emergency Radio Receiv rs by GUY DEXTER Satisfactory radio receivers may be made easily by taking advantage of the innovations discussed within this article. ILENTY of 1943 radio men never as good) and requires only a 6 -volt heard of a tubeless receiver. filament battery. At the same time, But we can readily understand it does not require the constant re- that, because the price of tubes somer- adjustment common to the crystal. saulted around 1924, long before the The well -known miniature copper - present crop of servicemen had gotten oxide rectifier, used by radio men to the bug, and we then completely aban- make d.c. meters read a.c., may be doned the simple crystal detector. used, as will be shown later, instead Prior to that time, however, there of the crystal. However, the rela- were few men or boys in any neighbor- tively high capacitance of this device hood who had not built a crystal set, limits its sensitivity and restricts its and many families relied upon this high- frequency use. Inherently, it is simple receiver for local broadcasts not as sensitive a detector of radio Fig. 3. for sometime after tube prices waves as is the crystal. dropped. The present value of simple sets nite, zincite, calchopyrite, etc. Of the There were also other simple detec- built around these elementary detec- synthesized materials, carborundum is tors in use in the early days; some tors lies in their importance as emer- the best known. preceding the crystal detector by a gency receivers. Such sets may be The "crystal" is not a perfect recti- few years and others trying for con- placed into immediate service when fier. Actually, it offers high resist- temporary recognition. They in- power lines have snapped out and "B" ance to current flow in one direction cluded (1) the electrolytic detector, batteries are not available for the (although not cutting off current in (2) the contact rectifier, (3) the me- family portable. They may be car- this direction completely), while al- tallic oxide rectifier, and (4) home- lowing more or less ready flow in the made diode tubes improvised from other direction. Inserted in a radio - au- frequency circuit, therefore, the crys- headlight bulbs. ANTENNA CUT TO FREQUENCY. tomobile tal will rectify the r.f. alternations "Horse- and -buggy" receivers em- OF TRANSMITTING STATION and deliver d.c. pulsations to a pair of ploying the simple detectors are just headphones. as workable today as they ever were. SIMPLE DETECTOR And, although they are less sensitive Crystal Sets than modern tube sets, the present A circuit to obtain this effect is war emergency may yet recall these shown in Figure 1 -A. The combina- simple sets from their resting places tion Ll -L2 -C1 comprises a conven- on museum shelves. tional receiver input circuit of the Threatened tube and battery short- HIGH RESISTANCE HEADPHONES coupled type. The primary coil Ll is ages, and the possibility of power -line the antenna inductor, serving to in- failures, remind us that cheap crystal duce a radio- frequency voltage in the sets require no power of any kind for secondary L2. The variable condenser operation and can pick up local sta- Cl tunes the circuit to the signal fre- 25- tions within a to 30 -mile radius Fig. 2. quency. with good volume. And they operate r.f. developed across L2 ried into bomb shelters where electric The voltage on short waves as well as broadcast is higher than that across L1 by a f ac- frequencies. power is not available. Particularly tor equal to the turns ratio of the The home -made diode tube delivers unique is the fact that they may be coils. in be transformer formed by the two almost as good a headphone signal as made so small size as to carried This voltage is presented to the cir- a crystal detector (in some cases, just easily in a pocket or handbag. and head- We in cuit comprising the crystal shall describe this article the phones, and is rectified by the crystal Fig. 1. theory and construction of the more delivers d.c. pulses to and con- which, in turn, efficient simple detectors the headphones. Capacitor C2 by- structional details of several receivers for r.f. The crystal incorporating them. passes the latter Fig. 4. Iloau the Crystal Detector Works Best known and most widely used of all the early rudimentary detec- tors was the crystal type. These de- tectors are still obtainable in some of the large city ten -cent stores for about 25c complete with crystal. Operation of the device is based upon the rectifying properties of small lumps of certain minerals which we have termed crystals. Common among these minerals are galena, iron pyrites, copper pyrites, silicon, graphite, bor- 28 RADIO NEWS .