> There is another "negative" aspect to using mesh, chicken wire, welded
> fencing, etc., in the ground system of verticals, and that is the losses
> these screens incur due to eddy currents created by the closed loops of the
> mesh. These eddy currents can be considerable, especially in higher power
> systems, and add loss to the overall system.
>
> 73, Dave, K1FK
> Fort Kent, ME
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Dave and all,
Mesh conductors are not a significant problem. Radar and satellite antennas
often use perforated reflectors to save weight. At the lower 'microwave'
frequencies, grid reflectors are common. The military and the FAA have
repeatedly proven the value of a copper grid or mesh as an 'artificial ground'
at HF and VHF. A widely held rule of thumb is that apertures less than 1/10
wavelength (~50 feet at 160M or ~1 inch at 1 GHz) are good approximations to a
solid surface at RF.
However, this does not answer the grid or mesh question for radials...
Unlike a reflector that sees a relatively uniform wavefront, a radial system has
increasing current density near the base of the vertical, where losses should be
minimized. This requirement is readily met with individual radial wires that are
automatically spaced more closely near the base. It can also be done with a
dense mesh of wires or metal screen with 'holes' similar in size to the spacing
of the radial wires. Chicken wire seems appropriate, except for the relatively
poor conductivity of steel. A better use of chicken wire is as a cheap way to
make a temporary improvement to a marginal or inadequate radial system until
more copper radials can be added.
73, Gary
K9AY
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