|‘Technical Shorts’ is a series of (fairly) short articles written by Gerry O'Hara, a CVRS Director and vintage radio enthusiast, each focusing on a technical issue of relevance in repairing, restoring or using Eddystone valve radios. However, much of the content is also applicable to non-Eddystone valve receivers. The articles are the author’s personal opinion, based on his experience and are meant to be of interest or help to the novice or hobbyist – they are not meant to be a definitive or exhaustive treatise on the topic under discussion….
References are provided for those wishing to explore the subjects discussed in more depth. The author encourages feedback and discussion on any topic covered through the CVRS forum.
The purpose of this ‘Short’ is to identify what coils, chokes and transformers, collectively termed ‘inductors’ are, why and where they are used in Eddystone receivers, some considerations in their design for particular purposes, and to identify problems that may develop and how to identify them. The article assumes the reader has a basic understanding of AC theory, the principle of inductance and terms such as ‘reactance’ and ‘resonance’. For information on such fundamentals, the reader should refer to standard texts on radio and electronics theory , eg. Terman, and Sid couldn’t give a cat’s whisker manuals such as those published by RSGB and ARRL about inductors - dozing in the (see reference section at the end of the article) winter sun is much more fun – oh come on, they aren’t that boring…
Inductors in radios fall into three broad types: coils, chokes and transformers. Coils and chokes are used where a circuit is required to have frequency dependant characteristics, sometimes with a capacitor in series or parallel, sometimes without. Transformers, however, whilst also being able to have frequency related performance, exhibit other properties that are exploited in circuits where frequency-dependence is not necessarily required or even desired. That said, it must always be remembered that the value of the reactance in either a coil, choke or transformer depends not only upon its value of inductance, but also on the frequency at which the circuit that contains it is operating. It should also be noted that the inherent distributed capacitance of a coils windings, ie, capacitances between adjacent windings and surrounding components, can form a resonant circuit with its inductance value. For RF applications, this effect limits the highest frequency to which the coil can be tuned.
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