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Variable Inductors
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Variometers A variometer consists of two coils arranged in such a way that the axis of one can be rotated with respect to the axis of the other. The coils are usually wired in series, and rotating one of the coils alters the mutual inductance. When the magnetic fields produced by the coils are in full or partial opposition, inductance cancellation occurs (mutual inductance is negative). When the fields add, the inductance increases (mutual inductance is positive). The mutual inductance is zero when the axes of the two coils are at right angles. The principal disadvantage of the variometer is that the conductor length remains constant as the inductance is varied. This means that the RF resistance is unnecessarily high for small inductances, i.e., the Q is poor at the minimum inductance end of the variation range. The long conductor length also gives rise to a lower than necessary self-resonance frequency (SRF) for a small inductance setting, i.e. the useful frequency range is limited. Open-circuit self-resonance occurs when the total conductor length is l/2 (estimates using c as the propagation velocity are usually accurate to within 3%). A variometer should not be used at more than about 87% (Sin60°) of its open-circuit SRF. Due to its severe practical limitations, the variometer is essentially a historical curiosity. A valid application is supplemental fine inductance adjustment in LF antenna loading coils, provided that the variometer inductance is a small part of the total. |
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Roller Inductors The roller inductor (known in the vernacular as a "roller coaster") has the advantage that the conductor length reduces as the inductance is reduced. This keeps the RF resistance of the coil at its practical minimum and allows reasonably high Q to be maintained at the low end of the inductance range. The big disadvantage of the roller inductor lies in the fact that the unused turns are coupled to the turns in use by transformer action. If N is the number of unused turns, and r is the radius of the coil, the length of the unused winding is approximately 2prN. If the unused winding is left open-circuit, the device becomes a Tesla coil when the unused winding length is l/2 at the frequency of opeation. The resulting corona discharge is likely to etch the wire and blacken or damage the coil former. One solution to the corona discharge problem is to short the unused winding. This however, gives rise to a large circulating current which reduces the inductance substantially. It also factors the resistive losses of the unused winding into the overall losses, and reduces the Q considerably. |
 
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Pseudo Roller Inductors. Some variable inductors have a stationary coil and an internal wiper arm, i.e., they use the same mechanical arrangement as a helical potentiometer. These devices do not appear to have a proper name. They are called "roller inductors", even though they are not (except in the sense that a rolling contact may be used for the wiper). These 'helipot inductors' have the advantage that they do not require brush contacts for the coil end connections. Hence the ESR should be slightly lower (or at least more consistent) than that of a roller inductor of comparable geometry. Another advantage is the absence of a ceramic (or worse still, phenolic-paper composite) coil former. |
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Binary Switched Inductors When two or more inductors are placed in series and arranged in such a way that there is minimal mutual inductance between the separate coils, the inductances simply add. By shorting out individual coils by means of relays, a stepwise variable inductance can be created. By choosing the component inductances according to a binary sequence, the relay drivers may be connected directly to a data register and operated by a microcontroller using the simplest possible program. The disadvantage of the switched inductor chain is that the degree of mutual inductance used in producing a given inductance is sub-optimal in terms of Q and SRF when more than one coil is in circuit, i.e., the same amount of inductance could have been produced with a shorter length of wire. The conductor length drawback is however, nowhere near as severe as with a variometer, because (in impedance matching applications at least) the larger inductors are not used at the high frequency end of the operating range. |
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Coil and Capstan The coil and shorting drum is the 'no compromises' solution to the problem of designing a variable inductor. For any setting, the inductance always has the shortest possible conductor length and hence the highest possible Q and SRF within the constraints of the chosen geometry. The downside is complexity. The rotation of the two cylinders must be synchronised using gears, and the conductor must be tensioned. An elaborate end-stop system is needed, since the device will fail if either drum is repeatedly unwound as far as the end connection. |
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Text and photogtaphs © D W Knight 2007
David Knight asserts the right to be recognised as the author of this work.
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