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art · November 3rd 07, 01:36 PM posted to rec.radio.amateur.antenna

On 3 Nov, 01:27, Ian White GM3SEK wrote:
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 at
http://www.w8ji.com/skindepth.htm.

The higher RF resistance of braid is very noticeable in tube power
amplifiers at HF, where the circulating currents are magnified by the Q
of the tank circuit. If the connections between the coil taps and the
bandswitch contacts are made from braid, they can run very hot, while
thin strips of solid copper give no problems at all.

For antennas, the main concern about using braid is corrosion, which
will insulate the strands from one another and greatly increase the RF
resistance. This is why the loss of normal braid-covered coax increases
dramatically if water gets under the jacket.

However, the original question was about a very short braid connection
in a non-critical application. There must be hundreds of 18AVTs out
there, all with corroded braid. Nobody notices the difference, so that
proves it's non-critical, right? :-)

The purpose of this braid is to prevent failures of the solder joint to
the SO-239 after a few months/years of flexing in the wind. John's
original suggestion of a small loop of insulated stranded wire would do
equally well - one turn around a pencil, say. Sealing the two solder
joints with hot-melt glue will finish the job nicely.

The tiny additional series inductance will not be significant at HF.
When the elements are adjusted to length, it will vanish completely.

--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek- Hide quoted text -

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