BE Radio November/December 1997 Contract Engineering Tower Lighting By William Fawcett ------------------------------------------------------------------------ William Fawcett is president of Mountain Valley Broadcast Service, Inc., a broadcast engineering firm in Harrisonburg, VA. ------------------------------------------------------------------------ The staggered FCC Antenna Structure Registration process is now about 2/3rd's of the way done. If you have registered a tower, or if you have an application pending, you probably have been thinking at your existing tower light situation. Even with recent changes there continues to be confusion about multiple specifications for lighting. Until recently, the FCC listed its own specifications in Part 17 of the FCC Rules. Furthermore, when a license was issued, pertinent paragraphs of FCC Form 715 would be cited, often with a copy of Form 715 attached to the license. Current registrations may quote the FAA specifications contained in FAA advisory circular numbers 70/7460-1H and 150/5345-43D which are now incorporated by reference into Part 17. Your license or registration may cite pertinent paragraphs or chapters of current or previous versions of any of these documents. Be sure to compare your new registration with the specifications listed on your license to make sure that they agree; some processing errors have occurred. Note that Section 17.17 specifically states that older specifications are grandfathered (as long as the current tower is not modified). Therefore, you may have a 399' tower with only one beacon level; or alternate flashing beacons, things that might not be permitted under the current rules. Keep the dollar cost of upgrading in mind when considering any modification proposal. Under WT Docket 95-5, the owner of the tower is responsible for compliance with lighting and marking requirements. In the case of a renegade owner, the FCC may require each licensee and permittee authorized on an antenna structure to maintain compliance in accordance with the Antenna Structure Registration. Therefore, it is critical that the station engineer have a working knowledge of lighting requirements and maintenance procedures, even when dealing with a leased tower. To Strobe or Not To Strobe It is amazing how the pendulum swings. When strobes were first introduced, the public outcry was great. Early designs did not attenuate strobe radiation below the horizon. With the proliferation of strobed cellular towers we are seeing a desensitization to these aesthetic concerns. Now many local ordinances discourage the orange and white marking required for red lit towers. Some local situations may call for the use of red lights at night and strobes during the day. This can be a costly compromise, but at least it will save you the expense of frequent re-painting and re-lamping. Rather than installing dual fixtures, you may consider a strobe design that mechanically places a red lens over the strobe at night. Not only is it likely that new towers will incorporate strobes, many red lit towers are making the transition. For various reasons it is becoming difficult to even find contractors that will bid on a tower painting project. Before making changes to a lighting system, you must file an FAA Form 7460-1. Additionally, the tower owner must also submit FCC Form 854, Antenna Structure Registration when such a change is made (generally after FAA approval is obtained). Later, FAA Form 7460-2 is used to report construction status. Maintenance Procedures You will likely receive a report of any problems encountered during a relamping procedure or a periodic on-tower inspection. Typical problems include damaged wiring, and fixtures broken by ice or errant bullets. Conduit system breathers should be checked and cleaned to avoid water accumulation. Apart from climbing the tower, there are a number of items that can be checked on a regular basis. The most basic of all inspections is the 24 hour daily check. By law, a visual inspection must be made an logged at least once a day. For remote sites, the installation may feature an automatic alarm. You must test the alarm(s) at intervals not exceeding 3 months. If lights are out (other than non-flashing intermediate side lights) you must immediately report the failure to the nearest Flight Service Station. Information concerning malfunctions and repairs should also be recorded in the station log. A trained eye can usually spot when one light in an incandescent dual-beacon fixture is out. Binoculars can be very useful for checking this, as well as looking for broken or open fixtures. Most incandescent alarm relays will dropout when only one lamp is burning. A clamp-on ammeter will quickly verify if any lamps are out; side lights typically draw 1 Amp each, beacons (620W) draw 5.2 Amps at 120 volts. Mechanical flashers should be periodically lubed, and their contacts burnished when necessary. The lights should flash between 12 and 40 times per minute, with a 50% duty cycle. Some mechanical flashers get very slow when cold weather causes the grease to harden. Most modern designs use a solid state flasher, which will not wear out mechanically, but may require replacement after a direct lightning hit. Strobe systems should be checked to ensure that the lights change from day to night mode. Although the public is getting used to strobes you will hear complaints if your system stays in day mode during the night. On either a strobe or red light system, you should make sure that the photocell eye is clean and is facing the north sky. On a hot AM tower you will likely have a lighting transformer to isolate the tower from the power lines. Occasionally, these transformers or the associated bypass capacitors will be damaged by a lightning strike. Controllers, photocells and flashers may be mounted on or off the tower. Obviously, those on the tower will be more difficult to service. solid-state flashers on AM towers may require special bypassing to avoid RF-related malfunctions. As the AM tower is also the radiator, any changes to the lighting system may cause a change in impedance. A substantial change may require retuning, and in many cases the submission of FCC Form 302. For stable operation, the neutral lead on the tower of an incandescent system should be bonded to the tower at the base, and at each junction box. Strobe systems on an AM tower should be installed according to the manufactures recommendations. National Electric Code Considerations Certain provisions of the National Electric Code may require the installation of a safety disconnect for the lighting circuit at the tower. Even though you could lock-out the circuit back at the transmitter building, a disconnect at the tower makes servicing the system easier and safer. Speaking of safety, one of the pen-sized AC power detector buzzers will help keep you off hot circuits. They also are the fastest way to find a blown fuse or open circuit. You can even use it to check the flasher and photocell in the daytime. The installation of fuses at the disconnect may afford some measure of lightning protection to the AC circuits away from the tower, but you may find yourself replacing the fuses on a regular basis. Wiring to the tower and on the tower should be sized according to voltage drop and not the minimum code requirements. Section 17.54 requires that the voltage rating of an incandescent lamp be no more than 3% of the actual voltage measured at the socket. Long runs may require the use of step-up transformers and/or 240 volt systems. Under Article 250 of the code, bonding of the neutral to ground usually only occurs at the service entrance. Because towers have their own grounding electrodes, they can be treated as a separate "building". This will allow you to bond the neutral to the ground at the tower, which is essential for lightning suppression and RF control. Your tower ground must also be bonded to your power system ground. As mentioned before, it is important to bond the neutral on an AM tower to the tower at regular intervals. This practice is not addressed by the NEC, and some interpretations might prohibit it. Fortunately, most jurisdictions do not inspect electrical wiring on towers; some specifically exclude such wiring. To get around this problem in some locales you might install a twist-lock receptacle and plug at the cold side of the lighting choke and call it "equipment", or alternatively supply a schematic plan with a professional engineer's seal with your permit application. Three-fold approach A broadcast engineer's job requires expertise in many different areas. Concerning tower lighting: you must have knowledge in regulatory issues, lighting hardware, and electrical wiring. Because of public safety concerns, it is critical that the station engineer is component in the theory and practice of all three. Sidebar: Tower Lighting Resources Antenna Structure Registration www.fcc.gov/wtb/antstruc.html FAA Form Info www.fcc.gov/mmb/asd/7460.html WT Docket No. 95-5 www.fcc.gov/Bureaus/Wireless/Orders/1995/fcc95473.txt CC TOWAIR software www.fcc.gov/wtb/towair.html