Well, it's less of a problem than I'd have guessed. It turns out that
I was already set up to measure inductance and Q of little toroid
coils, since I'm winding some for some filters. These are on T25-6
cores. The one I measured for the results below has 22 turns of AWG28
wire on it, essentially evenly spaced around the core, with a gap of
about 45 degrees between the wire ends. That is, the wire occupies
about 7/8 of the core. I won't claim high absolute accuracy, but the Q
measurement should be well within 10%, and the relative accuracy (from
one condition to the next) should be much better than that. In
particular, I can guarantee the direction that Q moves as I add
shielding. The first measurement is with the toroid inductor standing
vertically off a copper ground plane, with the core at the gap in the
winding about a millimeter up from the plane. The second measurement
adds a piece of copper foil tape stuck to the ground plane, and bent up
at right angles to the ground plane so it's immediately adjacent to one
side of the coil. The third measurement folds the copper foil over the
top of the toroid and down the other side, so the coil is surrounded by
copper foil. These are labelled 1, 2 and 3 below.

Inductance Q
1 1.355uH 160
2 1.349uH 158
3 1.335uH 157

The actual drop in inductance is almost certainly a little more than
indicated. That's because the copper foil adds capacitance, which
would lower the resonant frequency and be seen by my measurement
technique as increased apparent inductance. I could resolve that by
making a similar measurement with lower resonating capacitance, but I'm
not set up to do that right now. Of course, the lowered inductance is
because the shield reduces the volume occupied by the magnetic field,
so there is less energy stored in it for a given current. But these
measurements tell me that even for the low-mu type 6 cores, there is
quite a low external field.

Cheers,
Tom

Roy Lewallen wrote:
John Hague wrote:
On Tue, 11 Jul 2006 08:29:05 +0100, Roy Lewallen wrote:

Ben Jackson wrote:
. . .
Anyone have any tips for securing fine-wire toroids in
Ugly/Manhattan construction?

A dab of hot melt glue or RTV. Or a Nylon screw through the middle. Or
a couple of holes in the board and a cable tie. The only thing to
avoid is laying it down flat on a solid copper plane.

Roy Lewallen, W7EL

Roy

Excuse my ignorance, but what is the problem with laying a toroid down
on a solid copper plane? I thought the magnetic field was contained
within the toroid and thus minimised external effects. I have completed
a couple of projects recently with some of the inductors like that and
didn't notice any real problem. Mind you, I guess they might have
performed better if not mounted that way )

Two potential problems. One is that the field isn't completely
contained. Leakage is greater with more sparsely wound toroids and ones
with lower permeability cores. The second is the "one turn effect" -
There's a net field equivalent to that of a single turn running
circumferentially around the core. A solid plane parallel to this would
act as a shorted turn. Both effects would act to lower the Q, and might
be the cause of some drift or microphonics if the inductor was in an
oscillator tank.

But to be honest, I've never run any experiments to see just how much of
a problem it might cause -- it's quite possible you could get away with
it in some or even most applications. I'll put it on my list of things
to do when time permits -- unless somebody else is willing to take on
the job.

Roy Lewallen, W7EL