WIRELESS

Wanganui Chronicle, Volume LXXXI, Issue 19007, 7 May 1924, Page 9

WIRELESS

Ay

The Grid

Ether Congestion. It is quite apparent to most wireless men that the time is not far distant when the number of stations in operation will be so great that some improvement in existing methods will be required if jamming is to be prevented. Almost every week now stations are being erected, and in certain parts of the world almost all the available wave lengths have been taken up. Wireless engineers have for some time been conducting experiments with the hope of arriving at somo solution of this serious problem. One method, invented by Scott-Taggart, involves the use of frequency multipliers, which step up the frequency of the received signals so that advantage can be taken of the greater selectivity on the higher frequencies. This method is of great use where long wave lengths arc employed in transmission, but it is no use whatever on short waves. By short waves we mean waves under 1000 metres or so. These waves are very largely used for the transmission of telephony because of the great range which can be covered with comparatively low power. With the great increase in the number of stations, however the result is that clashing between two stations often occurs, ruining the transmissions of both. Mr. H. J. Tyzzer, an American scientist, has recently developed an entirely new method of preventing jamming, which also to a large extent eliminates static disturbances. In the past many engineers have suggested what is known as “note tuning,” i.e., obtaining a beat note of definite frequency and passing this through suitable filters, but unfortunately this system is not entirely effective. Similarly, interrupted c.w. has been used, and the note of interruption passed through filters, but this method is no better than the last. In the system proposed by Mr. Tyzzer interrupted continuous waves are used, but instead of interrupting them at audio-frequency, a ra-dio-frequency is used. For example, if the carrier wave is at a frequency equivalent to 200 metres, the interrupting frequency may be arranged at 100,000, which corresponds to a wave length of 3000 metres. At the receiving end the 200-metre signal is received in the usual way, except that instead of telephones in the first detector circuit another circuit tuned to 3000 metres is inserted. This is connected if desired to several stages of radiofrequency amplification, and finally a detector is employed. To explain the method in another way we may say that a continuous wave of 200 metres is used as a carrier and oscillates in the aerial. This wave is broken up by a lower frequency, which may be any frequency desired so long as it is well separated from the carrier frequency. If telegraphy is being sent the interrupting frequency is keyed, while if telephony is desired the interrupted frequency is modulated by the usual choke control system. By inserting a trap in the aerial circuit the direct radiation of the interruption frequency is prevented. In the receiving set an ordinary detector valve (non-osciilating) is employed for the first stage. In the plate circuit of this valve a tuned ra-dio-frequency transformer is inserted. This is arranged to operate at the interruption frequency of the transmitter and its secondary is connected to a radio-frequency amplifier which is also tuned throughout to the interruption frequency. The radio frequency amplifier is followed by a detector valve and if desired by audio-frequency stages. Those amateurs who have some knowledge of the Armstrong supersonic hy terodyne will notice that the new method is somewhat similar, although the actual idea is quite different. The Armstrong method creates a beat note at radio-frequency in the receiving set by using an external heterodyne, but the new method creates the interrupted frequency in the transmitter instead of the receiver. It will be seen that it is possible to receive signals only when the receiver’s first valve is tuned to the carrier frequency, and the subsequent valves to the interruption frequency. On an ordinary receiver only the pure unmodulated carrier would be heard. With a little consideration it will become obvious that the new system has several advantages. In the first place several stations can work with the same, or nearly the same, carrier frequency, but will not jam each other provided their interruption frequencies are well separated. Thus the number of stations working within a given area may be increased several times. Furthermore static distdrbances will not be of great importance, since these tend to make the aerial circuit oscillate at its natural frequency, which is not reproduced in the following stages. Another important benefit to be obtained with the new system is that of comparative secrecy. The receiver tuning must be correct in two places at two different frequencies and it is unlikely that any receiver accidentally hitting on the carrier frequency would also hit on the interruption frequency. As was before mentioned the ordinary self-heterodyne receiver wojild pick up only the pure carrier wave, and this would give no indication whatever either of the station transmitting or the interruption frequency used. There seems little doubt that the method outlined above will be of great assistance to stations in crowded areas. For broadcasting the new method is not of much use, since the receiving station will be unduly complicated, but for secret duplex telephony the method has undoubted possibilities.

K.G.0., Oakland, California, v Quite a number of local amateurs have heard this station during the past month or so; in fact, reception from it is now quite an ordinary matter. “The Grid” has recently seen a particularly full description of the plant, and is pleased to be able to give hereunder a few particulars which should be of interest. The station is the property of the General Electric Company, which also operates station WGY, situated near the Atlantic Coast. The size of the station is amazing; it may convey some idea to the amateur to say that the complete circuit diagram is 18ft. 6in. long and 2ft. wide. In designing the station 500 drawings were required. In describing the plant it is convenient to divide it into four general groups, comprising (1) the aerial system, (2) the power station, (3) the control room,

