Topband: Binocular transformer design

[Dan Zimmerman N3OX]

>
> My concern is that if I bend the one
> wire around the outside of the core to make for a reasonable connection
> with the other wire, characteristics will change since the wire will be
> necessarily in close contact with the core wall


On the low bands you're probably talking an ohm or less of stray inductive
reactance.

Just try it.  I just did.  I took about three inches of wire and put that
across my MFJ-259B

It reads 0+j0.

When I bring one of those binocular #73 cores  near the wire, it continues
to read 0+j0

When I make a half pass and just thread the wire through one side of the
core, it reads 35+j43

This is a pretty typical property of magnetic circuits, actually, when air
is involved.  Even if the wire is taped right to the side of the core, the
field encircling the wire is only concentrated in the core for less than
half of its way around the wire.  The rest of the way, the field goes
through air.

When the wire passes through the hole, all of the field along that length of
wire is contained in the core.

It takes a surprisingly small air gap to totally change the properties of a
magnetic core like that.  Even a few thousandths of an inch air gap in a
toroid would cause the impedance to plummet.    It's why split beads are
ground so flat on the mating surfaces.

In fact, you can test this with a split bead if you like.  Measure the
impedance of the bead with and without a couple sheets of paper in between
the mating faces... bet you'll be surprised.

I just tried it with a big (I think #31) clamp on on 4MHz.

Impedance with the core snapped shut, one pass through?  40+j70

Impedance with ONE sheet of paper on ONE side of the core?  13+j38

So 0.003 inches of gap more than halves the impedance.  If you consider that
the "gap" involved when you've got wire *outside* the core is basically 2/3
or something of the magnetic path around the wire at least, you can see that
the core might as well not be there.

I think you'd find it hard to measure the difference even with good lab
equipment.

73
Dan