Winding the Optimum Coil

Radio Record, Volume IV, Issue 9, 12 September 1930, Page 26

Winding the Optimum Coil

A Correspondznt Asks Questions es

Answered by

Cathode

A CORRESPONDENT has raised cer tain points in connection with the papers on coil design which were printed in thése columns some time ago. As these little difficulties are such as may

have been encountered by other enthusiasts, it has been considered worth while

bringing the answers to the queries inte some degree of prominence. The first question reads as follows: "In constructing any inductance coil, what is the best or most efficient ratio of length of winding to diameter of coil?" Now, in strictest theory, given a coil of certain diameter, the resistance deereases (ie, the efficiency improves progressively as the length of the coil is | inereased, provided that the optimum diameter of wire is always used. In practice, however, apart from the inconvenience of such bulky coils, there is no real improvement in efficiency after the length of the coil equals its diameter. In fact, a very efficient coil wil’ result from a winding length three-quar ters of the diameter. Thus we may say that for practical purposes efficiency is greatest when the winding length is equal to the diameter, while a reduc. tion of the winding length to three. quarters the diameter makes a more compact coil, and has but little effect on the coil resistance. This point is well illustrated in the charts appearing herewith. Our correspondent next asks the number of turns of 24 s.w.g. to wind on either a 38-inch or a 23-inch former and the most suitable winding length in each case; the tuning condenser is to be of .0005 mfd. maximum capacity.

From information given in the issue of thé "Radio Record" of August 23, 1929 (Vol. III, No. 6), we know that for a tuning condenser of this size an in ductance coil of 200 microhenries is necessary. For a .00035 mfd. condenser, 280 m.h. would be more suitable, while a .0003 or .00025 mfd. condenser would call for an inductance of 340 m.h. Aiming at maximum efficiency, referencé must be had to the charts. Fig. 1. Knowing that 24 s.w.g. has a diameter of 0.5585 millimetres, a line may be

drawn across the 200 ‘microhenry chart at this point, and it will be seen tha it cuts line A (38-inch diameter coils at a point corresponding to a windi length of 13 inches, while it cuts line B (23-inch diameter coils) at a point equivalent to a winding length of just on 2 inches. Reference to Fig. 2 shows that to give the required inductance with the diameters. and winding lengths decided upon, the Sin. coil would need about 45 turns (the chart does not extend quite this far), while the 24in. coil would call for 60 or 62 turns, which would, of course, be spaced over the previously ascertained winding length of. 2in.; the 8in. coil will not accommodate the necessary turns in the space of idin., so that here a close-wound coil is the most efOur correspondent submits a ‘suggested circuit for a crystal receiver in which he proposes to use this coil’; he will find this circuit quite successful. The third query submitted-is as follows: Given the inductance required, the size of the wire to be used, and the: size.of former, how does one arrive at (a) the number of turns to wind on, (b) the length of the former to space out the winding over? These two: points have really been covered in the-practical instance just given. It may be noted that choosing a gauge of wire and then ascertaining the optimum dimensions of the coil for that particular gauge is a rather "back-to-front" proceeding, and will frequently lead, as in the instance just. given, to a coil of unsuitable shape. A much sounder process of design is to first decide upon the most suitable and efficient shape and dimensions for the coil. then ascertain from the the number of turns to obtain the r quired inductance with those dimen» sions, and the most efficient wire’ diameter; the nearest gauge to the optimum diameter can then be chosen for winding the coil. HE next query is whether the efficiency of a tuned circuit is improved at all by increasing the ratio of inductance to capacity. It is, of course, although with a crystal receiver it is probable that the damping imposed on the circuit by the crystal would nullify any benefit which might otherwise be gained. When the tuned circuit is in the plate circuit of a valve the advantage of using a preponderance of inductance is readily understood. The amplification depends ov the magnitude of the plate load, and since the impedance of a tuned circuit depends on the magnitude of a factor L over Cr (where L is the inductance, C is the capacity and rthe high-frequency resistance), so the /amplification increases as the inductance L is increased. The improved amplification is paid for by a decrease in selectivity. Lastly, our correspondent wishes to know how the "Shape Factor" curve in the above-mentioned article is arrived at. This is actually a derivation and plotting of Nagaoka’s constants for inductance calculation, but the development of the formulae is somewhat outside the scope of these columns.