Gary wrote:
>
>I am wondering why even bother with mesh wire. Radials provide a return
>current path back to the feed point so current should mainly flow along
the
>length of the radial wires. Having shorts between the radial wires as a
mesh
>provides, would seem of little benefit. Any current flowing between
radials
>via the shorts would seem to result in loss current.
>
The wires at right-angles to the main direction will probably have
little electrical effect; but if the currents in those wires are small
then the losses (depending on I-squared) are negligible.
But the mesh construction does change a collection of parallel wires
into something *mechanically* different, that can easily be rolled out
in a single broad sheet that does not require the wires to be pegged
down individually. At this QTH with very uneven ground, these features
allowed quick and easy deployment over very uneven ground. Some of the
lengths of mesh were weighted down with stones for a season, after which
they were permanently held in place by new undergrowth. Three of the
other lengths are rolled up every summer to allow mowing, and then
rolled out again for the winter DX season.
I am aware of the advantages of conventional radials, having a 40m
4-square with four sets of 64 conventional radials elsewhere on the
property. But that required the entire area to be cleared first using a
backhoe blade, which is simply not possible where the mesh radials are.
>As to using any kind of steel material for radials the loss due to eddy
>currents is going to be rather high, the same problem that steel
antenna
>wire presents.
>Yes, steel radials are better than none, same as a steel wire antenna
is
>better than no antenna
You have only one antenna wire, but hopefully the number of radials is
very much larger so the two applications are not the same. As I already
pointed out, the more radials you have, the less current each individual
wire has to carry and the less important the wire conductivity becomes
(that I-squared effect again).
Also the skin effect means that more of the RF current flows in the zinc
coating (in fact the magnetic permeability of the steel helps to expel
the RF current outward into the zinc layer) and zinc is quite a
reasonable conductor - admittedly not as good as copper, but once again
don't forget the point I just made in the previous paragraph.
>but for all the effort to put the steel wire in or on
>the ground it would seem a much better case for copper.
>
It all depends on the local ground conditions. In the end, that is
probably the *only* generalization we should all agree on.
G'night all.
73 from Ian GM3SEK
>-----Original Message-----
>
>> 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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