Hi, Bob- I'm really seeing two or more issues here. There has been some debate as to whether or not tying in a ground screen to elevated radials is a good thing or a bad thing. I'll leave that discussion to other topbanders who have a lot more experience in that area. My understanding is that even a ground screen NOT attached to the antenna ground return conductors (radials, counterpoise, etc.) may still be of benefit.
Regarding the elevated radials, I'd suggest that you might want to weigh the possibility of using a K2AV FCP rather than the rather skimpy radial count of only four wires. I'm surprised there have been only a few replies in this thread. Regarding the reinforcement wires in your driveway, of course they could be connected to other ground screen conductors. It might help how well it all works; the only fly in the ointment is possible concerns about a direct lightning hit. If you get that unlikely big direct hit, there can be a huge current surge to ground, on the order of ~100,000A. Looking at how tower grounds are deployed (ideally) we see multiple large diameter conductors bonded to at least three ground rods near the tower legs (or often wide copper strap). Often the ground is extended out to more ground rods, sometimes splitting to two connected rods, etc. The idea is to spread the current out symmetrically away from the tower base. Connections are often done using 'cad weld' techniques, where a very hot exothermic reaction essentially melts the conductors to the rods. This is to handle the huge currents in a direct strike. If you do connect to your concrete reinforcement wires, I would guess that multiple interconnections might be good, if they do contribute to current spreading during a direct hit. For sure, if you ever do get that direct hit, there are going to be large currents in all nearby conductors, including buried ones. I'm having a hard time visualizing how you figure out where to drill to hit those embedded wires, then how do you get enough room to get in there and make a good connection to the wire? I'd guess it really ought to be some sort of compression connection. It needs to stay together if a huge current spike happens. It would also need to be weatherproofed, somewhat, to prevent rust and eventual failure to maintain an 'ohmic' connection (non-rectifying). I would think that just wrapping a few turns of wire would eventually result in a non-ohmic connection- especially if the wire types are dissimilar (Cu/Fe). I think I'd want to do that with some sort of clamps. Do you have enough room down inside a bore hole to do that? What would scare me the most would be if you made a single connection of the driveway wires into a heavy (proper) tower ground. During a lightning hit, a LOT more current might try to pass through that one joint, dissipating through and in the center of that big block of concrete. Odds vastly favor your never taking such a hit, but if you do... For lightning mitigation, having little or nothing to do with the overall quality of your RF ground, I suggest that keeping 'current spreading' in mind is always a good idea. 73, David K3KY Dave, I see your point. I wouldn't want to chance damaging the concrete. I am not thinking of attaching to the tower with it however, I am thinking of it as a ground screen and actually adding more welded wire mesh also at about a 15 foot radius around the base of the tower which would be connected to the slab mesh. The ground screen would not be attached to the tower and will help with near field losses from the shunt feed tower. What I am thinking of for radials is 4 elevated radials at 14 to 16 foot above ground. This height works well with my trees, etc on the property. Radials will not connected and completely independent of the ground screen. I don't have a huge lot so the radials will have to be bent at 90 degrees when they approach the property lines. Not ideal but I'm hoping with a little help from the ground screen I will have a decent signal. I did an exploratory hole in the concrete slab and the 6x6x no 8 wire grid is like new, no rust. I guess the concrete preserves it. I have two 8 foot wide by 70 foot long runs of the welded wire mesh imbedded in the parking slab. I will make sure each of the two runs is electrically tied together. So what I will end up with my proposed ground screen is wire mesh ground screen extending out from the base of the tower with a 15 foot radius and it will attach to the adjacent 15 foot by 70 slab ground screen. I figure why not use the 15 x 70 slab too if doable ? Sound OK ? Any other ideas suggestions ? I appreciate the input guys ! Bob K6UJ _________________ Topband Reflector Archives - http://www.contesting.com/_topband
