Transformers as band pass filters

An interesting article describing in detail the theoretical basis of the technical design of transformers for communication apparatus. For practical purposes it is possible to replace a transformer having self-capacity by a network which consists of a half-section of a known type of band filter. In the absence of dissipation the transformer, when working between its image impedances, passes a band with no attenuation. The ideal theory is applicable in practice to carrier and long-wave transformers above 5000 ~, up to 100,000 ~ for impedances of the order of 20,000 ohms, and for ordinary telephone transformers. The ideal theory fails for high-quality audio-frequency transformers passing wide-frequency ranges, and for radio transformers below 3000 metres, except where the valve-anode impedances are not higher than 10,000 ohms. Usually the frequency limits and the forward and backward impedances are given as the basis of the design. In practice it is found that the self-capacity and the coupling factor with a given core do not vary appreciably with appreciable changes in the number of turns in each winding. Design formulaelig are deduced from these considerations, and examples of their use are appended.