On 3 Nov, 07:02, art wrote:
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.
Ian
Reading the two analysis on braid and RF. It seams to be missinbg
something
or I have things totaly wrong about this. With multiple wires with DC
the
resistance will go down but with RF you have competing SLOW WAVE
phenomina,
bevause each wire is now insulated from each other which prevents wire
hopping.
In other words the corrossion or insulation would DECREASE resistance
would it not? If it is used as a dipole I can see the IMPEDANCE going
up since
the radiator lengths has DOUBLED!
As far as skin depth is concerned I would imagine/think the skin depth
is some what altered
because of the compensating increase in area exposed to RF despite the
fact that
braid presented only half of its useful area!
Could you please point out the errors in my logic?
Regards
Art
However, the original question was about a very short braid connection
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73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek-Hide quoted text -
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Ian, to put things more clearly the model I was analysing
was for the frequency rate of 1 Mhz where the analagy was
the use of Litz wire where slow wave contra helices were introduced
and where radiation only oceres in places where the wire is
exposed to the air.
Regards
Art