RADIO AND ITS RECEIVERS

Sun (Auckland), Volume II, Issue 397, 4 July 1928, Page 14

RADIO AND ITS RECEIVERS

Conducted for THE SUN by R. F. HAYCOCK

Alarm Signal for Ships

OPERATORS CALLED BY BELL Wonderful New Invention OF the part that radio has played in enabling many thrilling rescues of ships in distress to be brought about, volumes have been written, but in the past the greatest drawback has been the fact that a radio operator has had to be continuously on the listen for the S.O.S. signal. This has all been changed now, and instead of having to remain in his cabin day and night, the radio operator can go about the ship, secure in the knowledge that an automatic alarm will summon him if a vessel is in distress. A number of the Union Company’s steamers have been fitted •with the new device, and others will be equipped 3s soon as is practicable. Ry courtesy of Amalgamated Wireless (Australasia) Ltd., I am able to give a description of the new apparatus.

After nearly five years of extensive marine experiments a truly reliable auto-alarm instrument has been produced, which can draw the attention of the wireless operator on a ship during his hours off duty, by a prearranged signal transmitted by some other ship within a reasonable distance. The chief difficulties encountered during these trials were the failure of the apparatus to respond, due to interference caused by atmospherics or other signals, and the danger of false calls. Both of these difficulties have now been overcome, and an instrument has been produced that will operate with certainty on receipt of the pre-arranged signal, and in which the chance of false signals has been practically eliminated. The Marconi auto-alarm is certified by the British Board of Trade, after exhaustive tests, to comply in every way with the requirements of its regulations. The general scheme for the employment of such a device is that it shall be brought into operation during such times as a qualified telegraphist is not available for watch at the wireless instruments, and that the pre-arranged signal shall be strictly confined to use as a preliminary to the wellestablished S.O.S. distress call. This pre-arranged signal will have the effect of attracting the tele-

graphists who are off duty and calling them to their instruments, for which purpose three bells ring. One of the bells is situated in or near the chart room, a second is placed in the operator’s cabin, and the third in the wireless cabin, and all continue ringing until attended to, and switched off in the wireless cabin by the operator, who would then take up watch for the vessel calling. One part of the trials, alluded to above, was to determine the best form of pre-arranged signal for this purpose, and this has been found to be a series of dashes each of four seconds duration, separated by intervals of one second, and to this combination the name of “alarm signal” has been given. The auto-alarm equipment, which is intended to be connected directly to the ship’s aerial when the telegraphist goes off duty, in place of the ordinary receiver, consists of two chief units, the receiver and selector. The receiver consists of a threevalve amplifier and tuning circuits, arranged so as to cut out as much as possible signals on wave-lengths other than 600 metres, and at the same time to be fully sensitive to any wave within 2£ per cent, of 600 metres —the limits of error which are now allowed in the tuning of ships’ transmitting instruments. Signals from the receiver are passed on to the selector, whose duty it is to pick out the alarm signal from all other signals which may be received. The peculiar arrangement of the selector combined with the nature of the alarm signal, enables the apparatus to pick out the alarm signal, even when two ships are sending Morse messages at the same time, and on the same wave-length, however close they may be. such a degree of interference does not hamper the working of the instrument, and trial has shown that if the alarm signal is continued for one minute the apparatus will almost always get hold of it, even when three ships are working ordinary Morse on the same wavelength in the vicinity. If one of the valves in the receiver unit burns out, or the battery runs down, the mechanism in the selector unit will act and set the alarm bells ringing, thus bringing the operator to the cabin, where indicating instruments will show the cause of the ringing. MR. D. W. TAPP’S SET

Mr. D. W. Tapp, of Rotorua, whose reception of 131 stations was recorded in these columns last week, has forwarded me further details. He says that the set he uses for the regular wave lengths is a “Radigla 20.” He adds: “On the short waves I use an adaptor of similar design to the New Zealand radio all-wave sets. I have used many short-wave sets, but find an adaptor gives results quite as good as a straight-out three'. Built to the original design, the adaptor had a grid condenser .00025, but I find .0001 to be much more satisfactory. The tuning condenser is a .00025 Ormond, with all plates removed, but 3-stator and 4-rotor, reaction condenser .00025. The adaptor is shielded with 26-tinned copper, and earthed through .00025 fixed condenser. The earth terminal on the aerial coil is not used—the shielding and rotors of both condensers being earthed. Wonderful volume is obtained from PCTJ, 2XAF, 2XAD, 2XG, etc., and absence of body capacity, silence of set, and smooth reaction control make it a pleasure to tune. Regeneration is obtainable from 10 to 80 metres without “blind spots.”