View Full Version : 110 watt 2M antenna RF exposure
09-24-2011, 06:06 PM
Just looking for a few words of advice from experienced people on this forum. I did some research and see that DaveInDenver has some posts on this forum and others.
Will I be safe inside a vehicle with a 5/8 wave 2M antenna connected to a 110 watt radio? If think as long as there is a metal roof between me and the antenna I should be OK.
However, I am thinking that if there are people near the vehicle, say within a few feet of it, there could be some safety concerns if the radio is transmitting.
As long as we don't touch the antenna and get an RF burn what distance should be safe...7 feet? If real close, and the TX is only for a short period (thirty seconds every three minutes), is there significant potential for harm to the bystanders?
09-25-2011, 09:19 AM
The FCC requires that any station using more than 50W on VHF do a MPE evaluation, but handheld and mobile stations are usually exempt from doing these due to their low duty and push to talk nature. If you run >50W VHF at home you should consider RF exposure in case someone ever asks. I should also note that even if there is an exemption from the analysis, that does not mean you are exempt from the exposure rules regardless of your power. It's possible that even with 5W through a lot of gain to create a sizable local power density somewhere that just as harmful as 100W through a low gain antenna.
OTOH, there is a reason that 50W was deemed concerning and so there is nothing wrong with considering it in a mobile station.
Don't forget that the whip is not the only part of your station that can radiate RF. The ground might, the feedline certainly can and even the radio can generate a field. It's not a good idea to operate a VHF or UHF radio with the enclosure open, for the same reason it's not going to end well to run your microwave oven with the door open. These sources will generally be lower intensity than the antenna itself, but not always zero. For example, antenna couplers (e.g. tuners) can be a strong local source (particularly when NOT grounded!), but this is not really relevant to a VHF mobile. The biggest concern after the antenna itself would be the feedline, in particular bad connections or compromised coax. A bad solder joint at a PL-259/SO-239 or cut dielectric can be a decent radiator, it's essentially a mini antenna.
The IEEE recommendations show that the frequencies of most concern span 30-500MHz with the worst region being 100-300MHz. The 2m ham band falls right in the middle of that... At low frequencies there's less effect on humans and higher frequencies the power is attenuated quickly (short wavelength) so it does not take much distance to lower exposure. The VHF range starts to negatively affect humans but can have significant power density within large physical regions due to a longer wavelength.
The best way to know is to measure the field strength, but this is not always practical and so most hams do this analytically.
For 110W at 146MHz through a modest gain of 3.8dBi (e.g. 3.8dBi - 2.2dB = 1.6dBd, a 1/2 w.l. gain antenna) at 5 feet gives a predicted exposure of about 2.3mW/cm^2. The FCC allowable exposure for VHF is 1 mW/cm^2 for controlled and 0.2mW/cm^2 for uncontrolled. This would be considered non-complying exposure for all cases. If you go back and run the numbers at 10 feet, the level drops to 0.6mW/cm^2, which is acceptable for controlled exposure but still not allowable for uncontrolled. Ultimately you'll find that 17 feet is the minimum distance for uncontrolled exposure at 110W for VHF. For comparison, at 50W, these levels are achieved at about 5 feet and 12 feet. Something to think about if someone was putting up an antenna at home, try to keep 12' clear around it.
The difference between controlled and uncontrolled is determined if exposure time can be controlled, which for your station means that you have the ability to not transmit when someone is with the critical range. For amateur applications controlled would be your truck, house and yard, uncontrolled is areas beyond your control. Like your neighbor who doesn't know when you're transmitting and so could walk into and stand in a high exposure area for an unknown length of time. It's about heating on a human body, which is combination of power and time.
I would not be too concerned @110W under the roof if the antenna is right in the middle, but I would be mindful of anyone within a few feet. Remember that glass is not a great shield. Incidentally, this is another reason middle of the roof antennas are preferred for VHF, you have a good shield through a concerning range.
09-25-2011, 09:59 AM
Why are you running 110 watts on 2 meters?
09-25-2011, 10:20 AM
Why are you running 110 watts on 2 meters?
I'm guessing it's an old business radio or something, 110W is typical in the Motorola models.
09-26-2011, 03:14 PM
Dave, thanks for the great information. I had seen the chart before but didn't really know how to interpret it. Your explanation really helped. Now I know that 12 feet, at least, when TXing is a good distance to keep people out of range. We are talking infrequent PTT in this situation.
Bruce, I run a 50 watt radio in my truck (Yaesu FT-7900R) and will not be running a 110 watt radio in my truck.
I asked the question as a friend of mine has a business band (151.???) radio that he wants to try in his vehicle. He is not operating in the 2M ham band. I just wanted to be able to give him a heads up on the potential for RF exposure.
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