Improvements in variable inductors or resistors

Abstract

 Conventional variable inductors used at RF frequencies employ moving contacts bearing on turns of a coil with attendant difficulties in obtaining good contacts and ensuring that the unused part of the coil does not absorb power nor create unwanted resonances. The turns of a coil (1) can be manipulated by screw threaded members (4) (only one is shown) so as to vary the number of turns not in contact with each other, and therefore the inductance of the coil (1). This arrangement avoids any unused part of the coil (1) forming an inductance and therefor overcomes the aforementioned difficulties.

Description

[0001] This invention relates to improvements in variable inductors or resistors, and more particularly to variable inductors used in medium and high radio frequency transmitter stages.

[0002] Conventional RF variable inductors employ moving contacts bearing on turns of a coil, such as sliding contacts or contact arms, with attendant difficulties in obtaining good RF contacts and ensuring that the unused part of the coil does not absorb RF power nor cause resonances which disturb the effects of the used part of the coil.

[0003] The invention attempts to reduce or eliminate such difficulties.

[0004] Accordingly, this invention provides a variable inductor or resistor comprising a coil and means for spacing apart a variable number of adjacent turns of the coil to vary the inductance or resistance:

Turns which are in contact with adjacent turns may constitute a high-conductivity shorted-turn for RF purposes in the case of a variable inductor, or a minimum resistance element in the case of a variable resistor.

[0005] By employing the invention it is possible to construct an inductor which has no sliding contact and in which there is no unused part of the coil to cause the absorption of power and undesirable resonances referred to above.

[0006] The means for spacing apart a variable number of adjacent pairs of turns preferably comprises a threaded member arranged so that, when turned in one direction, the thread progressively engages and separates turns of the coil. When turned in the other direction the threaded member preferably drives the turns progressively back to a position where they all lie together in contact with adjacent turns so as to give the minimum inductance or resistance.

[0007] The invention is further described by way of example with reference to the accompanying drawing which illustrates apparatus constructed in accordance with the invention, parts of which are shown in side elevation and parts in vertical cross-section.

[0008] A variable inductor includes a coil 1 comprising a number of turns of helically wound silver-plated half hard copper strip. Two insulating bars 2 (shown in cross-section) are arranged parallel to the coil 1, and are held in position by two end plates 3 at each end of the bars 2. The coil 1 is carried by the bars 2 and lies between the end plates 3. Two diametrically opposite lead-screws 4, of which only one is shown, are of insulating material and each have a threaded part 4A and an unthreaded part 4B. The screws 4 are held in position by the end plates 3 so that their threads can engage the turns of the coil 1 and are parallel to it.

[0009] The right hand end (as illustrated) of the coil 1 is in contact with the left hand end (as illustrated) of the threaded part 4A of each of the lead screws 4 and the left hand end of the coil is in contact with an insulating plate 5. A non-ferrous leaf spring 6 is located between the insulating plate 5 and the end plate 3. It holds the insulating plate 5 against the coil 1 to maintain those turns of the coil 1 not engaged between turns of the threaded portion 4A in close contact with each other.

[0010] When the first turns (i.e., left hand turns) of the lead-screws 4 are engaged with the right hand turn lA, of the coil 1, all the turns are in close contact with each other and there is minimum inductance. This close contact is aided by a conductive cylinder 7, which surrounds and is in contact with that part of the coil 1 where adjacent turns are in contact with each other. The cylinder 7 is set into the bars 2 so that the track followed by the coil 1 is of constant width.

[0011] To increase the inductance presented by the coil 1 the lead-screws 4 are rotated by a mechanical drive (not illustrated), so that the end turn lA is moved along the threads, guided by the bars 2, with subsequent turns also becoming engaged until the required inductance is reached. Only those turns engaged by the threads, and hence no longer in contact with adjacent turns, contribute to the inductance, the other turns being kept in close contact by the action of the leaf-spring 6 and the insulating plate 5.

[0012] In the illustrated embodiment of the invention the coil 1 forms part of a tuning circuit in a radio transmitter other components of the tuning circuit being indicated at 8, and is connected via a flexible lead 9 and terminal 9A to an RF source of voltage, indicated schematically at 10, and also via another flexible lead ll and terminal. 11A to an RF load at 12.

Claims

1. A variable inductor or resistor comprising a coil and means for spacing apart a variable number of adjacent turns of the coil to vary the inductance or resistance.

2. A variable inductor or resistor as claimed in claim 1 and wherein the said means comprises a threaded member of material which is relatively insulating compared with the coil and arranged so that when turned the thread progressively engages and separates turns of the coil.

3. A variable inductor or resistor as claimed in claim 1 or 2 and including a conductive member arranged to surround those of the turns in contact with adjacent turns.

4. A variable inductor or resistor as claimed in any preceding claim and whereing the coil is connected between two terminals which are connected in a radio frequency circuit.

5. A variable inductor or resistor as claimed in any preceding claim and wherein the terminals are fixed with respect to the coil.

6. A variable inductor according to any preceding claim.

₇, A variable inductor or resistor comprising a coil and a screw-threaded member arranged so as to engage and manipulate the turns of the coil so as to vary the number of turns not in contact with each other and therefore the inductance or resistance of the coil.

Drawing