RF Radiation Hazards
You have a responsibility to protect yourself and others from RF radiation hazards. RF radiation from amateur radio equipment could heat up human tissue. That can cause harmful RF burns or tissue damage.
RF exposure evaluations help keep everyone safe. That’s partly why it’s included in all three license exams. In 2023, the FCC required all ham stations in their jurisdiction be evaluated for RF exposure. Does that mean you have to evaluate your old 80 meter station? Yes, an evaluation must always be performed. There is one exemption. Hand-held transceivers sold before May 3, 2021 don’t need RF exposure evaluations.
The amount of risk of RF radiation depends on the strength of the radiation as well as the frequency. Frequency is important because the human body absorbs radiation more at certain frequencies. We use the “Specific Absorption Rate” or “SAR” to measure the rate at which RF energy is absorbed by the body.
The FCC has published Maximum Permitted Exposure (MPE) limits. “Exposure” is also a key word, because we are trying to make sure the absorbed radiation is at a minimum. Think of how you want to avoid exposure to the sun to keep from getting a sunburn.
A standing ungrounded human absorbs the maximum amount of RF radiation at about 70 MHz. Meaning the SAR is at a maximum around this frequency. So, FCC human body RF exposure limits are most restrictive over the range of frequencies from 30 to 300 MHz. That includes the 6 meter and 2 meter bands.
FCC OET Bulletin 65 outlines all of these details. It’s the guide to follow and we’ll put a link in the written lesson. There’s a table that shows when evaluation is needed on each amateur band. It’s based on the transmit power of your radio on that band. In some cases, exposure evaluation thresholds are as little as 50 watts. If you run 10 or 12 Meters at 100 watts, you should have a completed evaluation.
There are two types of exposure levels to be concerned about. Controlled exposure and uncontrolled. Controlled exposure is about you. Operating your station and being fully aware of the RF hazards.
Uncontrolled exposure might be for nearby neighbors. You may need to evaluate RF exposure levels from your station at a neighbor’s home. When you do, ensure signals from your station are less than the uncontrolled Maximum Permitted Exposure (MPE) limits. The key words here are “uncontrolled” and “exposure”. The “uncontrolled” limits apply here. Your neighbors, co-workers, or the general public don’t know when you are operating. This means they can’t manage their exposure.
MPE limits have unique electric and magnetic field limits (E and H) because:
- The body reacts to electromagnetic radiation from both the E and H fields
- Ground reflections and scattering cause the field strength to vary with location and
- E field and H field radiation intensity peaks can occur at different locations
All of these are correct when asked about MPE limits on the exam.
Hams may share transmitter sites. At the top of a tall building there may be different operators on different bands. MPE evaluation is important at these busy sites. So with multiple transmitters, who’s responsible for that evaluation? The operators and licensees of each transmitter that produces 5 percent or more of its MPE limit in areas where the total MPE limit is exceeded are responsible for mitigating over-exposure situations. The key to remember here is the 5% limit.
Microwave bands are another particularly sensitive area to watch for. What type of hazard should microwave operators watch for? One is that the high gain antennas commonly used can result in high exposure levels.
Tower Safety and Grounding
Even if you don’t have a tower, you may help work on one, maybe at a club station? Keep in mind safety for any tower and having good lightning protection.
Working on a tower is fun, but strenuous. You definitely have to have fall protection. When you are climbing, use OSHA rules as your guide.
OSHA’s 100% tie-off rule is about fall protection. You should have it any time you are 4 or more feet off the ground. For tower safety that means at least one lanyard attached to the tower at all times.
When attaching that lanyard, they should be attached to tower legs. That’s the most substantial part of the structure. Use a shock absorbing lanyard to limit your fall distance. One model of those has elastic in it to absorb impact. Think of a bungee cord. Always attach those above the climber’s head level.
In an earlier lesson we talked about the single point ground panel. That’s a location to bond all your RF and electrical items in one spot. You’ll want to connect your tower to that.
That single point panel will be connected to an external earth connection or ground rod. That is to give you lightning charge dissipation. How does this work? Say lightning strikes your antenna, tower or other exposed surfaces. The ground rod helps by transferring the energy into the actual ground. The ground rod and associated lightning suppressors should give the lightning the lowest resistance path to ground. That helps minimize damage.
A full stack of antennas and radio gear can cost thousands of dollars. One lightning strike can fry it all. Lightning suppressors for your coax are around $30 and a ground rod is about the same. Now this isn’t everything you need for your single point ground panel. The point here is protection is not too expensive compared to replacing all your gear.
So this is the last lesson in the course! I’ll leave you with two quick tips before you take your practice tests. First, this course is always here for you. Want to look up something about Smith charts? Weak on calculating the resonant frequency of a circuit? Come back and review the material any time. Even after you pass your exam.
Second, consider networking with fellow students. They may have different experiences or ideas to reinforce a concept. There’s more information on that in the sidebar.
So, thanks for studying with us! I’m Jim, N4BFR and on behalf of the entire Ham Radio Prep Team, 73 and we hope to hear you on the air soon.
