Radio Broadcast (Nov 1923-Apr 1924)

352 Radio Broadcast fig. 8 The dotted lines bound the original Colpitts Oscillator circuit and the radio-frequency amplifier is at the left. Very little alteration in apparatus is necessary to obtain this R. F. amplification by the dotted lines on the diagram, but which is not required when a standard transformer is used. (Incidentally, employing the arrangement outlined for the homemade primary and secondary, the amplifier is changed to the pure-tuned type, and while adding an additional control, it is, on the whole, the more efficient system.) The 200 to 400-ohm potentiometer is that usually employed with radio-frequency apparatus. Our readers are familiar with the remainder of the circuit. The theoretical operation of the amplifier, and the apparent lack of rectification in the first tube (viz., the passing on of the radio wave as an alternating current) is of interest to the student reader. It is suggested that he study the article on R. F. amplification published in the August, 1922, issue of Radio Broadcast. Several subtleties coincident with this system of amplification are there explained. The truth is, as our correspondent has suggested by referring to the audion's single-way conductivity, that the radio wave is rectified in every tube through which it passes. The alternating currents or radio oscillations which are passed to the succeeding tubes are not the original wave (which of course was effectually blocked by the first tube), but is its amplified duplicate which is generated in the plate circuit of the tube. When the grid of the R. F. amplifier is adjusted in a certain way (see the article just mentioned in the August issue) by the potentiometer, the direct plate current will rise and fall in perfect synchronism with and in proportion to the alternations of the incoming wave. The primary of the amplifying transformer is a part of the plate circuit, and it follows that the rise and fall of the plate current, which flows through it, is accompanied by a corresponding rise and fall of the magnetic flux which is set up by the current. (Electricity in motion always "generates" a magnetic field.) This magnetic flux necessarily "cuts" the secondary of the R. F. transformer, and by the law of induction induces an alternating current therein — an alternating current of the same wavelength or frequency as the original wave intercepted by the antenna. It is quite obvious that this arrangement will block the passage of any oscillations generated in the detector circuit back through the R. F. amplifier and out to the antenna — thus eliminating radiation, the greatest objection to this "flivver" circuit. For the audio amplifier data, our inquirer is referred to the July 1923 issue of Radio Broadcast. Wavelength and Size of Coils / am sure that many of your readers, in common with myself, have many occasions for winding coils for various wavelengths. Will you please publish a formula for determining the wavelengths of different coils, or, more correctly, the method for designing inductances for different waves ? O. J., New York City. THERE is, of course, a formula (in fact there are several of them) for determining the required number of turns of wire in a coil for a desired wavelength. However, the application of these formulas is a comparatively difficult task, involving calculations of distributed capacity, spacing, diameter of the coil, inductance, etc., to say nothing of various correction formulas. These are complications that make such formulas useless to any one but the radio engineer, and luckily, they are really unnecessary except when calculations must be made with mathematical precision. An explanation, which would necessarily accompany the publication of these formulas, would be interesting to only a very small percentage of our readers. Such readers interested in the theory of coil design, are referred to The Wireless Experimenter' s Manual, by E. E. Bucher, where this subject is treated very comprehensively. For the average radio enthusiast, the correct winding of inductances is much more conveniently described in simple tables, which involve no subsequent mathematical calculations. Chart 1 is quite self-explanatory, indicating the number of turns, size of winding forms, etc., for primary, secondary, and tickler coils on the wavelengths in which the broadcast enthusiast is most interested. These calculations are only approximate, and are based on average values of antenna capacity, etc. In order to secure the indicated wave range,