Radio Broadcast (May 1928-Apr 1929)

~Z RADIO BROADCAST portant thing. Sooner or later they will appear whether their constants resemble those give above or not. Accuracy of Variable Condensers LAST month we spoke, of the effect on the tuning of a receiver in which one of several ganged condensers was incorrect in its capacity. We have a letter from M. H. Bennett, Electrical Engineer, Scoville Manufacturing Company, which states that an engineering laboratory has determined that on a hundred-degree dial, a discrepancy of 3 mmfd. at 100° is equivalent to detuning one of the condensers by one dial degree, and that such a detuning will cause a reduction in signal strength of about 5 per cent. Another prominent engineer remarks that the figure of one quarter of one per cent, is too high to be maintained in production — we suspect the question of cost enters here — and that the rest of a radio receiver at the present time cannot be built with such a high degree of accuracy. New Radio Tubes in England TABLE I gives some data on new Marconi tubes which are available in England, the land of many tubes. Unfortunately all of the data on these tubes are not available, but interest in this country will be directed toward the low filament consumption of some of these samples of foreign economical tubes. Importance of Reducing A.C. Hum WE HAVE spoken about hum several times. Here is a problem in hum voltages. Let us suppose the power tube delivers 1000 milliwatts to the loud speaker at the loudest signal to be received, and that the weakest signal will be 40 db below this value. This is the normal range of broadcasting, 40 db, corresponding to a power ratio of 10,000 times. Now suppose that at the lowest signal to be heard, the hum output from the loud speaker is not to be objectionable. This means that it ought to be about 20 db below the signal output. This makes the hum power output 60 db below 1000 milliwatts, or one microwatt. If the resistance of the loud speaker to the hum producing voltage is 4000 ohms, the voltage across it is 0.063 volts. Suppose the amplifier is a conventional two-stage transformer-coupled affair using transformers with turns ratio of 3:1 each. The voltage gain of such an amplifier, from resistance output to the primary of the first audio transformer is about 150. The hum voltage across this primary is 0.063-=150 or about 0.00042 volts or 0.42 millivolts. All of this indicates that the maximum hum appearing across the first audio transformer must be no greater than 0.42 millivolts — and yet we remember reading somewhere that the hum voltage in the plate circuit of a heater-type detector tube is of the order of several millivolts. Let us look at the plate-supply device. If the total output potential is about 200 volts, the hum output will be about 50 millivolts. Across the 45-volt tap will be roughly one. quarter of this hum voltage or 12 millivolts. For the sake of argument, let us assume that 90 per cent, of this voltage, which is impressed across the plate-filament circuit of the detector finds its way across the primary of the transformer. This means that due to the plate-supply device alone 10.8 millivolts of hum appears across the input. This voltage multiplied by 150 becomes 1.62 volts across the loud speaker. Let us suppose there is already this much there from the use of an a.c. tube detector, or 3.24 volts in all. This amounts to 2.53 milliwatts, or only 26 db below the maximum output of the amplifier! An assumed hum voltage of 10 millivolts across the 45-volt tap is high because the filter con Fig. 2. — Mr. Browning uses this diagram to demonstrate his selectivity-formulas densers across it, and those across the 90volt tap, get rid of much of the a.c. voltage creating the hum. It is certain, however, that a fairly large capacity should be across the 90 and 45-volt taps. This analysis may account for the fact that many experimenters who have built amplifiers to go down to 120 cycles prefer to use a d.c. tube for a detector and obtain its plate voltage from a 45-volt B battery. And it is surprising how much the low frequencies come up in apparent volume when the a.c. hum is cleaned out of one's receiver, amplifier, power supply, and loud speaker. It is true that so long as the receiver ensemble hums, no signal frequencies lower in pitch than this hum can be heard, and that few signals of the same frequency as the hum can be heard. They must be much louder than the hum — all of which points out some interesting psychological facts. A quiet receiver will always seem to have a much better lowfrequency response than one which has a lot of hum in its output. Receiving on 600 Meters With Lab. Set DUBING the Veslris disaster we listened-in to the traffic to and from ships in the vicinity of the wreck, and discovered many interesting things about the chaotic condition of the ether during such periods. We used our Lab. Circuit receiver with the addition of two fixed condensers which could