It is often necessary in rf circuits to have an inductor which can be adjusted over a frequency range. Variable coil forms are a high grade product with iron powder slugs and shielding pots. The assembly is enclosed in a tinned copper shielding can. These coil forms are available in three sizes using four material grades ( 2, 6, 10, 17). Material type is indicated by a coloured dot on the base of the former and slug. The above materials cover a frequency range of 1 - 200 MHz, and are suitable for pc board mounting. Type L-33-( ) form is 0.31" square and normally has 4 pins, L-43-( ) form is 0.43" square and has 5 pins while the L-57-( ) is 0.5" square and has 6 pins. Details of these coil forms is given in the diagrams. The AL values are given for each type and enable you to make a close approximation to the required inductance when winding a coil. Use the formula to calculate the turns required.