Ham Radio Q & A is a new type of amateur radio themed online show were I’ll answer your ham radio questions. You too can get your ham radio question answered, just send me an email to email@example.com or leave your question in the comments below. Do you have a question about antennas, equipment, or operating practices? Or is there some ‘How do I. . . “ or ‘Why is it . . .’ question that you’d like answered. Drop me a line and I’ll put it on the show.
Joe from New York asks the following question:
Hi I’m a new ham and I’m doing some research on attic antennas because of homeowners association rules. I have a few concerns that you might be able to help clear up. I will be running a mobile unit as my base station. Question 1- will this produce any harmful rf exposure to my family? 2- will this interact with any electrical in its area? 3- how would it get mounted?
Thanks for the letter Joe. lets talk a minute about attic antennas.
Restrictive deeds and covenants and HOA rules are becoming more and more commonplace. If you are looking at buying a newer home, you will most likely have to contend with some type of rules regarding the installation of antennas. Mounting your antenna in the attic is a great way to get on the air when you live in a restricted environment. And as long as you don’t have a metal roof or metalicized mylar vapor barrier, You really won’t lose any, if at all, performance with your attic antenna. And newer homes with a high pitched roof offer plenty of space for antennas.
Electrical wiring, and HVAC ductwork shouldn’t pose a problem to your antenna system. Typically if the wiring and ductwork are below the antenna, it won’t affect the antenna. If there is wiring running above the antenna or along the peak of the attic, there may be issues, but it can be hard to say, as each antenna installation is a little different. My best suggestion: try it, check things out with a meter, and see what happens.
For mounting the antenna, if you have exposed ceiling joists or trusses, you can set up a cross member and use some clamps or brackets to secure the antenna. Otherwise a short TV tripod or other support will work to hold the antenna up. On my blog I’ve got some picture of antenna stands that other hams have built out of wood or PVC. The link to that article can be found in the show notes below.
As for the RF exposure, the FCC has written a comprehensive document on radio frequency radiation exposure. Called the OET Bulletin 65. This document is full of graphs, charts and formulas to help calculate exposure levels and keep your station within permissible limits. There is a lot of information in this document, and the rules are designed to keep you and your family safe. But for simplicity sake, I’m going to break things down a bit.
Basically RF exposure limits boil down to three factors:
- The frequency of the radio waves you are transmitting
- The length of the exposure (also known at the duty cycle)
- And the power of your transmitter and antenna system (or it’s effective radiated power)
Higher frequency radio waves are more readily absorbed by the body than low frequency waves. That’s why microwaves are able to cook food- the radio waves excite the water molecules in the item, creating heat. But power also plays a big part in exposure. For example, take your wifi router. Your wifi network uses a similar frequency of radio waves as a microwave oven, but the power is so low and unconcentrated that that they have no affect on the body. It would take an awful long time for those low power waves to have an effect- which brings me to my third point, duty cycle. Duty cycle is the length of time you are transmitted as opposed to receiving. If you transmit for one minute and receive for three, your duty cycle is 25% for that 4 minute period.
So what does this all mean? The same radio waves emitted with a high power and duty cycle can cook food, but spread out with a low duty cycle and power level are relatively safe. All of these factors play into the total level of allowable exposure.
Now you can calculate your exposure by either using fancy and expensive field monitoring equipment, or computer RF modeling programs. But the easiest method is the use the charts and tables in the OET guide to estimate exposure.
So if you work though the forumlas and tables in the OET 65 guide, you’ll find that with a 50 watt transmitter on the 2 meter band connected to a 3db gain antenna, in an uncontrolled environment you should keep people at least 10 feet away. Now this chart assumes a duty cycle of 100%, so these values are a worst case scenario.
I don’t want to downplay the safety aspect, but the good news is that if you keep your transmit power relatively low, that is 50 watts or less, and maintain a reasonable duty cycle, having antennas in your attic are perfectly safe on an RF exposure standpoint.
For more information on RF safety, I do have an article on my blog about RF safety and exposure and how to calculate the maximum permissible limits of exposure. You can find the link in the show notes below, or email me and I’ll send you links for everything we talked about today.
We’ll, that’s it for today’s episode for ham radio Q & A. Another episode will come out next week, so please send those questions to firstname.lastname@example.org and they may have a chance of being read on the show. In the meantime, please subscribe to my youtube channel and follow me on Facebook. You can also read my blog online at www.jpole-antenna.com.