I love reader questions and I had a great question this past week that I wanted to share with the rest of my readers. I’ve recommended installing antennas in the attic if you don’t have an adequate location outdoors or if you have to deal with homeowners restrictive covenants or HOA rules.
But could putting the antenna in your attic put you a risk of increased radio frequency energy exposure?
RF exposure limits is something you should be concerned about though. This is especially important if you are going to be running high power transmitters (over 100 watts) for extended periods of time. The FCC requires that every station do a self evaluation if you are going to be running over the minimum power threshold. For the 2 meter band that level would be over 50 watts Peak Envelope Power (PEP). As long as you transmit under 50 watts, you don’t need to evaluate your setup.
But that doesn’t preclude you from creating a safe environment for yourself, your family, and neighbors. The ARRL has a great set of articles full of tables, formulas and information on creating a safe RF environment.
There’s a lot of formulas and tables, but let’s break this information down into a practical example. Say we’ve got a 2 meter J-Pole antenna that we want to install in our attic above the 2nd floor of our house.
With RF energy exposure there are two environments: controlled and uncontrolled. The controlled environment would be the people living and working within the vicinity of your antenna system. They would know the limits of exposure and could be protected from exceeding those limits. Yourself and your family would fit under the controlled environment. The uncontrolled environment are any neighbors or bystanders that you may not be able to directly protect from exposure. The rules are more stringent for uncontrolled exposure.
RF energy exposure boils down to three factors:
- The power of the transmitter
- The gain of the antenna system
- The duty cycle of the transmissions
These factors will help you estimate distances people will need to be kept from the antenna based on these three factors.
For controlled environment duty cycle is calculated in 6 minute intervals and uncontrolled its 30 minute intervals.
For example if you transmit 50 watts FM for 3 minutes on and three minutes off in a six minute interval, your duty cycle is 50% or only 25 watts for the average time interval. RF energy is non-ionizing, so its effects are not cumulative as long as you stay within the averages.
Lowering your transmit power also lowers your exposure. You may only need 10 watts to solidly hit the repeater. A 3on/3off transmit cycle calculates to 5 watts exposure.
Next consider the gain of your antenna. A J-Pole is 2.5 db, so your 50 watts translates to approx 85 watts, or 10 watts to 18. (After the duty cycle calculation, your exposure drops to 42 and 9 watts respectively.
The FCC only requires you to make a station evaluation if you are running more than 50 watts on the 2 meter band. Doing the antenna gain and duty cycle calculations, in most circumstance you’ll find that you are still within the 50 watt threshold for station evaluation. A station evaluation doesn’t have to be filed with the FCC, you just need to keep a copy with your station’s records in case there is any question about your operations.
Finally, let’s talk about distance from the subject to the antenna. According to the charts in Table 4b of page 23 in the OET bulletin, for the 2 meter J-Pole antenna with about 2.5db of gain. To limit exposure you would need to keep bystanders at least 1.4 meters, or seven feet away from your antenna in a controlled environment.
If you are mounting the antenna in your attic, you’d need about 7-10 feet of space between the antenna and the occupants in the house.
Distance, gain, and duty cycle are the three factors to consider when installing and safely operating an antenna system, be it outdoors or in your attic. Safety and RF exposure are important topics to consider. But following the formulas and duty cycle limits you can safely use the antenna in your attic.