Gary, K9AY is quite correct in his response that [copper] mesh used as an
artificial ground at HF and VHF has proven quite effective and that radar
and satellite antennas use perforated reflectors to save weight [I presume
Gary is refering to aluminum mesh reflectors in this case due to weight
concerns].
However, I interpreted the original point of discussion to be related to
ferromagnetic materials, such as typical chicken wire, fence wire, etc.,
which is usually uncoated or galvanized soft-iron wire. In my statement to
the effects of eddy currents, I was specifically referring to eddy current
losses in ferromagnetic materials, aka, iron wire (chicken wire mesh, iron
fencing, etc.). Significant Joul heating of the material is caused by eddy
currents flowing which are proportional to the square of the frequency,
square of the AC (RF) flux density and inversely proportional to the
resitivity of the material (iron vs copper in this case). Ferromagnetic
materials which exhibit low eddy losses typically exhibit very poor
performance at RF due to their high losses and RF resistivity.
In order to reduce the eddy losses in iron materials, the electrical
resistance of the material is usually raised to as high an electrical
resistance as possible (by manufacturing processes involving the addition of
non-conductive binders to the conductive material...I cite for example, the
silicone-iron materials used in transfomer laminations for [the] specific
purpose of reducing eddy losses, and this is counter-desirable for RF return
currents to the feedpoint of a vertical element. It also makes the material
VERY brittle and expensive to manufacture, and really doesn't apply to the
materials being discussed here, aka chicken wire, etc.
In the case of a 1/4-wave vertical radiator with it's typically low base
impedance, the electromagnetic field predominates (versus the predominant
electostatic field of a high impedance radiator, such as a 1/2-wave
vertical). A radial wire ground system constructed of copper wire will
minimize the I-sqrd-R losses while a significant mesh ground system
constructed of ferromagnetic material will contribute additional system
losses over and above those of a relatively small radial field, as W8JI has
indicated in his comments.
73, Dave, K1FK
Fort Kent, ME
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