Dear Mike, LF Group,
Placing a ferrite core in a coil will always increase the inductance,
and will also result in losses due to hysteresis, eddy currents, and
dielectric loss in the ferrite, all of which result in heating of the
core. Whether the Q increases or decreases depends on many
things, such as the type of core material, the geometry of the core
and the number of turns and disposition of the windings. At LF,
ferrite losses are usually quite low. For a given overall size of coil,
the reduced number of turns required to obtain a given inductance,
and the thicker wire which can be used as a result, reduces
resistive losses in the wire, and usually more than compensates
for the additional losses due to the ferrite. So the ferrite cored coil
can have a higher Q. However, you can usually make an enormous
air-cored coil if you want, wheras very big ferrite cores are hard to
come by, and are very heavy. But if small size is important, ferrite
cores could be worth a go.
One problem with ferrite cores is saturation - if the magnetic flux in
the core exceeds a certain point, the core effectively loses it's
magnetism over the peaks in the current waveform in the coil,
which can give rise to harmonic generation as well as excessive
heating of the ferrite. If you put a ferrite core into a coil carrying a
particular current, the magnetic flux density there is in the core
depends on the shape of the coil and core - a long, thin rod along
the axis of the core will be subject to much greater flux density than
a short, fat core. So poking ferrite rods into loading coils gives
them a really hard time, and a given type of rod should show less
heating if several short ones in a bundle are used instead of one
long one . The flux density also depends upon the permeability of
the ferrite - lower permeability means lower flux density, which may
be why some ferrite rods work better than others. However, lower
permeability or increased cross-section means that more ferrite is
required to reach the neccessary inductance. The inevitable result
is that a large mass of ferrite will be required for high power use.
Another factor that is important when deciding the type of coil to
make is the voltage it has to withstand. A 7mH coil with, say, 3A @
136kHz going through it has about 18kV across its terminals, So
insulating an inductor wound on a pot-core would be quite a
challenge at this level, especially for outdoors use. Although it may
give lower inductance for it's size, a solenoid has the advantage
that the high voltage points are as far apart as possible.
Cheers, Jim Moritz
73 de M0BMU
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