Radio Broadcast (Nov 1926-Apr 1927)

354 RADIO BROADCAST FEBRUARY, 1927 y A MESSAGE SENT AT 25O WORDS PER MINUTE This is an actual record of a message received at a speed of 250 words per minute during official tests of the beam wireless circuit between England and Canada Many parts, however, had to be designed especially for the work in hand. Vacuum tubes are used throughout for generating the high frequency current. The main power oscillators are cooled by means of oil and are designed particularly to reduce tube capacity and resistance losses. A master oscillator tube is provided which determines the frequency or wavelength at which the main transmitter tubes will oscillate. The voltage used on the tubes has an approximate range of from 8000 to 10,000 volts, which is supplied by special rectifier tubes. The method used in keying, or interrupting the circuit in order to transmit the dots and dashes of the signals, is of interest. When a signal is being sent, the high frequency current is allowed to go out into the antenna, while, when no signal is being sent, the power is absorbed by an equivalent load made up of resistances placed in a small house near [Z3 Coupling Bo* = Copper Tube Feedet _ I Feede/ System ljuncboni THE FEEDER SYSTEM This may be studied in conjunction with the pictures on page 352 and at the foot of this page. The antenna wires are taken in pairs to the coupling boxes and thence to the copper-tubing feeder system. The distance from the transmitter to each antenna wire is the same THE FEEDER AND COUNTERWEIGHT SYSTEMS The copper tubing which constitutes the line between the transmitter and the antenna system, together with one of the junction boxes, is shown in this picture. To the left may be seen one of the antenna coupling boxes to which two of the antenna down leads are taken. The weird wooden structures, which in windy weather are apt to cultivate bobbing propensities, and which may be seen dotted about the field, are the automatic counterweights which provide a certain amount of slack to the antennas when necessary the transmitter. This makes it possible to keep the load on the transmitting tubes the same at all times. The absorption system is controlled by means of vacuum tubes the grids of which are thrown positive or negative by the keying relay through other amplifying tubes. By using vacuum tubes in place of mechanical relays it is possible to speed up the system to a remarkable degree. Only one small mechanical relay is used to tie up the land line with the transmitter. Everyone who has followed radio to any ' extent in the last few years knows something about the super-heterodyne system of receiving signals. The English engineers use this idea in a new way. It will be remembered that the super-heterodyne changes over the incoming signal from a short wavelength (high frequency) to a longer wavelength (low frequency) in order that it may be more easily amplified. It is again detected and operates an audio amplifier. This system is extended for use in the receivers at the beam stations by using two heterodying systems, or what is practically the equivalent of two super-heterodynes in series. The signal is collected on the antenna, which, being of the reflector type gives a signal of considerable strength, and is fed to the first detector through a very loosely coupled tuned unit. The loose coupling is resorted to in order to cut out interference and reduce the pickup of static and other noises. This first detector is coupled with an oscillator which changes the short wave of 26 meters (11,538 kc.) over to a wavelength of about 1600 meters (187 kc). The signal then goes through a three-stage amplifier at this frequency and is again detected. At this point another heterodyne oscillator is provided and the wavelength changed from 1600 meters to 10,000 meters (30 kc). Again it is amplified through three stages and again detected. This second heterodyne may be tuned to an audible note so that the operator may listen-in and tune the signals as received through the first part of the receiver. The output of the receiver operates a highspeed relay which in turn operates the recording mechanism. Each stage of amplification is of the push-pull type in order to provide distortionless amplification throughout. The output works through a bridge system which insures