TABLE OF CONTENTS Cylindrical cores longer than a solenoid winding are used to increase inductance for a given physical size. Cylindrical cores shorter than the solenoid winding and moved along the winding axis are used for inductance tuning with the greatest inductance occurring with the core centered.
The effective permeability of a core, ? e, is the factor by which the inductance is increased over the value with an air core. For a cylindrical core, the effective permeability is as much a function of the l/d ratio of the core as it is the initial relative permeability of the core, ? r. The effective permeability is related to the material permeability by the relation [8]
| (25) |  |
where N f is the demagnetization factor. A curve fit to the demagnetization factor data in Snelling is
| (26) |  |
where l/d is the length to diameter ratio of the core, not the winding. The validity ranges are 1 < l/d < 100, 1 < ? r < 1000 and a winding which is evenly spaced and occupies 85 to 95% of the core length. For example, ? e of a cylindrical core with ? r equal to 125 and an l/d ratio of 4 is 16.1 and with an l/d ratio of 8 is 34.9. Plots of ? e versus ? r for various l/d is given in Figure 3-10 for solenoids which are evenly spaced and occupy 85...
