The 20 m Beam and the Guy Wire
You are installing a 3-element 20 m Yagi on a 50 ft push-up mast. The boom is 12 ft long; the mast is guyed with three sets of 1/4-inch galvanized guy wires anchored 30 ft from the base. The beam will rotate, and the lowest element tip will pass within 3 ft of one guy wire at certain headings. The transmitter runs 1.5 kW PEP. You have a clamp-on RF ammeter, a field strength meter, and the ARRL RF exposure calculator (or equivalent) on a laptop. The station is in a residential area; the nearest neighbor's property line is 40 ft from the mast.
1. Identify the RF exposure and safety concerns and state how you would evaluate compliance.
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Two concerns: 1) Controlled/Occupational vs. uncontrolled exposure - the operator and anyone on your property are 'controlled' environment; neighbors are 'uncontrolled.' You must evaluate both, using the maximum power (1.5 kW PEP) and the antenna gain/pattern. 2) The guy wire near the element tip - a conductive guy wire within a few feet of a driven element will couple RF, re-radiate, distort the pattern, and can carry significant RF current. It also creates a localized high-field region and a shock/burn hazard if touched. Evaluation: Use the RF exposure calculator with: frequency 14 MHz, power 1.5 kW PEP, antenna gain and height (50 ft), and the distance to the nearest uncontrolled location (40 ft to the property line, plus the distance to the neighbor's house). Compute power density at the neighbor's location and compare to the uncontrolled MPE (for 14 MHz, the power-density limit is 1 mW/cm² averaged over 30 minutes for uncontrolled; controlled is 5 mW/cm²). Also evaluate the field at the operator's position and at the guy wire. If the calculated field exceeds the MPE at any accessible uncontrolled location, you must reduce power, raise the antenna, change the pattern, or restrict access.
2. The guy wire is found to carry RF current. What are the practical fixes, and how do they interact with the exposure analysis?
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Fixes: 1) Move the guy anchor or re-route the guy so no element tip comes within a safe distance (typically keep conductive guys well clear of the element tips; several feet minimum, more at 1.5 kW). 2) Break up the guy with insulators (e.g., a strain insulator every few feet, or at least at the top and bottom) so it cannot carry a continuous RF current. 3) Replace the section near the antenna with non-conductive guy (Phillystran or similar) for the top span. 4) Bond and ground the lower guy sections properly so any induced current is drained, not radiated. Interaction with exposure: Breaking the guy with insulators removes the re-radiating conductor and the localized high-field region, which reduces exposure and restores the intended pattern. Re-routing the guy may change the distance to the neighbor's property line, so re-run the calculator. Any fix must be re-evaluated: after changes, re-measure field strength at the property line and at the operator position, and confirm the MPE is met at 1.5 kW. Do not rely on the old calculation.