My comments are interspersed.
-Bill

straydog wrote:



On Mon, 18 Jul 2005, dave.harper wrote:


Is this the reason a lot of coils are air coils?


1. Air core coils will be cheaper, lighter in weight, easier to make.

A typical medium-sized ferrite toroid coil for BCB use, FT-82-61 for
instance, costs about US$1 and can't weigh more than an ounce and uses
up about 4-5 feet of wire.


Easy of calculations?


2. Definitely.

Different calculation but one is as easy as the other. Just look for an
online calculator

iron, for example, might be good at audio frequencies and maybe up to
x00,000 Hertz, but you need powdered iron to go into the megacycle
range. There are other core substances that get you up higher. Anyone
else care to add to this?

True. There are two main substances used in ferrite toroids - and I
can't quote either name - and they have vastly different permeability
characteristics. I think the CWS-Bytemark website goes into some of
these details.


Don't forget that winding a torroidal coil is not so easy. Some cores
are available in halves so you can make "pies", otherwise the "bobbin"
carrying the wire has to pass through the hole of the doughnut many times.

I've been referring to the simple doughnut cores. A BCB coil takes
around 50 turns on -61 material. Even with my fat fingers it only takes
about 10 minutes.

-Bill