Dear Uwe, LF Group,
At 02:06 06/04/2003 +0200, you wrote:
Hi Walter,
quotation Terman ed.1943, page 128 on shielded air coils:
...shields... increase the losses as a result of energy dissipated
in the shield by the eddy currents. However, if clearence between
the coil and its shield is everywhere at least equal to the radius
of the coil, the reduction in inductance and in the decrease in
coil Q by a copper or aluminium shield will not be serious.
end of quotation.
grounded or not grounded, the eddy currents in the shield cause losses
(short-circuit).
Adding a conducting shield will inevitably lead to some additional eddy
current losses - however, it will also shield the electric field of the
coil, which will reduce dielectric losses in the environment outside the
shield. Because of the high voltage/high impedance nature of most amateur
LF antennas, dielectric losses may well make up a large proportion of the
total loss associated with the coil - so adding the shield could well
reduce total loss.
Cheers, Jim Moritz
73 de M0BMU
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