dave.harper wrote:
-ex- wrote:


I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise.


Is this the reason a lot of coils are air coils? Easy of calculations?
I assume you can get higher performance from ferrite coils than
air-core coils, right?

Thanks again!
Dave

Most crystal radio builders go with air core coils...partly because
thats the way it has been always done and such plans are available and
thats the way its supposed to 'look'
And you CAN make a better Q air coil than what is attainable with
ferrite.

When I say unpredictable about the toroids, the calculated
turns/inductance comes out the same but not the Q. I've wound the same
coil on half-dozen 'exact same' ferrite cores and gotten Q ranging from
below 200 to nearly 400. Those numbers (in my set) cover the range of
not-so-good to pretty-darn-good. I'm not sure why that is other than
its not something intended to be. You won't readily find Q charts for
ferrites like you do for iron powder cores. As a consequence of this
you don't have any guarantee that the cores you get are going to make it
to the p-d-g range.

There's some advantages and disadvantages with using ferrite cores. The
size is the most obvious advantage. Among other advantages - they are
not affected by nearby components and do not pickup signals from the air
(self-shielding). Thats why I got started with them...I have a 5kw BCB
tower about 1600 feet from my QTH! With air core coils the band is
totally swamped.

On the negative side...the ultimate Q-limitation seems to be about 400,
inability to have variable loose-coupling to traps and other stages are
most notable. They also don't have the "looks cool" factor like a big
coil

Now back down to earth. A random ferrite core inductor is going have a
MUCH higher Q than the average coil wrapped around a toilet tissue tube.
You're lucky to get 80-100 with that type of coil. It may not play out
as being important in a very simple circuit that has other shortcomings
but as mentioned before it also helps to have a high starting point on
as many of the components as possible.

Anyhow...its something else to tinker with!

-Bill