No. 23: Some Notes on Antenna Safety

L. B. Cebik, W4RNL

What would you like to protect from harm? Yourself? Your family? Your home? Your station? Your neighbors and their property? I guess that is enough to start a column on safety. Obviously, we can never finish it, but perhaps a few ideas might help you fill some gaps in the safety net around all you wish to protect and preserve.

Static Build-Up: As the wind and weather pass over the antenna elements, a charge builds up. On long wire antennas, it can generate enough of a charge to draw a visible spark across a gap to ground. On smaller antennas it can create unnecessary static. The best way to avoid problems here is to be sure that the antenna is at DC ground. Some antennas are designed with a DC ground; others are not. For low impedance antennas (less than 150 Ohms feedpoint impedance) fed with coax, the simplest way to insure a DC ground is to place an RF choke across the antenna terminals. This gives you a DC ground path but leaves the antenna RF-hot. At 10-meters, a 100 micro-H RF choke provides about 1800 ohms of reactive isolation, but a low resistance to charge build-up. Just be sure your coax braid is grounded somewhere along the line so that the charge has somewhere to go other than to your equipment.

Parallel feedline systems, such as all-band doublets require a different treatment, since the impedances they may see can range up to 5000 ohms and more. Here we can use high value resistors where the feedline enters the house. Run a 1 Mohm resistor from each line to a ground rod where the line meets the shack. It is also useful to add a knife switch or other disconnector (with the alternate contacts directly grounded) to remove the connection to the equipment when the antenna is not in use.

Station Grounding: Everyone is familiar with the need for a common ground bus for all station equipment with as short a lead as possible to a ground rod outdoors. If you have a tower, it should also be ground rodded. All ground rods (including the house ground at the electrical service entry) should be connected together by the largest copper wire you can obtain. Some folks use copper flashing cut into long strips. These days, braid is not recommended because its very large surface area open to weather deteriorates it quickly. Use solid mechanical connections, not solder (which melts in the presence of lightning like a teenager in the presence of a rock star: instantly).

If ground rods are widely separated, add ground rods in a row from end to end, spaced about the length of each ground rod apart. This is not over-kill--just the opposite. It is over-safe by holding the resistance between everything in your house and shack at the absolute minimum. Even a slight resistance may give a lightning surge an alternative path to ground through your "stuff."

Towers and Coax: Some studies have shown that the bulk of lightning current travels in the shield of the coax. A considerable voltage difference can exist between a tower leg and the coax braid if the first connection between the two is at the bottom of the tower. You may want to consider breaking your coax near the top of the tower, having one section to the antenna with the flex loop to accommodate rotation and another section down the leg of the tower. Securely fasten a plat to a tower leg and use a double female connector with chassis nuts (often called a bulkhead connector) to join the two coax lengths. This gives you a connection between the coax braid and the tower leg. This is not the only ground you should use, but an extra one.

Total Disconnection: During violent electrical storms, surges can enter the station equipment from several sources: the antenna lines, the AC power lines, and ground. When dealing with surges, do not think in terms of complete electrical paths. A surge can charge components, cases, and chassis, and create high voltages across components that are designed for low voltages. Hence, during violent storms, do a complete disconnect.

Outdoors, disconnect all antenna feed lines and reconnect them to a ground rod in the station ground system. Indoors, disconnect antenna feedlines from the station equipment. Unplug all equipment--or that power strip you use as a master switch. Most power switches are single pole, so one must unplug in order to break the neutral and ground line paths along which surges can travel. Also disconnect the station equipment from the master ground lead to the outdoor ground rod system. If you do some careful planning, you can make all these moves in under a minute. Just be sure that all connections are accessible and require no tools to connection and disconnection.

While it may be true that a direct strike is rare, most damage occurs from nearby strikes that place heavy voltage surges on power lines, antennas, and the ground. You can do a lot to keep them outdoors, where they belong.

Towers: Towers, whether free standing or on the roof top, require some special thought. Too few hams actually study tower installation before putting up the first one. Here are some things to think about:

1. What are the requirements for the tower base, including the concrete, rebar, excavation, etc.? Never under-support a tower at its base.

2. What, if any, are the guying requirements? If a tower is a guyed model, install the guying system to at least the manufacturer's standards. Be certain that guys are correctly and adequately anchored--and that they do not present a hazard to those who use your yard. If the tower can use a building support, be sure the building is up to the job. Most roof-line facia boards on a house are not.

3. Where does the tower go? Place it where it cannot hurt neighboring property (or people) if the tower and antenna fall under the worst conditions imaginable. Also try to place it where it will not hurt your own home and family if it falls.

4. Where do the feedlines go? Be sure that feedlines do not create either an electrical or a physical hazard for family members or visitors.

5. What are the codes? The more urban your setting, the more you may be subject to codes and ordinances that require permits, special requirements (for example, conduit for control voltages over a certain value), inspections, and, in some cases, licensed installers. Do not under any conditions bypass these requirements. Investigate in advance to know what you have to do and what it will add to the cost of the installation.

6. What about intruders? Towers are in some places classified as attractive dangers, which is what they might be to a neighborhood child with an urge to climb. Hollering is a deterrent, but not protection. Consider fencing in or cladding the lower tower sections so that the tower cannot be climbed.

7. Are any of the tower conveniences dangerous? Crank-up and tilt-over towers are convenient, but usually rely on cables, pulleys, and gearing to go up, come down, and stay in position. Analyze the stresses on these auxiliary parts to ensure that they can safely handle the loads. Develop a maintenance schedule for inspecting and for replacing cables before their lifetime is ended. If they die on the job, disaster often results.

RF Safety: RF hazards come in two varieties. One is radiation. Current FCC regulations provide standards for safe amounts. Use the worksheets and analyze your station, even if you think you may be exempt by virtue of power levels and spacings. Be sure.

A second hazard comes from direct contact with antenna elements carrying RF currents. Ground mounted verticals and inverted Vee ends provide the greatest hazards, since both children and adults can come into contact with them--either directly or with some implement having a long metal handle.

Isolate ground-mounted verticals with fencing or other barriers which are effective especially in preventing children from touching a potentially active antenna. A vertical in or adjacent to a children's play area may mean station silence during periods when children are present: if contact is not a danger, radiation may be.

Elevate and insulate horizontal antenna ends above the level where anyone can touch them with a metal rod. Never underestimate the ingenuity of a child with a small mean streak or a large curiosity.

Insurance: Carry all you can afford that may be relevant to hazards that antennas may present. However, never let the existence of insurance be a substitute for the best possible practice in the installation and maintenance of antennas and their supports. The lives you preserve may include your own and those of your loved ones.

This is not a complete look at all the facets of antenna safety. We have not even reminded you to keep your antennas and supports well away from power lines. But perhaps we have said enough to prompt you to do a periodic inspection of your own safety measures--and perhaps some reading into the handbooks and other literature. The three key words are these: protect, divert, and prevent. They are three ways of saying that you care enough to do the very best.



Updated 3-2-99. L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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