Radio Broadcast (May-Oct 1925)

How to Design Radio Coils A Simple Non-Mathematical Method Which Can Be Applied by Any Radio Constructor By HOMER S. DAVIS ONE of the most frequent problems confronting the amateur radio builder is the design of the inductance coils of a new receiver. Often the size of tubing or kind of wire specified by the designer is unobtainable, or one may prefer to re-design a coil to conform to the principles of low-loss design. "Cut and try" methods are crude at best, and if the amateur has some means of easily computing the inductance of a coil, he can save both time and expense by its use. The solenoid, or single-layer winding, is a common form of coil, and it is rather generally agreed to be the most efficient for a given value of inductance. But although the simplest to design, the formula for a solenoid is rather difficult to use unless one is quite familiar with mathematics. Fortunately, there are several methods of representing formulas graphically, and of these, the alignment chart is probably the easiest to use, and therefore offers the best solution to our difficulties. A pencil and a ruler are all that are required to use these charts. A discussion of the manner in which they were worked out is given below for the benefit of any who may care to follow it through, although it crohenries, d the diameter in inches, n the number of turns per inch, 1 the length of the solenoid in inches, and K the shape factor. The latter depends upon the ratio of the diameter to the length, and its value may be obtained from tables in the Bureau of Standards Bulletin No. 74 and elsewhere. Thus it is not especially difficult to solve for the inductance of a coil when d, n, and 1 are known. But the factor K causes no end of trouble when we try to use the formula in the reverse direction, solving for 1, since K is then an unknown also. Cut and try methods must be resorted to. We can express K in a formula in terms of d and 1, but the relationship is not simple. However, in most cases, the value of the ratio lies between \ and 2, and between these limits we may express K as approximately: K = o6?4 is not in any way essential to the use of the charts. The reader may skip entirely over this discussion if he so desires. The formula for the inductance of a single -layer solenoid is: L=.o2 507d2n2lK where L represents the inductance in mi A Coil Calculator Is what the chart which accompanies this article of Mr. Davis's, really is. Any number of constructors who tried to build a radio set from directions have been stopped short by their inability to secure a certain size coil and had no way of calculating its size, except by mathematics. And mathematics, to many of the radio constructing gentry, is not a desirable part of the picture. Many of the advanced radio calculations lead one directly into the calculus. Those who wish nothing more than a good rule-of-thumb will find the chart will allow them to build coils to the specifications of the various construction articles, will allow them to build a coil to attain a certain wavelength range with a condenser of given size, and by reversing the process, it is possible to find out what size condenser should be used with a given coil to attain a known wavelength. The chart and the wire table should be of great help to the builder of sets. — The Editor Substituting this in the first formula, we have: L=.oi69n2d1707H-293 and K has been eliminated. It is now possible to solve for any one value when the three others are known. It is still a. formidable looking equation, but it may be charted with ease. It is hardly necessary to explain here how the charts are constructed, but suffice it to say that they are based upon the same theory as the slide rule, which was described in the "R. B. Lab." department of the January 1925 number of Radio Broadcast.