be thrown across the tuning condensers. These fixed condensers brought the maximum wavelength that could be received up to nearly 800 meters, and thereby permitted the reception of all of the ship-toshore traffic on the several channels between 600 and 800 meters. The circuit diagram is given in Fig. 1. The Yaxley switch is a simple double-pole double-throw unit and the condensers had a capacity of 250 mmfd. Selectivity in the Browning Drake Set IN SEPTEMBER R\dio Broadcast, Glenn Browning described some of the engineering behind the 1929 Browning-Drake receiver. This receiver uses somewhat closer coupling between primary and secondary of the interstage r.f. transformer than has been attained heretofore, with the result, according to Mr. Browning, that better selectivity is secured. This statement "closer coupling, better selectivity" bothers many readers, and so we have asked Mr. Browning to expJain it. We reproduce some of the mathematics below. A brief statement of what happens is as follows: for a fixed amount of amplification in a tuned radio-frequency transformer working in conjunction with a given amplifier tube, the selectivity may be increased by advancing the coefficient of coupling and at the same time decreasing the number of turns on the primary so that the amplification remains the same. This is due to the fact that as the Table I number of turns is decreased and the coupling increased the resistance reflected into the secondary circuit from the primary decreases, and hence the selectivity of the secondary circuit approaches more nearly its selectivity when standing alone and not connected to the plate resistance of the previous tube. Let rjP and t)s be the ratio of resistance to reactance of primary and secondary circuits and Vni be the ratio of resistance to reactance of the secondary when the primary is present. Let RSi be the apparent resistance of secondary when primary is present. Rsi = Rs + M2 oj2 Rp Rs Lsd) TABLE R=i = ^5 + — in general VP .(1) • (2) .(3) T2 = KT/p TT/s for a given amount of amplification. ... (4) Where Rs = Secondary resistance Rp = Plate resistance of tube Rsi = Apparent resistance of secondary with primary present T = Coefficient of coupling K = Proportionality factor less than I. Therefore Tjsi = l)s K T)s and as t is increased the selectivity factor of the secondary increases. It is worth noting that if unity coupling prevails and if the proper number of turns are used for maximum amplification, K =1, the selectivity of the tuned circuit is halved, as all mathematics and experience dictates. Removing Noise in Shielded Receiver THE following letter from a reader in Farmington, Michigan, may help others who have shielded receivers. "I would like to pass along a discovery I made regarding the Sargent-Rayment Seven. I had considerable difficulty at first owing to instability in the r.f. stage. At one time it would work perfectly and the next day it would fly into oscillation for no reason at all. I finally found this to be due to poor electrical contact between "he partitions and the removable cover. 1 procured a piece of aluminum 8 inches wide and long enough to cover the r.f. stages and bolted it securely to the partitions. This cured my trouble." Many experimenters find their receivers do not work after shielding has been added. The trouble lies not with the shielding material of the coils but in the fact that the whole arrangement has not been properly designed. A coil too near a metallic plate will not only lose inductance at an alarming rate but have an astonishing increase in resistance as well. League of Nations to Broadcast Type No. Ef It Gm Rp Ip Ep Eg DEH 210 2.0 .10 35 700 50000 DEL 610 6.0 .10 15 2000 7500 p 625 6.0 .25 6 2500 2400 21.5 160 13 p 625a 6.0 .25 3.7 2300 1600 25 140 18 p 425 4.0 .25 4.5 1950 2300 22 140 14 HL 610 6.0 .10 30 1000 30000 H 8 .8 .8 40 730 55000 HL 8 .8 .8 17 1000 17000 p 8 .8 .8 6 1000 6000 The Secretariat of the League of Nations intends to resume the short-wave broadcast trials which took place in Geneva in May and June of last year. The special purpose of this second series will be to examine the possibility of transmitting speeches from Geneva to the Americas, Japan and Australasia. The trials will take place in the same technical conditions as those held last year. A studio in the League Secretariat in Geneva will be connected by ordinary telephone cable with the Dutch station of Kootwijk (call letters pcll) kindly put at the disposal of the League by the Dutch Post Office authorities. Sixty-minute speeches will be broadcast at 5:00 p.m. (E.S.T.) on 38.8 meters in English, French, and Spanish on March 12, 19, and 26. Thirty minute speeches will also be broadcast in Japanese on March 13, 20, and 27 at 8 :40 p. m. on a wavelength of 18.4 meters, and on March 14, 21, and 28 special thirty-minute Australian programs will be broadcast in English at 8:40 p. m. on a wavelength of 18.4 meters. —Keith Henney • march, 1929 page 301