Radio broadcast .. (1922-30)

144 RADIO BROADCAST DECEMBER, 1927 very specific and delicate conditions of operation are maintained for it. A piezo-electric substance for radio crystal control purposes must have certain internal atomic properties, and it must be hard, durable, and not easily broken down physically or electric- ally. Quartz is the best commercial product so far offered to fill these requirements. The manu- facture of quartz crystals for radio purposes is a specialized subject and, as few broadcasters are likely to attempt grinding their own crystals, need not be discussed here. The crystal should be optically ground to oscillate at only one fun- damental frequency. If the frequency is to remain constant, the crystal must be maintained under constant physical conditions as a prerequisite. This includes an unvarying contact pressure and temperature. When the temperature changes the dimensions of the crystal change and the natural frequency varies proportionately. The crystal must be kept clean; a drop of oil or water in- troduced between the holder and the quartz slab will usually stop oscillations altogether. It follows that in a broadcasting station installation the crystals are usually kept in a dust-proof box whose temperature is thermostatically controlled. Some commercial crystals, in addition, are sealed into small individual containers, provided with lugs designed to slip under binding posts. The actual contact with the crystal is inside the con- tainer. If springs or screw clamps are used to make contact with an open crystal care must be taken, to secure parallel movement of the metal surfaces, so that the crystal is not subjected to pressure on part of its surface and left untouched elsewhere. A loose contact leads to brushing, heating, and possible cracking of the crystal. The capacity of the crystal holder should be small in order that its piezo-electric variations may exert the maximum governing effect on the cir- cuit of which the crystal is a part. It will be noted that there is some design analogy between the old-style crystal detector stand and the quartz crystal holder of a modern tube transmitter; each has protean forms. Some illustrations of actual crystal holders will be found with the RADIO BROADCAST papers mentioned in the bibliography, and Crossley includes a detailed description of the contact requirements in his paper. The radio-frequency energy in the initial crystal circuit may amount to a fraction of a watt, while the final stage may deliver many thousand watts to the antenna. It is clear that great care must be taken to prevent feed-back, parasitic oscillations, and unstable circuit con- ditions along the line. Under some conditions of circuit imbalance the crystal is likely to overheat and be damaged. The transmitter may stop oscil- lating. In the endeavor to control regeneration and secure circuit stability, designers have fre- quent recourse to shielding and neutralization of successive amplifier stages, and sometimes push- pull radio frequency amplification is employed, resulting in a series of balanced circuits anal- ogous to those of low frequency telephone practice. In a crystal-controlled telephone transmitter, modulation may take place in the final stage or at an intermediate point after the crystal but before the final stage. The advantage of modu- lation at a low power level lies in the possibility of securing ample modulator capacity relative to the radio frequency energy to be modulated. But if, for example, modulation takes place in one of the earlier stages at a power level of, say, 50 watts, care must be taken not to impair the audio-frequency characteristic of the transmitter by cutting of the side bands in successive tuned stages, and of course the power tubes must have sufficient capacity to handle the peaks of modu- lation. The Bell Telephone Laboratories engin- eers seem to incline toward low power modula- tion, while the Westinghouse and General Elec- tric engineers prefer to wait until the full radio frequency power is developed before impressing the audio frequency on the carrier. The power of successive stages depends on tube characteristics and the use to which the transmitter is to be put. Crossley shows a i 50- 6oo-kc. telegraph transmitter in which the crystal controls a y.j-watt tube, which is followed by a 5O-watt impedance-coupled amplifier, a i- kw. tuned amplifier stage, and the final 2o-kw. stage feeding the antenna. These figures and some others in this paragraph represent the nominal oscillator ratings of the tubes in ques- tion; the powers actually developed are generally less. Meissner mentions a telephone transmitter in which, after the crystal tube, 5-watt,75-watt, joo-watt, and j-kw. stages are found, the last named supplying 3 kw. to the antenna. The Na- tional Broadcasting Company's Bound Brook telephone transmitter (made by Westinghouse for R.C.A.) uses a y.j-watt tube in the crystal stage, swinging a 2jo-watt tube, followed by two 250*5 in parallel (500 watts) before the final 4O-kw. bank. Bellmore, built by