Radio Broadcast (May 1928-Apr 1929)

23S RADIO BROADCAST ADVERTISER Jenkins & Adair Condenser Transmitter For Broadcasting, Phonograph Becording, and Power Speaker Systems ^^HIS transmitter is a small condenser which JL varies its capacity at voice frequency, and is coupled direct into a single stage of amplification, contained in the cast aluminum case. The output, reduced to 200 ohms, couples to the usual input amplifier. The complete transmitter may be mounted on the regulation microphone stand. It operates on 180 v. B and 6 or 12 v. A battery. This transmitter contains no carbon, and is entirety free from background noise. Its yearly upkeep is practically nothing. It is extremely rugged, and will withstand hard usage. I'ricc, complete with 20 ft. shielded cable, $225.00 F.O.B. Chicago. J. E. JENKINS & S. E. ADAIR, Engineers 1500 N. Dearborn Parkway, Chicago, U. S. A. Send for our bulletins on Broadcasting Equipment ATTENTION! B Eliminator Constructors Owners Solve all B eliminator problems. Stop motor boating. 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No. 211 Radio Broadcast Laboratory Information Sheet August, 1928 Soldering Irons HOW TO CARE FOR THEM pRACTICALLY all commercial soldering irons are designed to heat rapidly so that they will be brought to an operating temperature within a short time after they are connected to the line. Unfortunately, however, if they are left connected to the line after they have reached an operating temperature they become too hot for satisfactory work, the tip blackens very quickly, becomes pitted, and in a comparatively short while the iron requires a new soldering tip. All this trouble can be easily overcome, and the manner in which it is done in the Laboratory may be of interest to readers. The arrangement used in the Laboratory is indicated in the diagram. The soldering iron is connected in series with an ordinary electric light bulb across the power line. A shortcircuiting switch, S, is provided across the bulb. The procedure when some soldering is to be done is to push the plug in to the light socket and close switch. S. With the switch, S, closed the iron is then connected directlyiacross the line and reaches a satisfactory operating temperature quickly. When this temperature is reached the switch is reopened so that the electric light is in series with the iron. The size of electric light used is such that the iron is maintained at the correct temperature and does not overheat even though the power is left on for hours without using the iron. If an arrangement such as this is used the tip of the iron will remain tinned for a long time, and a better soldering job can be done. The wattage of the electric light bulb that is used depends upon the soldering iron. The particular irons used in the Laboratory work best with a 75-watt lamp. The switch, S, may be almost any type although it is a good idea to use some kind of enclosed switch designed for use on 110-volt lighting circuits. No. 212 Radio Broadcast Laboratory Information Sheet Equalizers August, 1928 WHY THEY ARE USED IN TRANSMITTING outside events (programs *■ that do not originate in the studio) broadcasting stations have to make use of wire lines to connect the control room of the station with the microphone and amplifier apparatus located at the point at which the program originates. These wire lines must transmit with equal efficiency a band of frequencies from about 100 cycles to about 5,000 cycles. In order to give a wire line such a characteristic it is necessary that it be "equalized" so that the transmission efficiency will be equal over the entire band of audio frequencies. The device used to give a line such a characteristic is termed an "equalizer" and its action will be explained in this sheet in conjunction with the diagram on Sheet No. 213. The frequency characteristic of a seven-mile length of cable is indicated in curve A on Sheet No. 213. This characteristic shows that the cable transmits the low frequencies much better than the high frequencies, due to the fact that there is considerable capacity between the two wires that form the pair of cables and this capacity tends to bypass the higher frequencies. Equalization is accomplished by introducing into the circuit a device that will lower the transmission efficiency at low frequencies to a value equal to the efficiency at high frequencies; this is the function of the equalizer. The equalizer consists of a network of resistances, capacities, and inductances of values such that they introduce a considerable loss at low frequencies where the transmission efficiency of the cable is high and practically no loss in efficiency at the high frequencies where the transmission efficiency is low. The result is that the efficiency of the entire system is reduced to approximately the efficiency of the cable at the highest frequencies to be transmitted. The curve B shows the characteristics of the cable with the equalizer in use; the frequency characteristic is sensibly flat from 100 cycles to 5,000 cycles. As was mentioned above this betterment in the frequency characteristic is obtained at a considerable reduction in overall efficiency. The low efficiency is then compensated by connecting repeaters (power amplifiers) in the circuit to raise the power level of the entire system. The broadcasting circuit connecting New York with Chicago contains about eight repeater points. Power amplifiers are located at these points and function to boost the power in the line to overcome the loss in the cable. As a result we frequently find cases where the final amount of power at the receiving end is considerably greater than the power originally introduced in the line at the transmitting end. No. 213 Radio Broadcast Laboratory Information Sheet August, 1928 Frequency Characteristic of a Seven-Mile Cable 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 FREQUENCY IN CYCLES PER SECOND