[TowerTalk] Solid 160 radial plates?

[Jeff DePolo]

Regarding using ground bus bars as tie points:

> You could probably  shove a bunch 
> of wires in one hole and crank down on the screw, but I'd feel better 
> with soldering them.

If you can still find them, copper ground bus bars with screw terminals
would be ideal as you could then braze the wires to the bar.  You could fit
several small (12 AWG or smaller) wires under one screw, and then braze them
collectively to the bus bar.  Avoid soft solder for anything buried, braze
it with silfos.  And of course, never bury uncoated aluminum..

In broadcast, we typically start with a copper mesh mat encircling the tower
foundation.  Around the "inside" perimeter is copper strap (typically 3" or
4"), and again around the "outside" perimeter.  The radials are wrapped
around, and brazed to, the outside perimeter strap.  The mesh we use is a
grid with around 6" spacing.  15% silfos and a mapp gas torch will get the
job done with a little patience (and no wind), but OE makes the work go even
faster.  The dimension of the mesh-covered area is selected based on a
combination of factors including base Z, frequency, and cost, but typically
is somewhere around 20 to 50 feet on each side if square, or a comparable
radius if trimmed to have a round or octagon footprint.

You can find somewhat cheaper copper mesh with larger grid spacing.  Copper
"pool mesh" used for equipotential bonding under/around swimming pools is
pretty easy to find, Google it.  However, the grid spacing is usually around
a foot.  I haven't modeled it to see how much less efficient a 12" grid is
compared to a 3" grid, but I'm guessing it's not going to be appreciably
different.

The purpose of the close-in ground screen is not necessarily to improve
efficiency, but to minimize the effects of changing ground conductivity as a
function of moisture/weather which, particularly in the case of directional
arrays, can be problematic.  For ham purposes, simply encircling the base of
the tower with copper strap (folded and brazed at the corners) and attaching
radials to it would be what I would do.  But if you're really gung-ho, mesh
is what the pros typically use, although I've seen plenty of AM's with only
copper strap around the tower base with 120 radials attached directly to it.

Personally, I would avoid any kind of mechanical connection from a radials
to a "grounding plate".  I'd use copper everywhere and braze it.  The cost
of all of that stainless hardware, lugs, etc. would be more expensive than
simply brazing.  It also takes less time to install.  And, a properly brazed
joint will outlive any of us on this list.

					--- Jeff WN3A


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