General Electric for the N. B. C., uses more stages; the crystal, likewise governing a y.j-watt tube, is coupled to another of the same size; then follow a 5o-watter, two fifty-waiters in parallel (100 watts), a looo-watt tube, a single 2o-kw. tube run at about a quarter of its rating, and the final jo-kw. stage. In both of these American transmitters, plate modulation of the final power stage is used. In the Bellmore transmitter the output of the crystal stage is purposely kept low as one of the design consider- ations, only about 0.5 watt being generated, while Crossley mentions getting 21 watts from a crystal-governed tube of the same type, in one of the U. S. Navy experiments. This divergence shows how design calculations determine operat- ing conditions. Fig. i is a schematic circuit diagram of a crystal-controlled telephone transmitter without the audio amplifier and the power supply to the modulator and r. f. output stage. The power rat- ings of the successive stages have been omitted because, as indicated above, various sizes of tubes might be employed in such a chain, ac- cording to the final output required, the tube characteristics, and other design factors. The Small Broadcaster IN A letter of some length, which we should print in full if the space were available, Mr. Robert A. Fox, formerly owner and manager of Station WLBP of Ashland, Ohio, takes me severely to task for my past animadversions on incompetent broadcast technicians, which he ap- parently thinks were aimed exclusively at small stations, and then goes on to a penetrating dis- cussion of the small stations' economic problems. Mr. Fox points out that in arranging high quality programs the small station is at a dis- advantage, "for the simple reason that musicians do not charge you for their time according to the power of your station, but in accordance with the number of hours required of them." The ad- vertiser, on the contrary, will pay more or less proportionately according to the power. The members of the station staff are in the same posi- tion as the musicians. The result is that one man must sometimes assume the staggering burden of acting as "station monitor operator, an- nouncer (doing a nemo job of it), operator; chief engineer as well as salesman, financier, publicity agent, and program director," all this with one assistant. Even then the structure collapses under the fixed expense, and Mr. Fox concludes, "The small broadcaster is economically doomed." But, he insists, the failure is economic, not per- sonal—"the fellows who have been operating under 1000 watts have more brains than those above 1000 watts." They may not have more brains—nature does not distribute brains according to antenna watts, either in direct or inverse ratio—but they cer- tainly have more courage. And, while economic- ally they may be sick, they may yet survive, on some other basis, to see a better day, No one can say, at this juncture, that the small neighbor- hood station will not find a place in community life, with some form of cooperative support, in a frequency band wherein it can serve local in- terests without interfering with the large stations and networks aspiring to national coverage, and be in turn protected from interference by them. As to the less material matter of my own atti- tude toward such enterprises, it is a curious commentary on our American attitude toward criticism in general that when a man states baldly, in public, unpleasant facts about institu- tions or people, he is immediately suspected of being hostile to those institutions and people. That it is his right and duty, once he has set up as a critic, technical or social, to discriminate be- tween what he finds good and bad, is a basic fact not sufficiently recognized among us, in radio and elsewhere. There is still a lot of bad broadcasting and incompetent operation going on. No one with a pair of ears and the tonal discrimination of a tomcat can think otherwise. There is also a large and growing element of good showmanship, efficient operation, and skilled personnel, among both large and small stations, and 1 have not been backward in giving credit to those responsi- ble for such progress. The standards have been lower among smaller stations, because of the lack of resources and, sometimes, because of the lack of time and skilled personnel. All these factors go together. If a man tries to act as announcer, engineer, operator, program director, and publi- city representative of a station he will inevitably turn out a half-baked job in each capacity. He may be a hero, but he is not a broadcaster by 1928 standards. One may admire his courage and still tell him what one thinks of his audio frequency band and the quality of his sopranos. As for constructive contributions, I have tried to do my part by writing technical articles which are of use largely to the smaller broadcasters, be- cause the information contained in them is com- mon property among the operators of the bigger stations. Let that be weighed against my refusal to be a member of a cheering squad.