Radio broadcast .. (1922-30)

350 Radio Broadcast windings, that more than a small fraction of the received signal is transferred to the detecting circuit in this manner. It is probable that the primary, acting as a transmitting antenna energized by the received signal, wirelesses the =o A VERY SIMPLE CIRCUIT !n which the received energy is transmitted from the primary coil of the loose (vario-) coupler, P, to the secondary coil, S, by induction ™ energy to the secondary which is virtually a receiving aerial, and which must be tuned to pick it up! The advantages of the loose coupler over the single circuit tuner are several. Aside from permitting delicate and sharp tuning by slight changes in the coupling, which simultaneously varies the wavelengths of the primary and secondary, the coupler is very efficient in eliminating undesired signals, a quality known as selectivity. If the primary is tuned to a certain station, the antenna will pick up its wave with a partial exclusion of all others. But at the same time, signals from a powerful or near-by transmitter, tuned to a different wave, will force oscillations in the non-resonant circuit. The secondary, which must be tuned to the same wave as the primary, will readily receive energy induced by the tuned station, but will discriminate against, and still further tend to eliminate, the forced signal. The primary and secondary are analogous to two water filters. While some impurities may flow past the first, few if any pass the second. HUNTING TROUBLE ON A CRYSTAL RECEIVER THOUGH your antenna is not all that could be desired (we suggest a single wire inverted L, from a hundred to a hundred and fifty feet long), you should certainly receive signals on your set, which is probably the most efficient combination of instruments for crystal reception. One or more of several things may be at fault. The condensers Ci and C2 should first be removed from the circuit, one at a time in the order named, and the set tuned after each removal. If signals are received, it is evident that one or both of the condensers is shorted. When the variable capacity is at fault, the experimenter can probably repair it by adjusting the relative position of the stationary and revolving plates. If it is the fixed condenser that is shorted, it is best replaced by a new one. The receivers should be next tested by touching the ends of the phone cord to the terminals of a dry cell. If the headset is in good condition, a loud definite click will be heard. Having once determined that the receivers are not at fault, the phones themselves may be used for further testing the set. It is possible that the antenna is grounded and this should be next ascertained by disconnecting the aerial from the coupler and transferring it to one of the receiver cords. The remaining phone cord is grounded through a dry battery. On making and breaking the connection, a fairly loud click will indicate the difficulty. The primary and secondary are next in order, and are individually tested by connecting the phones to one end of the winding, and the other to the switch lever through the dry cell. As the circuit is interrupted by running the lever on and off each tap, a loud click should be heard in the phones. The connections between the various instruments, particularly soldered joints, should be carefully examined. Inexperienced enthusiasts often use rosin as a poor substitute for soldering pastes. Aside from being an unsatisfactory flux, if the iron is slightly cool it will remain between the metals, merely sticking the solder on with a highly insulative glue, with the result that an ostensibly perfectly soldered joint is no connection at all! After having determined that the condensers are not broken down or shorted, a battery substituted for the crystal detector will indicate, by a definite click, that the essential secondary connections are well wired. The last possibility is that the detector crystal may not be a sensitive one, a condition that should be ascertained by BUZZER HOW TO CONNECT A BUZZER For use in testing your crystal set buzzer test, an arrangement that is at any time a desirable addition to a crystal set. A small, high-toned radio buzzer should be connected as here shown, where the only deviation from the conventional announcing buzzer hook-up is the single wire running from the stationary contact to the ground lead on the receiver. If the pushbutton is replaced by a telegraph key, the buzzer may be used for code practice. The detector is adjusted while the buzzer is vibrating, and the note will be plainly audible in the receivers when the cat-whisker is on a sensitive spot. What Would You Like to Have in Radio Broadcast ? The editors would be pleased to hear from readers of the magazine on the following (or other) topics: 1. The kind of article, or diagram, or explanation, or improvement you would like to see in RADIO BROADCAST. 2. What has interested you most, and what least, in the numbers you have read so far.