Braid
On 2 Nov, 21:19, Roy Lewallen wrote:
Stefan Wolfe wrote:
Actually braid has muxh less 'resistance' than a flat or round conducor at
RF due to greatly increased surface area and skin effect.
Sorry, that's simply not true, except at lower frequencies where the
skin depth is comparable to the wire diameter. It's at those frequencies
that Litz wire can provide some advantage. But it's made of separately
insulated strands. Because of skin effect, the current at HF (or
wherever the wire diameter is at least several skin depths) is only on
the outside surface of the braid, not the outside surfaces of all the
wires. The extra loss comes from the necessity of the current moving
from one set of wires to another as the original set goes under an
adjacent group. Surface roughness in itself can significantly increase
RF resistance (cf. Johnson and Graham, _Signal Propagation - Advanced
Black Magic_, Sec. 2.11), but the braid structure increases the
resistance more yet.
I know Tom, W8JI, has measured the impedance of solid strap and compared
it to braided shield, and confirmed that the shield has substantially
higher RF resistance. I don't see measurement results on his web site,
but he briefly discusses the phenomenon athttp://www.w8ji.com/skindepth.htm.
One other note - for the same surface area, a round conductor has the
least RF resistance of any conductor shape. The reason is that the
current is evenly distributed on the surface. On other shapes it's not,
resulting in higher resistance per unit length.
Roy Lewallen, W7EL
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