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How to make homebrew
coil inductance

homebrew coilFor beginning hams it would be desirable to tell about features of coil inductors design a little. The basis of any coil is any pipe, named the coil-former on which the wire in the form of a spiral is wound. Normally beginning radio amateur repeats a construction in which description it is specified that it is necessary to wind N-turns on a former in diameter D. But it is very frequent the necessary coil-former available is not present, and there is another. Then there are following questions:

  1. How many turns it is necessary to wind on other former?
  2. Is this former suit and how device characteristics will be changed?

Program Coil32 easily solves the first question. Knowing circuit parameters into which the inductor enters, or its constructive sizes and number of turns from the device description, it is possible to calculate its inductance, and knowing inductance - to calculate number of turns for a new former.

The second question is necessary to understand more in detail. What parameters characterize the coil inductance?

  • First of all, it is the value of inductance
  • Q-factor of the coil inductance, characterizing value of losses in it
  • Parasitic own capacity of the coil inductance
  • Temperature instability of inductance

Value of inductance is normally directly proportional to diameter of the coil and a square of number of turns. pot core inductor To reduction of overall dimensions of the coil and number of turns apply magnetic cores - ring, armored. The pot core inductance is shown on a picture. However they have frequency restrictions and on power. For example, in filters for acoustics their application is inadmissible, since at the big power because of singularities of a magnetic material, value of inductance will depend on signal amplitude and it, accordingly, will strongly be distorted. In output cascades of transmitters and acoustics filters to reduce overall dimensions of coils it is impossible, losses thus increase, and you do not want that power of the amplifier would go on wire heating.

Q-factor is important for oscillator coil inductances. It inversely proportional to value of resistance of losses in it. Once, I with surprise found out that my site was searched by Google query - "Unit of measurements of Q-factor of the coil inductance". Q-factor is measured in the relative units and has no special unit of measure (type the Ohm, Kg). Strictly speaking, Q-factor is a relation of reactive impedance of the coil inductance
XL = 2πƒ L

to its resistance of losses. Losses are added from losses in wires, dielectric, the core and the shield.
Losses in wires are caused by three reasons:

    • First, the winding wires have ohmic resistance (to DC), so the coil should be wound wire with the lowest resistivity (copper, silver)
    • Second, the wire resistance to AC increases with increasing frequency, which is due to skin-effect, the essence of which is that the current does not flow over the section of the conductor, and on the outer ring of the cross section.

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  • Third, in the winding wires, the suite into a spiral, occurs proximity-effect, the essence of which consists in forcing the current to the periphery of the wire adjacent to the former, resulting in cross-section, in which the current flows, the character takes the crescent, which leads to an additional increase in the resistance wire . Reduce losses due to the proximity effect can be applied to the winding pitch. There is an optimal winding pitch depends on the geometry of the coil.

 

On frequencies to 1 - 3 MHz to reduce losses in wires it is possible applying a Litz wire consisting from larger number of thin wires, twisted in a bundle. At small diameter of thin strands the skin-effect is weakened, and twisting of strands in a bundle weakens proximity-effect. On high frequencies the wire should be polished since roughness increase by its surfaces its length for a high-frequency current and accordingly a loss resistance. See more about q-factor calculation in Coil32.

 

Dielectric losses due to the fact that an electromagnetic wave passing along the coil loses energy by interacting with the material of the former. cap These losses similar to losses in capacitors or coaxial cables and are dependent upon the quality of material(tgδ). Reduce these losses can be applied ribbed formers, resulting in a form of coils become polygonal, or complete failure of the former.

 

Losses in the core are directly proportional to frequency and power transiting through the coil and depend on a core material. On high frequencies, to reduction of losses apply non-magnetic brass insert cores, or apply air core coil inductance without any coil-former.
Losses in the shield are caused by that the current flowing on the inductor, induces a current in the shield. For their reduction the shield should defend further from the coil. Diameter of the shield should exceed diameter of the coil not less than in 2,5 - 3 times. Under the influence of the shield is reduced inductance. The degree of this reduction can be estimated using the screen plugin

For single-layer coils - increasing the dimensions, keeping constant the value of inductance and coil shape, Q-factor is approximately proportional to the square root of the diameter of the coil. In addition, the quality factor depends on the ratio of length to diameter of winding and has a blunt maximum at l / D ≈ 1. For such an optimum coil winding pitch substantially equal to twice the diameter of the wire (in other words the distance between the turns must be equal to the wire diameter).

The Self-capacitance is stray parameter onelayer of coil inductances, restricting possibilities of its application first of all on frequency, Since this capacity is added to a LC circuit capacity. The self-capacity together with self-inductance of the coil are forming LC-circuit whose resonance frequency is called a self-resonance frequency of the coil. Above this frequency, the use of the coil is meaningless, because in this case it has been capacitance. Clearly, the need for being able to reduce self-capacity. The minimum self-capacitance have single-layer air core coil inductance.

In singlelayer coil self-capacitance is proportional to the diameter of the winding, and also depends on the ratio of length to diameter of winding and has a blunt minimum at least l / D ≈ 1. Increasing spacing between the turns reduces the inductance of the coil, and the self-capacitance does not change.

multilayer The self-capacitance of the multilayer coil inductance is much more, to its reduction apply basketweave winding, or the partitioned winding. Wire covered in a silk also reduces this capacity.

 


Choice of formers of coil inductance depending on service band and assignment can be made by the most various methods and from various materials (a paper, a press-spahn, organic glass, high-frequency ceramics and various high-frequency materials). The coil-former material influences inductor Q-factor. Concerning electrical characteristics the best, not demanding impregnations and moisture resistant coating polystyrene formers are. Then as impairment of dielectric qualities it is possible to name following materials for formers: high-frequency ceramics, ultra-porcelain, impregnated tubes from a cable paper.

For coil inductance in master oscillators on the first place there is a parameter temperature instability of coil inductance and mechanical strength of the coil. Thus it is desirable to have good Q-factor. The highest qualities on these parameters coils on a continuous former from high-frequency ceramics with a winding put by a method of a burning out of silver in a former possess.

Flat coils on PCB apply on high frequencies to reduction of overall dimensions of the device. To frequencies of 100-150 MHz it is possible to apply the glass fiber coated with a copper foil. To ground in such coils the exterior output follows. If the printed circuit board double-sided from an underside opposite to the coil there should not be a metalization.

Summing up, it is possible to note that the coil inductance design depends on device features in which it works. However it is possible to make one principal conclusion – reduction of overall dimensions of the coil inductance always conducts to decline of parameters of the inductor and, accordingly, device general options into which composition it is included. For example, miniaturization of inductors in receiver input cascades worsens its image frequency rejection ratio.

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