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

Date: 18 Jul 2005 14:18:03 -0700
From: dave.harper
Newsgroups: rec.radio.amateur.homebrew, rec.radio.amateur.misc
Subject: Simple questions on receivers

-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?

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

Easy of calculations?

2. Definitely.

I assume you can get higher performance from ferrite coils than
air-core coils, right?

You don't need as much ampere-turns to get a given amount of inductance
and thus, ohmic resistance will be less, therefore higher Q (in theory).

caveat: the magnetizable material you use for the core (i.e. iron,
ferrite, and other stuff that I think other guys here surely know better
than I) will have a big effect on useable frequency on up to some cut-off
threshold that may be sharp or spread out. Pure solid sheet 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?

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.

Thanks again!
Dave




Art, W4PON