Topband: Topband Re: Ground Screen Question

[k3ky at radioprism.com]

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