>On Thu,10/27/2016 10:10 AM, Kevin Kidd wrote:
>> If it's not copper or some alloy thereof, don't waste your time for a
>> permanent installation.
>
>Some pretty sharp engineers disagree with this advice. Rob Sherwood,
>NC0B, first published on the usefulness of galvanized ground screen in
>1977, and talked about it in presentations in the Dayton Antenna Forum
>in 2008 and 2009. Rob is best known for his work on quantifying the
>receive performance of modern HF rigs. Like anything making contact
with
>the chemical makeup of soil and local moisture at any given QTH,
>materials that work in one location may not work in another.
>
>73, Jim K9YC
Good points from Jim.
1. Mesh grounds have multiple current paths in parallel, so the
individual wires don't need to have especially high conductivity. What
does matter in a mesh screen is that the individual crossover joints are
reasonably good, to create those broad distributed current paths. For
all those reasons, good-quality galvanized steel mesh can be quite OK.
2. Local soil chemistry is critical. Previous inhabitants at this QTH
left behind some 'test samples' of old chicken wire that had
disintegrated into sharp rusted pieces, ruining the ground for any
further cultivation... which made me look for something else instead.
What does work here is strong galvanized pig fencing, about 2.5ft wide
and available in 150ft rolls. This material is made from heavy
galvanized wire and has quite a large mesh varying from about 4 to 6
inches. The mesh crossovers are rolled but the connections remain very
tight, and after about 5 years under acid leaf mold the galvanizing is
still good.
Thinking about mesh size, 4-6in is not as bad as it looks. The usual
mesh sizes of 1in or less are far smaller than is needed for HF. With a
traditional fan of ground radials, we generally aim for a spacing on the
order of 0.01-0.05 wavelengths at the outer edges, which on 80m would be
well over 2 feet. From that perspective, a 4-6in mesh size isn't bad at
all. Nearer to the base of the vertical radiator, the overlap between
the rolls of mesh will also provide closer conductor spacings where they
are needed the most.
Where all the rolls of mesh meet at the center, they can be bonded
together using these little gadgets:
http://www.rappa.co.uk/products/151-four-way-line-clamp-25-pack These
clamps are strongly made and heavily galvanized, so they can be cranked
up tight to form solid cold-welded bonds, zinc to zinc. Again the same
principle applies: apply cross-bonding in many separate places to create
broad distributed current paths.
Obviously a few rolls of mesh won't make an ideal ground, but at least
they have made an *achievable* ground, in a location where traditional
radials would have been impossible.
73 from Ian GM3SEK
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