the handheld antenna to be less than 1. The power would be lower, too. Try analyzing with a power of 5Watt and an antenna gain of 0.5. 
jimh 
posted 03202012 12:27 PM ET (US)
Let us estimate the RF exposure from a handheld radio. We will assume the power at the antenna is 5watts, and the antenna has a gain of 0.5, that is, the antenna is not as efficient as an isotropic, which is a reasonable assumption due to the small size of the typical handheld antenna. For a distance we can assume the antenna will be quite close. Let's use a distance of six inches or 15cm. Our estimate of the field is thus S = P x G / 4 x pi x R^2 where: P = 5,000 mW S = 5,000 x 0.5 / 4 x 3.14 x 15^2 S = 0.88 mW/cm^2 Using the dutycycle of 2minutes ON each 30minutes, we derate the exposure to an average of 0.06 mW/cm^2 which is well below the limit of 0.2 mW/cm^2 recommended by a factor of three. We ought to be reasonably safe using a 5Watt VHF Marine Band handheld transmitter at a distance of 6inches as long as we don't have the key down more than about 12minutes per hour. 
Tom Hemphill 
posted 03202012 05:32 PM ET (US)
Thanks for doing all the number crunching on this. I feel safer. I'm wondering, are the same exposure guidelines used for casual radio users like us as for those who have exposure through their profession? It seems to me that the cumulative effect over many years may be of concern. 
jimh 
posted 03202012 09:08 PM ET (US)
In the FCC's literature there are also guidelines for occupational exposure. The limits are higher. Here is an excerpt from page 9 of the guidelines:
quote: I used the general population guidelines, which are lower, in my examples above. The power density limit in the 30 to 300MHz 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. One could say that the operator of a VHF Marine Band radio transmitter could be viewed in the occupational category, but perhaps only if the operator were informed about the RF exposure and aware of the level of RF he may be exposed to. I suspect most boaters are completely ignorant of the RF levels and the recommended guidelines. 
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