David:
I have another message lost in the aether on the subject. I strongly
suspect the Drake tuner has a ferrite core balun. Ferrites can be
conductive, with a bulk resistivity ranging from 1E9 down to 50 ohm-cm,
depending on the material.
Hence if the balun is handling any degree of power, there may be a
significant turn-to-core voltage that causes arc-over. The voltage
turn-to-turn is less, of course, as the total voltage is more or less
evenly divided amongst all turns. But if the core has even moderate
conductivity, then the total voltage across the first and last turns
will appear between the first and last turns, insulated only by the wire
insulation.
Hence, spacing or wrapping the core is useful.
Jack K8ZOA
David Cutter wrote:
Your comment about spacing off the core is interesting: my Drake AMU
has an attached balun which is constructed with the wire wound on a
former which stands the turns off the core. The wire is 1mm diameter
bare wire and does not touch at any point. I have never seen this
anywhere else. The core is also bare, ie no visible coating. I do
not know its material.
David
G3UNA
----- Original Message ----- From: "Jack Smith"
<[EMAIL PROTECTED]>
To: "Ron D'Eau Claire" <[EMAIL PROTECTED]>
Cc: <[email protected]>
Sent: Monday, February 26, 2007 6:17 PM
Subject: Re: [Elecraft] Toroids - Effect of Winding Spacing on
InductanceandDistributed Capacitance
As far as cores being non-conductive, that is true for powdered iron
(the typical red and yellow cores) but it is not necessarily true for
ferrite cores. The bulk resistivity of ferrite materials varies
widely from 1E9 down to 50 ohm-cm, and not all (few, actually in my
limited experience) ferrite cores are coated with a insulating
surface. In addition, many ferrite materials have an extremely high
dielectric constant, which suggests the need for a bit of spacing
between the core and the winding to reduce distributed capacitance
arising from paths through the core (obviously more important when
the core is more
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