Topband: Feeding a Base Insulated Tower
Most, as in nearly all of the licensed AM broadcast stations in the U.S. use all three of the devices listed below (together) to reduce the probability of lightning damage to their transmit systems when using base-insulated towers. 1. A static drain choke leading from the base of the tower to a low-Z "lightning" ground path buried in the earth. 2. A conductor with a single-turn loop of ~15- to ~30-inch diameter connecting the bottom of the tower to the output terminal of its physically adjacent Z-matching network. 3. An arc gap crossing the base insulator connecting to that good lightning ground of item 1 above -- the gap spacing set to flash over when the peak voltage there exceeds the peak voltage present during normal operation, by a safe percentage. Modern, commercial AM broadcast transmitters also include SWR protection that will instantaneously/momentarily quench their r-f output into a load Z exceeding a safe value for the duration of that excessive SWR. The end result is that the hardware of commercial AM broadcast stations rarely is affected by lightning events near their transmit antenna systems, and as far as their listeners might discern. R. Fry _ Topband Reflector Archives - http://www.contesting.com/_topband
Re: Topband: Feeding a Base Insulated Tower
I have a couple questions I have never been able to find any direct answer for... so... out of curiosity... 1: I have never found or read any information concerning the best way to feed a base insulated tower. Is it better to just feed one leg from the matching network or would it be better to feed all three tower legs at the same time... OR... does it even make any difference at all? Any out there that have tried both ways? I've had a dozen or so series fed towers since 1970. Except for the wire length and size from the tower to the matching network, it makes very little difference how you feed the bottom. 2: Also..Now there is a static bleed choke across only one leg insulator of the vertical... Is it better with one on each leg across the base insulators? (each leg is insulated with T10 Garolite solid rod type insulators about 18 in long and milled to fit the Rohn 25 tower legs.. 4 in. insulated gap left between the upper tower and lower 2ft of tower left out of concrete base.) _ It makes zero difference. The static drain can be in the house, at the tower, a resistor, a choke, or whatever. All that matters is you have some dc path to ground to prevent charging. The charging causes a tic or arc when the voltage exceeds insulation at some point. The dc current is typically in very low microamperes as a storm approaches, or the conditions are right (I've measured it several times), but can range up to very low milliamperes in severe stuff. The transients from nearby strikes can be amperes, but the slew rate is so high they will not go through a choke or resistor anyway. Reducing any voltage from close discharges takes a close gap or a low impedance RF path. 73 Tom _ Topband Reflector Archives - http://www.contesting.com/_topband
Topband: Feeding a Base Insulated Tower
I have a couple questions I have never been able to find any direct answer for... so... out of curiosity... 1: I have never found or read any information concerning the best way to feed a base insulated tower. Is it better to just feed one leg from the matching network or would it be better to feed all three tower legs at the same time... OR... does it even make any difference at all? Any out there that have tried both ways? 2: Also..Now there is a static bleed choke across only one leg insulator of the vertical... Is it better with one on each leg across the base insulators? (each leg is insulated with T10 Garolite solid rod type insulators about 18 in long and milled to fit the Rohn 25 tower legs.. 4 in. insulated gap left between the upper tower and lower 2ft of tower left out of concrete base.) _ Topband Reflector Archives - http://www.contesting.com/_topband
