RF Exposure and Your Greyline: The Science, the Rule, and a Five-Minute Check
The Signal Lab · Station Safety
RF Exposure and Your Greyline
Every transmitting antenna makes an RF field around it. A Greyline is no different, and no more. Here is what is actually being measured, what the FCC asks of every station, and a calculator to check yours in a few minutes.
The Short Version
At 100 watts on SSB or CW, the distance the FCC limits call for around a 24-foot Greyline is about one to two feet. On FT8 at 100 watts, a few feet; at legal limit on digital modes, more, which is why the check exists. The formula does not know who made the antenna. It knows power, time on the air, frequency, gain and distance, and a Greyline's gain is a dipole's.
What RF Exposure Is, and Is Not
The energy an HF antenna radiates is non-ionizing. It does not carry enough energy per photon to break chemical bonds, the way X-rays and ultraviolet do. The established effect of strong RF on the body is heating, the same physics as a microwave oven, at far lower levels.
The expert bodies the FCC relies on (NCRP, IEEE, and internationally ICNIRP) identified the level at which harmful heating effects may begin: a whole-body absorption rate of 4 watts per kilogram. The FCC then set its limits well below that line: ten times lower for people who know the antenna is there (you and your household, the "controlled" limit), and fifty times lower for everyone else (the "uncontrolled" limit). The limits change with frequency, because the body absorbs RF most readily in the VHF range.
So the question an evaluation answers is not "is RF dangerous?" It is "how far from the antenna does the field fall below a limit that already carries a wide margin?"
What the Rule Asks of Every Station
New FCC rules took effect on May 3, 2021. The exposure limits did not change. What changed is that amateur stations lost their old blanket exclusion: every station is now evaluated against the limits unless it qualifies for an exemption. Stations already on the air had until May 3, 2023; a new station, or one changed in a way that affects exposure (a new antenna, a new spot in the yard, more power), is evaluated before it transmits. It is a short exercise, and you keep the result with your station records.
Check Your Station
This calculator uses the FCC method from OET Bulletin 65, the same math as the ARRL RF Exposure Calculator, and gives the same answers. Run it once for each band you use. Each input is explained right where you enter it.
Distances are measured from the antenna. Like the FCC method itself, the result is a conservative far-field estimate; it is not meant for anything closer than about 8 inches. Keep a copy of your numbers with your station records.
Worked Examples: A 24-Foot Greyline
Ground reflections included, full power at the antenna, transmitting half the time, on FT8's usual frequencies for the digital rows.
| Station | You and household | Neighbors and public |
|---|---|---|
| 100 W SSB, 20 m | 0.9 ft | 2.0 ft |
| 100 W CW, 40 m | 0.6 ft | 1.4 ft |
| 100 W FT8, 20 m | 2.0 ft | 4.4 ft |
| 100 W FT8, 10 m (3.5 dBi) | 4.6 ft | 10.4 ft |
| 1,500 W SSB, 20 m | 3.5 ft | 7.8 ft |
| 1,500 W FT8, 20 m | 7.7 ft | 17.2 ft |
These reproduce ARRL's published sample results exactly when given the same inputs.
As Safe as Any Antenna, Placed With Care
Nothing about a Greyline concentrates RF beyond what any dipole of the same size does. What a ground-mounted vertical changes is where people can stand: a flagpole lives in the yard, not up on a tower. So site it the way the numbers suggest.
- Put the pole where people do not linger inside your public distance: off the walkway, away from the patio chairs and the neighbor's fence line, especially if you run high power on digital modes.
- Do not transmit while anyone, you included, is handling the flag, the halyard or the tilt base, or working on a remote tuner at the base. Touching any antenna while it transmits can cause an RF burn; that is a contact rule, separate from the exposure check.
- Re-run the check whenever you add an amplifier, a new band or the whip, or move the pole.
The Shelf: Where the Authorities Explain It
ARRL RF Exposure page: the evaluation and exemption worksheets, an FAQ, and a video walkthrough.
RF Exposure and You, by Ed Hare, W1RFI (ARRL), free as a PDF: the whole subject, written for hams.
The FCC's 2019 Report and Order, the rule itself, and FCC OET Bulletin 65 with its Supplement B for radio amateurs, the method behind every calculator.
Want a second set of eyes on your numbers? Call 435-200-4902 with your power, modes and where the pole stands. An operator answers.
73 · Jon KL2A · Greyline · 435-200-4902
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