Radio Broadcast (May 1929-Apr 1930)

frequency oscillators, and, in fact, in any type of circuit where it is required that the coupling between coils and other apparatus be at a minimum. Very compact sets may be built incorporating these coils without being troubled by excessive oscillation. As stated previously, self-shielded coils may be placed quite close to each other and at any desired angle without harmful coupling effects; hence they are ideal for use in compact sets. In tuned radio-frequency sets when selfshielded coils are used, less than the usual amount of osculation may be expected. In fact, such a tuned radio-frequency set when carefully built will, in many cases, operate like a neutralized set in that almost complete freedom from internal oscillation will be experienced. Also, as these coils are not susceptible to outside influences, the individual coils will not pick up near-by broadcasters as other coils often do, thereby causing much interference. Radio-frequency oscillators require that no energy be radiated by the tuning coils of the oscillator. Radiated energy will cause serious interference when radio-frequency measurements are in progress. Self-shielded coils will be found to be ideally suited for oscillators, because the energy radiated by the coil is almost [negligible in comparison with that radiated by the usual type of inductance. For short-wave work the self-shielded coil should give very satisfactory results. At short waves especially, magnetic and static coupling between coils and other parts becomes very strong. This is the cause of much of the energy loss and inefficiency on short waves. Design Data THE design of a self-shielded coil will be found to be more involved than the design of a single-layer solenoid, but by following these instructions the experimenter who can work out the constants of the single-layer coil will be able to determine the constants of a selfshielded coil of the desired inductance. The first step in the design of any inductance coil is to determine the inductance needed. This may be calculated from a fundamental formula much used by radio engi where 3.55 x 10« x C L = inductance in microhenries. X = wavelength in meters. C = capacity in microfarads. In this case, where the inductance of a coil which is to be used with a variable condenser is desired, X is the highest wavelength to which it is expected to tune, and C is the fullscale capacity of the condenser. In order to compensate the effects of distributed capacity in the coil, and other capacities such as the tube input capacity, several microhenries may be deducted from the inductance of the coil just calculated. The outer tube diameter should now be decided on. For reasons of economy of space small diameters may be used, but from the viewpoint of efficiency a larger coil is preferable. Coils for covering the broadcast band may have an outside diameter of from 2\ inches to 3j inches. The length of the outer coil winding should be within 1.2 to 2.2 times its diameter in order to maintain high efficiency. The dimensions of the inner tube depend upon the area and length of the outer tube. As stated previously in this article, the length of the inner winding should be a little less than the length of the outer winding. It was also stated that for high elliciency and good shielding, the ratio between the areas of the inner and outer coil windings should be 2 when the outer winding has a ratio of diameter to length of 1.26, 2.1 when the ratio of diameter to length is 1.58, and 2.2 when the ratio is 2.1. The diameter of the inner tube may be calculated easily from the area and length of the winding. After determining the dimensions of the inner coil winding, calculate the number of turns needed on the inner tube to give about one third more inductance than desired for *"1tr*1f "Tf * Tf High Potential / end J P Ground — end Fig. I — Detail of the self-shielded coil. the completed coil. For this calculation Nagoaka's formula may be used. This formula for the calculation of inductance of solenoids may be found on page 252 of the Bureau of Standards Circular No. 74, Radio Instruments and Measurements. The number of turns needed on the outer tube is based on the ratio of the turns and areas of the inner and outer coil windings. The following formula gives the number of turns to be used on the outer winding: As N2 Ni = — — Ai where Ni = number of turns on outer tube. N2 = number of turns on inner tube. Ai = area of outer winding. A2 — area of inner winding. After the number of turns on the outer tube has been determined, calculate by means of Nagoaka's formula the inductance of the winding. Subtract the inductance of the outer winding from that of the inner winding, and the result is equivalent to the total inductance of the coil. In the self-shielded coil it is permissible to subtract the inductance of one Inner Winding , High Potential Outer Winding ^ Fig. 2 — In the self-shielded coil the mutual inductance between the inner and outer coil sections effectively neutralises the self inductance of the outer coil; therefore, the outer coil is at the same potential throughout and has no external magnetic field. For the same reason the outer section serves as an excellent electrostatic shield for the inner coil. winding from the other to obtain the total inductance, because the inductance of the outer coil is equal to the mutual inductance. This may be clearly demonstrated by means of the formula for the inductance of coils in where Lo = total inductance. Li = inductance of coil No. 1. L2 = inductance of coil No. 2. M = mutual inductance between both coils. In the self-shielded coil, Li is numerically equal to M ; and, as M is negative because the fields of both coils are opposed, therefore, the formula may be rewritten: Lo = Li + L2 — 2L1 Reducing the formula we have Lo = L2 — Li This is true only in a properly designed selfshielded coil. In order to obtain the desired inductance for the coil, it may be found necessary to repeat the calculations several times, using a different inductance for the inner winding each time until the proper inductance for the entire coil is obtained. It will not be necessary to make more than three calculations for a given coil if good judgment is used. When the entire inductance has been calculated properly a wire table should be consulted, and the size of wire to be used for the inner winding decided upon. The wire size used is limited only by the number of turns which must be wound within a given space. The outer winding may be space wound with No. 18, 20, or 22 wire. To obtain higher efficiency the coil should be constructed of litz wire throughout, and the inner winding bank wound. Extreme care should be taken that each strand of litz is unbroken, and is properly soldered at the ends. Broadcast-Band Coil THE following data may be used for constructing a coil which is to be used in conjunction with a 0.00055-mfd. condenser for covering the broadcast wave band : OUTER COIL SECTION Diameter Length of winding No. of turns Wire 33 No. 20 double silk covered INNER COIL SECTION Diameter Length of winding No. of turns Wire U" 1" 72 No. 32 double silk covered Lo Li + Ls + 2M If instead of winding the inner coil with fine wire, a heavier wire is used and the coil bank wound, a higher efficiency will be obtained. The bank winding should cover the same area and have the same number of turns as the winding specified in the above table. The outer coil shoidd be arranged in respect to the inner coil section so that it slightly overlaps the high-voltage end of the inner coil in order to produce more thorough shielding. Also, the inner coil section must be concentric with the outer coil. Both coils must be wound in the same direction and connected in series at one end as shown in the diagram. For coupling purposes a few turns of wire may be wound over or near the low-voltage end of the inner coil. When such a winding is used, the end farthest away from the highvoltage terminal of the inner coil should be connected to the plate of the preceding tube or to the antenna. The other end of the winding goes to the plate battery or to the ground. The author leaves the constructional details of the self-shielded coil to the builder's judgment. No doubt, many ways will be devised for fastening the coil sections together, and for mounting the completed coil. Unlike other forms of "concentrated-field" inductances, the self-shielded coil just described is unique in that it is electrostatically shielded, and has an extremely limited magnetic field; while other coils having concentrated fields are not shielded statically. • may, 1929 page 22 •