The development of a small variable air condenser compensated for rapid changes of temperature

Various methods are discussed of obtaining compensation for temperature variation in air-dielectric condensers of small capacitance value suitable for radio-frequency purposes. It is found that appreciable air-gap spacing is necessary for high stability and a high degree of compensation. It is concluded that a parallel-plate system affords the most satisfactory means of achieving high stability. A description is given of a variable air condenser of small dimensions and weight which has a capacitance range of 17¿68 ¿¿F and a high degree of temperature compensation; the overall dimensions are 2¿ in. × 2 in. and the weight is 5¿ oz. The temperature-coefficient of capacitance can be adjusted to have any value within the range zero to ¿120 parts in 1 million per degree Centigrade, and the compensation is effective at any rate of temperature variation not exceeding 5 deg. C. per minute. When the system is used in association with a ceramicformer coil, its frequency coefficient can be made as low as 1 part in 1 million per deg. C., and the frequency stability over long periods with large temperature-change is of the order of 10 parts in 1 million.