Radio Broadcast (May-Oct 1922)

The Selective Double-Circuit Receiver FIG. 2 In this form of double-circuit tuner the damping produced by the detector is reduced shown in Fig. 2, which differs from Fig. i only in the connection of the detector and telephone circuit across a part (instead of the whole) of the secondary tuning coil. In this manner the effect of detector resistance upon the secondary tuning may be cut down substantially, with a corresponding gain in resonant discrimination between arriving waves of slightly different frequencies. Another method, which has the additional advantage of providing easy variation in the detector coupling, is to use a second oscillation transformer as shown in Fig. 3. The less the inductive coupling between the coils of this second transformer, the smaller will be the damping introduced into the tuned secondary cirrcuit on account of detector resistance. In both these circuits (Figs. 2 and 3) the telephone shunting condenser may well be of rather large capacitance; instead of the value 0.005 microfarad suggested for Fig. i, a condenser of 0.02 microfarad may be used with some improvement in signal strength. The operation of any one of these three circuits, but perhaps particularly of that shown in Fig. 2, will be a pleasant surprise to anyone who has come to believe that receivers using crystal detectors are necessarily poor in tuning or selective qualities. For best results it is of course necessary to adjust carefully, and the apparatus itself must be well made in order that losses of received energy in conductor resistance, condenser leakage, poor connections, etc., will not overshadow the gains to be secured through the secondary tuned circuit with reduced detector coupling. A properly built crystal receiver of this type, accurately adjusted, will give signals as loud as or louder than those from ordinary singlecircuit crystal sets and, in addition, a degree of selectivity which can be surpassed only by the best vaccum tube outfits. Moreover, the absence of batteries and the freedom from tone distortion which are characteristic of crystal receivers may be taken together with the selectivity obtainable in the manner just described to recommend such arrangements for most reception over short or medium distances (up to about 25 or 30 miles from broadcasting stations or 100 miles from radiotelegraph plants) where good receiving aerials may be erected and when ordinary headtelephone listening is satisfactory. When it is desired to receive over longer distances, one should take advantage of the greater sensitiveness inherent in the threeelectrode vacuum-tube detector. As has been pointed out, this instrument draws very little power from the tuned receiving circuit to which it is connected, and, consequently, modern audion receivers are generally more highly selective than those using crystal detectors. The vacuum tube of course requires a battery for lighting its filament and for supplying telephone-circuit current; on the other hand it is at least several times as sensitive as the crystal (a critically adjusted " gassy" or soft detector tube may give responses some fifteen times as loud as would a crystal, to the same weak signal) and so may be used with smaller receiving aerials or for working over longer distances. The vacuum tube detector will give fairly good resonant selection in a single circuit re FIG. 3 The detector may be placed in a third circuit inductively coupled to the tuned secondary