Yes, Generator Hum of Southern Cross was broadcast

Radio Record, Volume II, Issue 10, 21 September 1928, Page 3

Yes, Generator Hum of Southern Cross was broadcast

Scepticism of Newspaper Critic Unjustified

HE radio writer of a certain Auckland paper wrote :- "1YA caused some amusement by asking listeners to listen to what the announcer said was the sound of the. Southern Cross’s generator. It probably goes down with many; experts, however, merely smile up their sleeves." If this "expert" critic will widen his sleeve, he may now have another laugh, for the facts are against him. The generator hum was. broadcast. Others have inquired why it was, if the generator hum was heard, the roar of the engines could not be heard. The answer is simple, and lies in the radio frequency waves. QIMPLY put for the man in the street, the position is that a current of electricity must be generated and sent out into the ether before any sound can be earried by it; but only sounds translated into radio frequency will be carried. The current is sent out at a definite frequency, and when received has a definite pitch. It is a note, as in music. If filtered suitably it will give a clear whistle in the receiver, and is only faintly audible. When transmitted without having first been so filtered there is a characteristic roughness which is easily distinguishable. Apparatus for the filtration of this wave would add extra weight if it were carried on a plane, and for this reason filtering apparatus is not so used. The result is that instead of a shrill, pure note coming in on the receiver from the Southern Cross a rough suppressed roar is heard. To modulate this before dispatch it would be necessary to carry a microphonic arrangement in addition to the filters previously referred to. If a microphone was installed it would be impossible for the a. ated

operator to make himself heard above the noise of the engines, which would then be transmitted into the radio frequency waves. The hum of the generator would still be in the receiver, but it would be drowned out by the noise of the engines impressed upon it. For these reasons it would be ridiculous to carry these instruments on such a journey as that just accomplished. Speech then being barred, some other device must be resorted to-plainly morse. CONTINUOUS current of electricity was available from the generator, driven by windmills on each side of the fuselage, which supplied two currents, one at ten volts for the filament of the transmitting valve, and the other at 65 volts, which was brought up to 900 volts by a transformer to serve the plate. The lighting of the plane was carried out by means of this generator. Modulation of the carrier wave being barred, one alternative was to interrupt the steady wave generated by the transmitter. By means of a key placed in the circuit, a series of dots and dashes (morse) could be transmitted at will-the duration of the sound representing the generator hum, the silence its absence. When the key was screwed down the uninterrupted earrier wave or generator hum of the plane’s transmitter could be plainly heard as one long dash. This was a prearranged signal which denoted "All’s well." ‘THIS is a signal used generally with planes in communication with the shore. When radio beacons are operwea — ----

ating the position of the plane is known, and if by chance the signal should stop, ships in the neighbourhood — could be rushed to the exact spot where the plane was when the signal stopped. HE swelling and diminishing of the note from the Southern Cross’s transmitter was caused by the swaying movement of the aerial wire trailed by the ’plane. "HE generator hum is the sound caused in the receiver by the impulses of electricity sent out from the transmitter at high frequency. ‘These impulses are given sound only by the specialised instrument known as_ the receiver. The generator makes no sound other than that caused by the whirling of the windmill. On the other hand, the engines cause a vibration of a lower frequency which is audible, but which, unless impressed on electrical impulses, is not heard at any great distance.. As before stated, there was no means available to do this, so that the waves from the transmitter went on their way unmodulated and without the sound of the engines. The generator hum was picked up by 2YA and put on the air just before the message came through that all was well. It coincided with the restoration of communciation. It was received on the short wave. The Wireless Equipment. EN addition to the short-wave apparatus mentioned, Captain Kingsford as

Smith carried auxiliary radio appara-tus-in all, three receivers and three transmitters. The 600-metre transmitter, which was intended for communication with ships, was built on the same lines as the ‘short-waver, but-it has a much shorter range. Both transmitters, fitted in aluminium cases, are mounted in the wall of the navigator’s part of the airplane, projecting only about 8in, from the wall. The Emergency Transmitter, ‘THE third transmitter is an emergency one, designed for use in case the machine had to land on the water, so it was made waterproof. It has a one-inch spark coil, and is tuned broadly to 600 metres. The key is under a rubber-covered -hole in the waterproof box, so that to send a message the rubber is pressed, thus working the key without letting water into the transmitter. The aerial for the emergency transmitter is 200 feet of stranded wire, which, if it had ever to be used, would be carried up into the air by a small balloon. A small hydrogen container to fill this balloon is carried in the *plane. The emergency transmitter, which is battery operated, will work for eight hours, and on test has been heard 200 miles away at night. The aerials for the other transmitters are lengths of wire, weighted at the free end, which are carried on reels. The Reeeivers. E1H- receivers are so arranged that any one of the three tuner and detector units can be switched across te the two-stage amplifier. . So well has the radio apparatus proved itself, that Commander Richard Byrd has ordered similar equipment for his Antarctic expedition. —