Radio Broadcast (Nov 1926-Apr 1927)

Record Details:

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302 RADIO BROADCAST ADVERTISER Sasy with the 1-1 Now you can have radio entertainment in any room, porch or lawn without disturbing your set! The E-Z Extension Connector hooks up to your set easily and quickly, enabling you to carry the loud speaker anywhere. Exclusive tension slot construction assures positive contact. Connector only 35C with 20 ft. cord— $1.25 with 35 ft. cord— $1.75 " 25 ft. cord— 1.50 " 50 ft. cord— 2.00 Raytheon Block Condensers Guaranteed lOOOvolt breakdown test. Finest materials and special impregnating compounds embodied in Polymet Block Condensers assure the lasting success of your construction job. Specify PolymetCondensers! Type 1000-14 Mfd. tapped at 1, 1,2,2, 8 $9.50 Type 1001-.1-C-.1 Mfd. . . 2.00 Poly Claro-Plug Every set deserves one — turns old set into new — improves tone — broadens the range — eliminates scrapes, rasps, hissing. Reduces static. " Over 125 manufacturers of receivers and power units use Polymet Products. THEY KNOW. Follow their lead — specify Polymet Products for best results. At all good dealers everywhere or send direct. Send for illustrated descriptions of all products. FREE on request. Polymet Manufacturing Corporation 599 B Broadway New York City "World's Largest Manufacturers of Radio Essentials" Polymet Products No. 62 Radio Broadcast Laboratory Information Sheet January, 1927 Antenna Power Dissipation DISTRIBUTION OF ENERGY THE power supplied by an oscillator to a transmitting antenna, is dissipated in three ways: First, in the form of radiation; second, in the form of heat due to resistance of the wires in the circuit; third, in the form of heat due to dielectric absorption. Only the first of these factors represents a useful dissipation. This radiation is the power which travels out from the antenna in the form of electromagnetic waves (as signals). Curve A in the accompanying drawing shows how the radiated power varies with the wavelength, it being [proportional to the square of the antenna current, and inversely proportional to the square of the wavelength. Curve B represents the power used up in the resistance of the wires. This is a straight line and does not vary with the wavelength. In actual practice, the eddy current loss and skin effect might be slightly greater at the lower wavelengths, but the variation is so small that it may be neglected. Curve C illustrates the variation with wavelength of the power absorbed in the dielectric, and, since this absorption is proportional to wavelength, the curve is a straight line. This loss is due to trees, buildings, masts, or other objects in the vicinity of the antenna which absorb power. Curve D represents the total power in the antenna, and is equal to the sum of the three separate curves. In taking curves such as this on an antenna, it is quite possible to obtain humps at certain wavelengths. This generally indicates the presence of some circuit in the vicinity of the antenna with a natural period of oscillation at that wavelength. WAVELENGTH Radio Broadcast Laboratory Information Sheet Line Power-Supply Devices January, 1927 calculation of resistance values TN ORDER to obtain four output voltages from a *■ line power-supply device we will place four resistances, Ri, R2, Rj, R<, in series across the total output of the device. One end of Ri will connect to the negative B and one end of R4 will connect to the maximum voltage terminal of the device. The positive voltage tap, E, for the detector (221 or 45 volts) will be taken off between Ri and R2. The voltage, E2 (generally 90 volts), for an r.f. amplifier, will be taken off between R2 and Ri. Voltage Ej, with a value of, say, 135 volts, will be obtained from a tap connected between Rj and FU. E4 is the maximum voltage of the unit. In order to calculate these resistance values, we must assume that a certain amount of current flows through the resistance Ri. An average value that can be assumed is 5 milliamperes, or 0.005 amperes. If we assume this current to flow through Ri, and that we desire 221 volts for the detector, then Ri = 22.5 -r 0.005 = 4500 ohms. If the voltage required is to be 45, then Rt = 45 0.005 = 9000 ohms. The voltage across R2 is 90 — 221 = 671, and as the detector plate current at 221 volts is usually about 0.0005 amperes, this current, plus the 0.005 amperes loss current, flows in R2, hence R2 = 67.5 -5 0.0055 = 12,300 ohms approximately. In the case of 45 volts on the detector, the R2 voltage would be 45 (90 — 45) but the detector plate current at 45 volts is now about 0.001 amperes, hence R2 = 45 0.006 = 7500 ohms. To determine the current in R3, we must know the plate current taken by all the tubes operating at 90 volts. Assuming there are two r.f. tubes (uv-201-a) only, the current taken by each when biased at 4.5 volts is 0.002, or 0.004 for both. The voltage across R> is 135 — 90 = 45, and the current flowing in R3 is 0.005 + 0.0005 + 0.004 (for a detector plate voltage of 221), therefore Ra = 45 -5 0.0095 = 4750 ohms approximately. In the case of 45 volts on the detector plate, with 0.001 amperes flowing, we have R3 = 45 -5 0.01 = 4500 ohms. The current in Ri is the sum of all the currents plus the current in the plate of the first audio tube ((UV-201-A). The plate current in an UV-201-A at 135 volts with 9 volts negative bias is 0.0025 amperes and, in the case of 221 volts detector, the total current in R4 is 0.0005 + 0 005 + 0.004 + 0.0025 or 0.012 total. The voltage across fU is 400 — 135 = 265. Hence, R4 = 265 0.012 = 22,000 ohms approximately. Case 1. 1 UV-201-A Detector, 221 volts. 2 UV-201-A R. F. £0 volts, Neg. bias 41. 1 uv-201-A A. F. 135 volts, Neg. bias 9. 1 ux-210 A. F. 400 vclts, Neg. bias 30. Ri = 4500, R2 = 12,300, Ra = 4750, R4 =22,000 Ri Case 2. Same as above except 45-Volt Detector. 9000, R2 = 7500, R3 = 4500, R( = 21,200 No. 64 Radio Broadcast Laboratory Information Sheet The Gang Condenser January, 1927 CORRECT CONNECTIONS SINCE the appearance on the market of gang condensers, that is, condensers having a common shaft and working in unison, many readers have requested information as to how they should be connected. The difficulty in connecting them usually arises from the fact that all of the rotor plates are connected together, thus making it impossible to get a positive return lead for the detector tube, while the radio frequency tubes of necessity must be negatively baised. By looking at the diagram, it will be noticed that the filament return of the detector tube coil does not connect to the variable condenser but to the positive filament lead. A path for the radio frequency current is provided through the bypass condenser, as shown, to the tuning condenser. An alternative way of connecting is to allow the coil to be connected to the condenser, making the grid positive by connecting the grid leak directly between the grid of the tube and the positive filament. The grid condenser prevents the short circuiting of the batteries. When a gang condenser is used, the coils must be carefully matched in order to minimize any inequalities between them. In spite of careful matching. there are bound to be some discrepancies, and it is frequently necessary to use a separate condenser in the antenna circuit, as outlined in Laboratory Sheet No. 33. Some gang condensers are provided with small condensers in parallel with the main condensers, which may be used to bring each circuit into exact resonance. ■J( Examined and approved by Radio Broadcast "A