Formers used for winding coils are ceramic or glass, because they are stable. Copper wire with silver plating is used up to 50 M Hz. Lit wire produces lower RF resistance than solid copper but it is effective below 2MHz. Above this frequency , RF currents flow through outside conductors of stranded wire. At these frequencies, universal winding is used to reduce self capacitance. In this winding, the wire is oscillated from side to side as the coil is wound.
HONEY COMB COILS: These windings are relatively widely spaced and turns per layer give honey comb appearance.
PROGRESSIVE UNIVERSAL WINDINGS:
It is similar to universal winding except this instead of spilling back and forth in same area, the spiralling also progresses along the former. With this type of winding, it is possible TO HAVE HIGH PERMEABILITY in tuned inductors.
QUALITY FACTORS OF COILS
It is figure of merit of coil.
Q = XL/R = tanφ
Q is dependent upon frequency. If Q is high, then the sensitivity of circuit is good.
Choice of size of wire: For audio frequencies, standard wire gives good response. At higher frequencies, it is desirable to use many standards to have high Q. Cost increases with more standards.
CHOICE OF WIRE INSULATION: The insulators control the spacing between wires and winding size. It is necessary to select insulators with low dielectric losses. Enamel coated multi stand wires insulated with flannel of silk, cotton, glass, etc.
Choice of formers: The size of former is decided by space limitation, winding size, etc. The material used for the former should be strong, dimensionally stable with temperature changes and electrically good but fragile and costly. Peonies are less costly but vary widely in their electrical properties. In transmitter coils, ceramic formers are used.
Choice of finish: Wax varnishes or lacquers are used for final finish to make them mechanically and electrically stable.
Choice of shielding: To prevent undesirable coupling between inductors, it is necessary to use a shield on it. Shield is a conducting material, copper or aluminium. Aluminium is less costly. As the shield is not a perfect conductor, losses add to the losses in inductor.
Coupling of coils: To obtain close coupling, second winding is wound is continuation with first winding, separated by a layer of tape. For loose coupling, winding are placed apart on the same former or placed side by side with ferrite rods.