Tuesday, June 22, 2010

I currently have a 52 foot crank-up tower that is about 5 feet from the back of my house ...

Q John Duncan, WA5ZVE, asks, “I currently have a 52 foot crank-up tower that is about 5 feet from the back of my house. Additionally, I live on a lot that is about 65 × 110 feet. I am looking at replacing my existing tower with a 72-foot model. I also want to make sure that both my current and future installations are in compliance with the new RF safety regulations. When I read the RF-exposure regulations, I noticed that they require hams to know their peak power, but then they talk about average power and average exposure. I also noticed that there is a 500-W limit on some bands. This appears to mean that I can’t run my 1500-W amplifier with either installation and still be in compliance. Did I miss something?”

A Determining RF safety compliance can be confusing, which is why we published the ARRL RF Exposure Book. (Please excuse the shameless plug!)

Acutally, there are two power levels that you need to consider. The first is the peak output power to your antenna. This level determines if you need to do a station evaluation. On 160, 80 and 40 meters those stations that run 500 W PEP or less do not have to be evaluated. On 30 meters, the level is 425 W. On 20 meters the level is 225 W, on 15 it is 100 W and on 10 it is 50 W. This doesn’t mean that you can’t run more power, but greater output would require an evaluation.

When you do your evaluation, you can use your average power. To calculate this, start with PEP. Multiply that by the duty factor of the mode you are using: 20% for SSB with no processor, 40% for CW or heavily processed SSB, 100% for RTTY or FM. Multiply that result by the percentage of time you might be transmitting during the averaging period. Let’s talk uncontrolled/general public exposure, so we will use 30 minutes. As an example, if you are a high-power conversational CW operator, you should probably use 400 W (1500 W × 40% × 2/3 [20 minutes on out of 30]). From that level, you can do your evaluation.

There are a lot of ways you can do an evaluation. The FCC published Supplement B that has a number of tables. These tables show you how far you need to be from your antenna to comply. For example, on 10 meters, the HF band with the most stringent requirements, if you are running 500 W average power to a typical 3-element Yagi, your neighbors must be 54 feet away from your antenna, diagonally. In your case, they would be, so the dreaded evaluation is over and you just passed! You may have other antennas to analyze but, as an example, again using the simple FCC tables, 500 W average power to a 40-meter dipole requires 6.9 feet separation between the antenna and neighbors. That’s an easy “pass,” too.

It looks like you can use your amp with either the 50 or 70 foot tower. What you probably can’t do, at least with the shorter tower, is transmit a 30-minute continuous carrier at full output on 10 meters. That would require that your neighbors be 95 feet from the antenna, so you can only do it when your neighbors are not standing for 30 minutes on the property line. Of course, there is also the issue of potential damage to your amplifier, your reputation on the air and so forth!

From January 1999