Radio Broadcast (May-Oct 1922)

RADIO BROADCAST circuit but also sets up a current in the secondary circuit which is within the magnetic field of the primary circuit. The advantage of using two separate inductances for the primary and secondary circuits is that their physical relation or coupling m.ay be adjusted. The looser the coupling — that is the further apart the two coils are held — the sharper and more exact the tuning of the two circuits. With a loose coupled set it is Fig. I. An oscillatory circuit usually possible to eliminate interference, which cannot be avoided with the single circuit set. The tuning of an inductively coupled set is as follows: The coupling is first set at maximum; that is the primiary and secondary inductances are placed in as close relation as possible. The antenna circuit is first tuned to the incoming wave; the secondary circuit is then adjusted to resonance. Since the antenna circuit has the large capacity of the aerial and the inductive effects of the long wires, it usually requires comparatively few turns or little inductive value in the antenna circuit to place it in resonance with a much larger number of turns on the secondary inductance. A typical adjustment, for instance, is ten turns of antenna inductance to fifty turns of secondary inductance. Exact values cannot be given as this is determined by the size of your antenna, the length of the lead-in, the diameter of the coils used, and other conditions which are not standard. Some sets are being manufactured which have but one control for wave length. In such sets close resonance is not always possible and for this reason they are not as selective as inductively coupled sets. The result of placing the secondary circuit in resonance with the primary circuit through which a received signal is coming in is to have the secondary condenser charged and discharged millions of times per second. For ininstance, if you are listening to a 300 meter wave this condenser charges and discharges 123 one million times per second. This high rate cannot be converted into sound waves because no mechanical device can move so rapidly, nor would the ear respond 1:0 air waves of such rapid frequency. For this reason we use a crystal rectifier. A crystal rectifier allows current to flow through it in only one direction. The function of the crystal detector, which is placed in series with the secondary inductance and capacity, is to allow an appreciable charge to accumulate on the plates of the condenser. It permits the plates to become charged in one direction, but prevents the discharge or equalization of the charges. When radio music is being transmitted on a 300 meter waye it is projected through the ether in vibrations of one million frequency. The voice or music is imposed upon this rapidly oscillating vibrating wave by means of modulation. Speech consists of intricate combinations of frequencies, ranging from 40 to 5,000 cycles per second. Modulation consists of changing the average intensity of the high frequency ether waves in accordance with the voice variations. Striking a second C above the middle C on thepiano produces an airwave of 500 frequency. When this is sent over the radio telephone at 300 meters, miodulation divides the one million frequency radio current into groups of 20,000 per second. The crystal detector allows a charge to accumulate on the plates of the condenser. The modulation employed by the transmitter varies the amount of this average charge at voice frequencies. Head telephone receivers are connected across the condenser. The low frequency variations of the average charge pass through the telephones causing the diaphragms to vibrate. This in turn causes air waves which you hear. <. Crystal detectors are of various types, but the process of adjustment in all cases is Fig. 2. Tlie antenna circuit as an oscillatory circuit