Rich, Even massive grounding systems do not keep a tower from being hit! It only mitigates the damage.
I was CE of a TV station in South Florida with towers of 1,000' a couple at 500' and several more at 100' to 200'. They all got hit at one time or other. Sometimes many times a year. We used massive grounding systems with the tall towers. The towers we rented or on buildings - we had to take what we got. Some were excellent some were non-existent. We operated 24/7 since 1955. We did have redundant equipment. We had two-way antennas as high as 900', microwaves from 100' to 950'. Tower cameras as well. The ENG receive sites (at 950' & 500') had very sensitive LNAs at 2, 7 & 13 GHZ. as well as the pan/tilt steering electronics and drive systems. When possible we did not put electronics on the tower; because of the surge current of a direct strike. A small (tiny?) strike will have over a thousand amp current surge. If the steel tower has a 0.1 ohm resistance, then the top of the tower is 100 volts above the bottom. Any electronics at the top that is connected at the bottom can have that potential on the equipment case (a DC ground loop so to speak) of 100 volts. Most times it is much higher. The least affected equipment were the microwave transmitters and receivers that used waveguide. The signal up and down the waveguide is transmitted rather then conducted. We felt that the best money we spent to mitigate lightning damage were dissipation arrays on all of our towers higher than surrounding structures. Over a 10 year period our lightning damage costs were reduced significantly over the 10 year period prior to that installation. It seems that dissipation arrays are in dispute at this time. It goes to the old adage - that just because you put out elephant repellent and then see no elephants still does not mean the elephant repellant works! My 40 years experience in South Florida as a Broadcast Engineer has taught me that you will find the weakest links in your plant's structure and make them less vulnerable but never be completely immune to lightning damage unless you eliminate all conductors of electricity. Fiber interconnections between studios and towers really helped us reduce the losses. 73 George AI4VZ -----Original Message----- From: Richard Fjeld ... Certain radio and TV services need to operate 24/7 and can't shut down when lightening threatens. Their grounding methods apparently prevent the towers from being hit . ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
