Ferrite cores in TV line output and switchmode power supplies often have an
air gap. This means that the effective permeability is not very high. This
is done because the transformers in these circuits are used as inductive
energy storage devices, not as true transformers. The inductance of the
winding is much lower than the winding of a true transformer with the same
number of turns.
For LF transmitter use, we don't want to store the energy, just have a
transformer with a high inductance which doesn't shunt the load, so we don't
want a gap in the core. In the case of TV line output transformers, the gap
is usually formed by means of two thin mylar spacers between the two halves.
Unscrew the clamp and the two spacers will fall out and can be discarded.
In the case of 'E and I' cores in switchmode PSUs, the same spacer idea may
be used, so if you are recycling one of these for LF transmitter use, just
remove the spacers. But in some cases the gap is built into the centre limb
of the 'E' part, and if you examine the E core carefully you will see that
the centre limb is shorter than the outer limbs and there's an air gap when
you clamp the E to the I. Cores like this will be no good for our purposes!