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## Re: [TowerTalk] Grounding Coax at Tower

 To: K4SAV Re: [TowerTalk] Grounding Coax at Tower Jim Lux Wed, 27 Feb 2008 12:31:28 -0800 mailto:towertalk@contesting.com>
 ```K4SAV wrote: > Having the cables exit the tower above ground is a really bad idea. > Voltage drop down the tower is very high during a strike. Anything > exiting the tower above ground will see a very large voltage impressed > on the cable at the point it exits the tower. But how high? And what needs to withstand that potential? The voltage would depend on whether resistance or inductance dominates. A 30kA strike on a 1 ohm tower would only be 30kV over the whole length of the tower.. call it on the order of 5kV over a 10 foot length out of a 60 foot tower. A 30kA strike with a rise time of 2 microseconds on a 20 microhenry tower (1 uH/meter) would result in a voltage across 10 feet of about 3E-6*30E3/2E-6 or 45kV.. so for the direct strike scenario, the inductance dominates. That translates into very > high currents on the cable. Those currents go toward the shack instead > of into the ground at the tower base. If the cable has a low enough impedance relative to the path through the tower. Considering the example above, where the di/dt makes the voltage 45kV. If the cable path has some extra inductance (say by coiling the coax a few times), you could make the path along the coax very much higher impedance than through the tower, so the majority of the current might flow through the tower. Using 30kA as your stroke current, say you wanted to limit the current through the jacket to 1kA (peak).. you'd need the coax path to have 30 times the inductance of 10 feet of tower. 12 turns, one inch apart on a 12" diameter form would be about 28 uH. 12 turns, 1/2inch apart on a 8" form is 23 uH. The former is about 10 times the inductance of the tower to ground from 10 feet up (I'd guess the tower is <3uH) (I used Wheeler's approximation for inductance) I chose 1kA as a round number, and on the assumption that the fusing current of the jacket is quite a bit greater than the AWG13 center conductor of a RG-213 style cable. 16 AWG wire has a fusing current of about 100-150 A steady state. AWG13 is twice the area and fusing current runs roughly as the area (for small changes in area). This is also a short pulse at 1kA (lasting, say, 50 microseconds) So, at least the coax won't melt, and if the voltage is 45kV across those 12 turns, it's about 3kV/turn. If the turns are spaced a bit apart, it won't arc over. > > Other effects: > 1. The cable at the point of exit, looses its mutual inductance with the > tower which increases the currents on the cable. But, one can fairly easily increase the inductance of the cable, which would reduce the current. So I think that effect is a wash. > > 3. The cable is now more suceptible to direct strikes. > Yes, but it's probably well below the top of the tower, so the tower provides some protection (whether you use the rolling ball or cone of protection idea, either way, a cable coming off at 10ft on a 60 ft tower would have to go fairly far to be a "good" lightning target) > 4. The cable is also more susceptible to induced currents due to the > large loop area. That is, I think, the biggest problem. > > 5. There are no good points. Not having to dig a trench, not hitting your head on the cable when you walk under it, etc. There are places where mechanical constraints force doing things like this. On commercial installations, they'll have a cable tray from tower to building, and presumably, that tray carries a lot of the lightning current. Several folks (N6RK does this, I think) have array setups where the feedline is open wire line carried across a field at some feet above the field (so that animals can graze underneath) VOA's big curtain array in Delano,CA is fed by open wire line supported well off the ground. It might be interesting to look at what sorts of lightning protection strategies they use. (Although.. they have a big enough budget to take a "if it doesn't hurt, why not do it" approach) _______________________________________________ _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
 Current Thread Re: [TowerTalk] Grounding Coax at Tower, jimjarvis Re: [TowerTalk] Grounding Coax at Tower, K4SAV Re: [TowerTalk] Grounding Coax at Tower, J. Gordon Beattie, Jr., W2TTT Re: [TowerTalk] Grounding Coax at Tower, Jim Lux <= Re: [TowerTalk] Grounding Coax at Tower, K4SAV Re: [TowerTalk] Grounding Coax at Tower, Jim Lux Re: [TowerTalk] Grounding Coax at Tower, K4SAV Re: [TowerTalk] Grounding Coax at Tower, Jim Lux Re: [TowerTalk] Grounding Coax at Tower, David Robbins K1TTT Re: [TowerTalk] Grounding Coax at Tower, Jim Lux Re: [TowerTalk] Grounding Coax at Tower, David Robbins K1TTT Re: [TowerTalk] Grounding Coax at Tower, Dick Green WC1M Re: [TowerTalk] Grounding Coax at Tower, jimjarvis