Radio Broadcast (May 1928-Apr 1929)

Detector Distortion Many times we , have read about radio receivers so engineered that the detector did not overload, or receivers in which the detector did overload, or some other reference to distortion due to detector overloading. This leads us to ask such questions as: When does a detector overload? What does the output sound like when such overloading occurs? Is it true that a C bias detector will handle much larger input voltages without overloading? If so, how much? The answers to some of these questions are being sought in the Laboratory, and as fast as the material is ready it will be presented in these columns. In the meantime, one of our friends has determined, empirically, that the average detector begins to overload when the detector delivers about 15 tu below 1.0 milliwatt. What does all this mean, you will ask? Let us suppose that a detector has an output impedance of 30,000 ohms and that it works into a load of this impedance, say a resistance or a transformer of proper characteristics. Fifteen tu below 1.0 milliwatt is equal to about 0.03 milliwatts, or about 30 microwatts. What voltage across 30,000 ohms will deliver this amount of power? This is the useful voltage, for it is what the amplifier boosts in value so that the final power tube in the system will deliver its rated output. This voltage may be found by extracting the square root of the product of the power by resistance. Or. the E = vWoXR = V0.03X 10— 3X30,ooo = ivolt (approximately) Therefore, a detector which will deliver 30 microwatts to 30,000 ohms without overloading will produce 1 .0 volt across the input to the amplifier. It now remains to prove under what conditions the detector will fulfill these expectations, and when overloaded, how the average experimenter can tell it by the sound of the output. Has any reader experience in this matter? The following letter apqA Short-Wave ropos of short-wave experioAdapter mental broadcasting and its reception is interesting: It comes from C. R. Strange, of Sydney, Australia. "On the page 'Our Readers Suggest' in the December Radio Broadcast, there is described 'A ShortWave Converter for any Radio Receiver' by Perry S. Graffam. " It will be of interest to your readers to know that out here in Australia we appreciate your journal and that several days ago I built this adapter to plug into my Grebe Synchrophase which was presented to me by the A. H. Grebe Company of Richmond Hill, New York, following my reception of their station, wahg, on 314.5 meters. "An hour after I had finished Graff am's adapter I was listening to the transmission from 2 lo (London) through the short-wave station 5 sw Chelmsford England, on 24 meters, using the two audio valves of the Synchrophase receiver and my model100 loud speaker. On the 25th (January) I listened wonderfully to this station for 55 minutes, the volume being audible some 15 feet from the speaker. Also on the 25th and last night I listened also with wonderful success; here, some 13,000 miles away, it is rather thrilling to listen to London broadcasting at trays Laboratory midday such items as selections from Cavalleria Rusticana, Percy Grainger's pianoforte arrangements, Carmen, and a fine tenor voice singing 'The Sargent Major's on Parade.' " We surely are in a wonderful age. Televison will be the next thing for Australia." There have been numerShort-Wave ous attempts to convince the broadcasting public and, we imagine, the Radio Commission as well, that broadcasting should take place on the highfrequency (short-wavelength) bands. Let us look only at the problem of keeping a station on its assigned frequency which, for sake of argument we shall assume to be 10,000 kc, or ten million cycles. Many broadcasters are having difficulty in keeping their present transmitters within one-half kc. of their assigned frequency. What would be their troubles if they worked at 30 meters? Five-hundred cycles in 10,000,000 represents an accuracy of one part in 20,000, or fivethousandth of one per cent. At the present time, a station operating on 1000 kc. must keep its assigned frequency to within 500 cycles in one million which represents one part in 2000 or five-hundredths of one per cent. In other words, it would be about ten times as difficult to keep a station on its frequency at 30 meters as it now is at 300 meters. We understand the Navy builds short-wave equipment that must be accurate to within 100 cycles at 30,000 kc. That is, they build a transmitter to this specification, and a receiver to go with it and the sum of their percentage inaccuracies must not be over 200 cycles in 10 million, or 100 cycles for the individual unit. This represents an accuracy of one part in one hundred thousand, or one ten thousandth of one per cent., an accuracy 50 times as great as that required of stations now operating within the broadcast band. These Navy units, it should be noted, are designed for code transmission and reception. 29 Radio Broadcast TShsetJlnamatie' takes pride in quoting ys em m ^ ^ letter from England f . Louis G. King, whose system of tuning, known as the Equamatic System, was first described in this magazine. Mr. King has just returned from Europe and that his trip was successful may be surmised from this part of his letter: "Recently we have entered into an agreement with Graham Amplion, Ltd., for the production of the Equamatic System in the British Isles. After carefully testing radio receivers produced by the leading manufacturers in all parts of the world, Graham Amplion, Ltd., adopted the Equamatic System." The system has, in this country, been most closely associated with receivers designed by the Karas Electric Company. Many times in the Finding Ore past year or so, the edi by Radio tors have been asked to forward information on devices useful in finding ore or hidden treasure by radio. Up to the present time it has been impossible to give any authentic information regarding such apparatus, and therefore, we are appealing to the readers of the magazine. What is wanted is information on methods and apparatus used, whether using radio or other electrical apparatus, results secured, and articles published whether in this country or abroad. This will enable us to help prospective treasure hunters toward a financially successful jaunt. Radio listeners in cornea/ Power munities where there is no from t-wo 112's a.c. power available need not feel that it is impossible to secure sufficient power to operate their loud speakers properly just because they cannot tap the house lighting system and get high voltages and considerable plate current therefrom. For example, two 112-A tubes will deliver considerable power without excessive plate voltage or current — which means that the farmer who has no power equipment may secure good quality and plenty of loud speaker power from B batteries, and do it economically. The table below shows the relative power output and necessary grid a.c. voltage to deliver this power from a single 171 or two 112's in parallel. Note that two 112's in parallel with 157 volts on the plate require 16 milliamperes from the B batteries and deliver 400 milliwatts of power to a loud speaker on only 10.5 input grid volts while a 171, taking the same current from 135 volts, requires a grid voltage of 27 to produce 350 milliwatts. Two 1 12's in parallel will have an output impedance of about 2500 ohms which will work very well into the average loud speaker: Tube Ep Ec Ip Watts 171 90 135 157 180 2— 112's 90 135 157 16.5 27 33 40.5 4.5 9.0 10.5 11 16 18 20 8.0 .08 11.6 .240 16.0 .400 Output .12 .35 .50 .65 Some tubes from Sylvanh Tested Products Co., have been Products measured in the Laboratory recently. The values given below are the average of six of each type of tube: Type Ip [>■ Rp Gm Ep Eg 201-A 2.2 9.2 12330 750 90 4.5 7.0 7.5 5160 1450 135 9,0 112-A 171 19.0 2.9 2240 1300 135 -27.0