[TowerTalk] Radial field question Single radial wires vs meshand more

[Herbert Schoenbohm]

Because of the high amount of copper theft in recent years some AM 
stations in Puerto Rico hjave resorted to using barbed wire in their 
ground system.  Its cheap and lasts a long time but hard to handle.

Herb, KV4FZ
On 10/28/2016 6:46 PM, Ian White wrote:
> 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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