and (4) the studio. The aerial is of the multiple tuned type consisting of three 14 gauge phosphor bronze wires spaced 12ft. apart. The flat top is 120 ft. long and is supported by two 150 ft. masts. The counterpoise used consists of a network of wires suspended 10ft. above the ground. Six equalising coils are used in the counterpoise for the purpose of balancing the various parts. The power station receives power from a 220 volt 60 cycle 3 phase source, and contains all the apparatus" necessary for the tion, modulation and control of the high frequency power. It is not possible to go into detail, but it may be said that the maze of apparatus is bewildering even to one with some knowledge of transmitting plants. The plate supply for the oscillating tubes is supplied through kenotron rectifiers at 15,000 volts, with a ripple of less than one-tenth of one per cent. The main generating valves are of the watercooled type, and have associated with them fans, radiators, and pumps very similar to those used in a motor-car. Special apparatus is installed for the purpose of determining very closely the frequency of transmission, on which the wave length depends. The tuning and coupling unit is of largo size, included being apparatus for eliminating harmonics. The modulation tubes are of the water-cooled type. Power amplifiers are used before the modulator tubes. For the purpose of determining the state of the plant an artificial aerial is provided, so that tests may be conducted without interference. The control room is located 1600 ft. from the power station, and contains apparatus which controls the whole operation of the latter. The operators in the control room are able to check the modulation, radiation, etc., and alter the operation of the units in the power station as may be desired. The studios are under the observation of the control room operators, who can thus make any adjustment required for a particular item. The microphone amplifier banks are situated in the control room and any microphone can be switched on to any bank as desired. In case of trouble one bank of amplifiers can be switched out and another substituted immediately. The studio equipment consists of the usual musical instruments, signal lights, microphones and controls. A separate director’s control, which is situated in the studio, places the whole apparatus within the control of one man. Every piece of apparatus in the station is duplicated so that if trouble occurs an immediate throw-over can be made. The station is rated at 5000 watts, which is about five hundred times the power used in our local station, 2AH! This will give some idea of the magnitude of the undertaking. Altogether the station is most complete in every way, and the expense incurred in its erectibn must have been tremendous. “The Grid” has been wondering for some days where the company secures a return sufficient to recoup them for the outlay involved. There must be a tremendous profit made in sales of apparatus in America.

Wireless Demonstrations. Those who attended the exhibition of apparatus conducted by the local Radio Club a few months ago will remember that the loud speaking demonstration was somewhat of a failure. “The Grid” has just received a report of a demonstration conducted in England a short time ago, in which the results were also somewhat poor. In the hope that some of his readers are prepared to accept a somewhat belated explanation of the failure of the local demonstration, he publishes the following particulars of the English results: The demonstration in England was held only three-quarters of a mile from 2LO, London, which station operates on 1500 watts. The initial experiments were made on an aerial some 15 miles from 2LO, and excellent results were obtained using a crystal detector and a four-valve resistance coupled amplifier. The apparatus was transferred to the demonstration room the evening before the opening, and tests conducted. To the surprise of the operators the energy received was negligible. An inside aerial was being used, so steps were taken to alter this. An aerial 100 ft. long was erected under the glass roof of the building, but the lead in had to pass near power and telephone lines. This aerial was even worse than the first, so an outside aerial was erected. This gave some appreciable improvement, but still no satisfactory results were obtained. The apparatus was hurried back to the laboratory and a stage back to the laboratory and a stage of high frequency amplification added. With this it was hoped to improve the strength of the detected current so that the resistance coupled amplifier would work. When this was connected, however, it refused duty altogether, so a new amplifier had to be procured. In the meantime everyone was enquiring for the broadcasting demonstration! The new transformer coupled amplifier was at length installed, and two loud speakers connected, with terrible results. The sound was echoing in the room and creating a terrible din. By altering the position of the loud speakers moderate success was obtained, but at any distance from the instruments the distortion was appalling. In the immediate vicinity the results were fair, so no alteration was made. The difficulties were roughly as follows: First, the aerial was bad; second, the power lines, telephone lines, flashing signs, spark coils and other electrical apparatus all interfered considerably; thirdly, the acoustic properties of the hall were bad. Incidentally it may be said as a point of interest to Wanganui amateurs that the English apparatus was in the hands of experts. From the experience they had it is easy to understand the reasons for the nonsuccess of the local demonstration. Almost the same difficulties existed, but the means to overcome them were not at hand. Anyone thinking of holding a demonstration would be well advised to make thorough tests on the spot as a preliminary measure. General Notes.

K.G.O. is reported to have been good last night until the local howlers got going.

2 A.H. is still in dock. “The Grid” can report some progress, as one of the new coils is almost completed. About the end of the month is sug-

gested as the opening date, but probably tests will be made before then. The Australian stations are coming through well lately, 2 B.L. being extremely good. Aussie amateurs can be heard at all hours. No doubt 2 C.M. will be more lively now since Mr. MacClurcan is just back from his trip to San Francisco.

Some of our local prohibited circuit users should get a twist up shortly, also those gentlemen (save the mark) who use a Ford coil and mother’s clothes line to perturb the ether. “The Grid” will not explain why these people will get their deserts; for certain reasons he deems it better not to say anything. But things are moving as most amateurs will soon find out.