Message-ID: <4E5CE126.2090203@iup.uni-heidelberg.de>
Date: Tue, 30 Aug 2011 15:09:58 +0200
From: =?ISO-8859-15?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: HF Litz wire for TX coils?
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LF,

I plan to build a rather compact LF TX coil with reduced losses. Now i
think about using a bigger enamelled wire (currently using ordinary
loudspeaker cable). A local distributor (http://www.elnatrafo.de/)
offers 2mm diameter enamelled copper wire for about 10 EUR/kg which is a
fair price. The current wire is 0.75mm^2, so about 1 mm diameter.

HF Litz wire may have lower losses at 136 k but what about the current
capability / ampacity? I searched for some data of the litz wire i used
for the ferrite antenna: A litz wire of 120*0.1 mm
(http://www.reichelt.de/index.html?ACTION=3;ARTICLE=57209;SEARCH=CLI%20200%2F120)
is 0.943 mm^2 but handles just 3.36 A.

A single wire with 2mm diameter (assuming 0.18mm skin depth at 136 kHz)
has a effective cross section area of 1.02 mm^2, so about the same as
the Litz wire i mentioned. But the single wire is more robust, cheaper
and easier to wind. Also the heat transfer could be better with a single
wire when using a PVC tube for the coil.

I need about 2.9 mH for resonating at 137 kHz  (yes, Mal, we know, you
have below 1 mH our youzr inv-L). This could be achieved by using a 20
cm diameter tube and about 120m of wire. http://dl.dropbox.com/u/19882028/LF/coil%20calculation.png

Any comments or suggestions?

73, Stefan/DK7FC

PS: What do you think about a coil body out of glass? I have to take
very special care here not to burn down the institute but i want to
remote control the system, so mostly i cannot watch the coil and have no
XYL walking arround in the garden, telling me if something begins to
smell ;-)