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Re: LF: Coil winding conundrum

To: [email protected]
Subject: Re: LF: Coil winding conundrum
From: "Peter Dodd" <[email protected]>
Date: Wed, 12 Apr 2000 21:19:05 +0100
Reply-to: [email protected]
Sender: <[email protected]>

G4JNT> Both coils were the same diameter, same inductance, and roughly the same length in total. So why was the one made of plain wire better ? Self capacitance ?
Gamal> No. Unless the "capacity" exhibits significant dielectric losses,
great self capacitance does not harm in lieu of Q.
What you probably have missed is to keep distance between
individual turns of the coil's winding. The effect of closely
winded turns is named proximitty effect. Current in one turn
tends to displace the current in the adjacent turn and vice versa.
When constructing loop antenna windings, I try to keep a clearance
of at least 2 times the wire diameter in between individual turns and
always get optimum Q. For solenoid type of coils I recommend a minimum of 1 wire's diameter of clearance.
As the self-made Litz wire has a greater outer diameter, it's
relative close proximitty to the next turn reduces Q much more
than in the case of the solid wire.
I use a 400uH commercial variometer in series with my main loading
coil for tuning. This single layer solenoid coil is wound with Litz
wire and the spacing is about 1.5 wire diameter.
The main loading coil is wound on plastic fencing material formed
into a cylinder. It was not possible to maintain a one wire spacing
so I compromised and wound the coil in 'bunches' of 10 windings.
A lot of effort goes into the design of professional LF coil design,
to reduce adjacent wire proximity while retaining a managable small
size as can be seen in the design of the Decca transmitter coil
formers (hopefully, more will be reveilled when I can figure it out).
Fairly complex coil design articles appeared in early publications,
which described basket and honeycomb winding patterns.
I will try to get some of this into the next edition of the LF Book -
the main problem is the illustrations!

--
Regards, Peter, G3LDO

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