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From: commgrp@silver.ucs.indiana.edu (BACS Data Communications Group)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Litz wire (long)
Message-ID: <41761@iuvax.cs.indiana.edu>
Date: 13 Apr 90 15:40:56 GMT
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>In article <1990Apr12.071836.17112@monu6.cc.monash.oz> 
>steve@monu6.cc.monash.oz (Steve Balogh) writes:
>>The individual strands of wire in Litz wire are NOT insulated from 
>>each other. Litz wire is essentially LOTS AND LOTS   A N D   L O T S   
>>of very fine (and sometimes silver-coated) wire all twisted 
>>together. Because of the greater surface area of such wire, it is 
>>supposed to have less impedance at RF frequencies than normal single 
>>strand wire.
 
Jeff Winslow replies:
>Oh, really? And how can it have greater (electrical) surface area if 
>all the surfaces are in electrical contact? What keeps the current 
>inside the fine wires and away from the bundle's surface? (Especially 
>if it's silver plated.)

>Maybe usage varies in different parts of the world... I looked it up 
>in a couple of electronics dictionaries as well. They both indicated 
>specifically that the wires are individually insulated.
 
Stranded wire has less skin-effect resistance than the equivalent 
solid wire, but much more than true litz wire with individually-
insulated strands.

Some large/expensive stereo speaker cables are being marketed as 
"Litz" and are made of many very fine (but not individually insulated) 
strands.

Skin-effect resistance can be significant in large-diameter conductors 
at audio frequencies.  Whether it makes noticeable difference over the 
length of speaker cables is another interminable argument for 
rec.audio (like the one about mercury-filled plastic tubes which were 
supposed to give your speakers a more "fluid" sound. :-)

Here's a list of AWG copper wire sizes and their "critical 
frequencies" (frequency above which skin effect causes resistance to 
be significantly greater than DC resistance).  I don't know the 
criteria for "significantly greater."  The table partially reproduced 
below was in the Canadian LF/VLF newsletter _Northern Observer_ issue 
#9 (April 1989).

AWG       dia.      critical 
size      mm        freq.
----------------------------
1         7.35      586 Hz
2         6.54      738
4         5.19     1017
6         4.12     1.87 kHz
8         3.26     2.97
10        2.59     4.72
12        2.05     7.50
14        1.63    11.9
16        1.29    19.0
20         .812   48.0
22         .644   76.3
28         .321   307
32         .202   775
36         .127    1.96 MHz
40         .080    4.96

In addition to skin effect, wire resistance can increase by "proximity 
effect" where a strong magnetic field forces the current toward one 
side of the conductor.  That's a reason why some cyclotron magnets are 
wound with flat "wire."

--

Frank Reid     W9MKV     reid@ucs.indiana.edu