CAPACITY VARIATION IN CONDENSERS

Sun (Auckland), Volume II, Issue 337, 24 April 1928, Page 16

CAPACITY VARIATION IN CONDENSERS

In considering variable condensers up to the present we have not deal with the values of the capacity or with the manner in which it varies on rotating the moving vanes. These points will be dealt with in this part. First of all we will consider the various ways in ■which the capacity is usually made to vary. The S.L.C. Condenser The abbreviation S.L.C. stands for “straight line capacity.” This type of condenser is now rarely used in "wireless receivers, though for laboratory instruments its use is general. The plates are so designed that on rotating the moving bank the variation of capacity is proportional to the angle of rotation. In other words, if we plot a curve showing the relation between the condenser reading in degrees and the capacity, th-e result should be a straight line. The shape of the moving vane necessary for this result is semi-circular. In actual practice it is generally found that the line is not quite straight. The discrepancies usually occur at the lower and upper ends of the scale, owing mainly to the edge effects of the vanes. There is always a definite minimum capacity. The disadvantage of a condenser of this type is that when placed in parallel -with an inductance coil, the ■wave-lengths to which the combination responds are not spread out proportionally over the whole scale of the condenser. They are crowded together at the lower end of the scale, due to the fact that wave-length is proportional not to the capacity, but to the square root of the capacity. The S.L.W. Condenser

To render tuning more easy, condenser designers set out to make a component which should have vanes so shaped that equal rotations of the moving bank in any part of the scale produced equal changes in wavelength. The result is the S.L.W., “square-law” or straight line wavelength condenser. The only way of judging the degree of success is to examine a curve showing the relation between condenser readings and wavelength. According to many manufacturers, this is an absolutely straight line. Actually, on plotting out the curve, it is usually found tha.t serious discrepancies are present. In the first place, the slightest error in shape of the plate or in assembly, may have an appreciable effect on the curve, particularly at the lower end, where slight variations of capacity are comparable with the total capacity. Effect of Stray Capacity

Apart from this, there is the fact that the minimum capacity of the condenser and also the capacity of the coil and leads with which it is in parallel have to be allowed for in designing the vanes. Most manufacturers cheerfully neglect this, yet they continue to advertise “absolutely straight-line wave-length” condensers! Some, it is true, correct the shape of the vanes for a definite stray capacity which they specify. Even so, one cannot always arrange for this definite stray capacity to be present. The whole point is that it is practically impossible to make an absolutely S.L.W. condenser in practice. But does this matter? So long as the wavelengths are spread out fairly evenly over the whole scale, it seems to us that this is all that is necessary. An absolutely straight line is the ideal, but a fairly close approach to it is quit© possible. The only point to remember is that the dodge of plotting two known stations one at each end of the scale, and drawing a straight line through the points, does not necessarily give the settings for other wave-lengths to any great degree of accuracy.

Incidentally it may be pointed out here that when we plot out wavelength (or frequency) curves of variable condensers submitted for test, we assume that the condenser is in parallel -with a coil of 200uH inductance and 20uuF self capacity. These average value are quite arbitrary, and were merely adopted as a means of comparing different components. If a manufacturer states that his condenser is corrected for a certain “stray,” this value is, of course, used instead of that mentioned above. S.L.F. Condensers

During the last year or so another type of condenser has gradually become popular, and now seems to be ousting the S.L.W. from popular favour. This is the S.L.F. “inverse square law,” or straight line frequency condenser. The plates of this are so shaped that equal rotations of the knob in any part of the scale produce equal variation of frequency. Since, according to the Geneva plan, stations are now being spaced out according to frequency, this type of condenser is now the logical one to use. In this type of condenser the change in capacity is very slow at first, and then increases rapidly. Most of the remarks regarding the straight line effect made when considering the S.L.W. condenser apply equally to the S.L.F. It is even more difficult to produce an exactly straight line with the S.L.F. condenser, owing to the fact that the minimum and stray capacities are quite large compared with the variation of capacity at the beginning of the scale.