SECRET RADIO

Evening Star, Issue 19789, 13 February 1928, Page 10

SECRET RADIO

TRANSMITTERS SEND FRAGMENTS ALTERNATELY Ordinary broadcasting is like shouting from the housetop. Every one within hearing distance gets the message. But there are other methods of transmission in which the message can be heard only by the person to whom it is sent (say the ‘Literary Digest’). These are explained in an article on ‘ Secret Signalling,’ contributed to ‘lladio News’ (New York). Owing to the supposed greater secrecy of the telegraph lino or cable, we are. told, many business firms and some Governments have not turned readily to radio communication; for it can be sasily understood that many times it is imperative that messages be sent and received without their contents being known to any one except those for whom they are intended. But the supposed secrecy of wire and cable is not flawless, and the supposed publicity of radio is being remedied. As the writer tells us: “ In the first place, messages transmitted by wire arc not proof against the skill of determined wire-tappers. “ A message that is secret can be sent in code or cipher. But it is a difficult task to find a system which can not be deciphered. This fact was proved numerous times during the World War. “In code transmission, used for commercial purposes, entire words or phrases are replaced by letters or numbers taken from a special code book, of which copies must be in the hands of both the sender and the receiver of the message. “ Even when elaborate secret codes are used, the mere fact that several copies of the book are in existence increases the chance of the key falling into the wrong hands. “ Having seen that the secrecy that telegraphic cyphers are said to enjoy is not so very secret after all, it will be of interest to consider one of the latest systems for ■ secret signalling in radio. Naturally cyphers can be used in radio in the same manner in which they are used in land wires, but the question at once arises: Can a radio signal be safeguarded against interception by an unauthorised listener to the same degree that an ordinary line can be protected from the wire tapper? “Obviously, while the land wire is localised along a particular path, which may be kept under supervision, radiated magnetic waves spread out in all directions and therefore over a wider area of attack. In the illustration will he seen a plan developed by Mr R. D. Bangny.

“Two separate transmitters are used, situated some distance apart. The antennen are of the kind which transmit a beam of waves; i.e., the radio Waves are concentrated in a narrow path instead of being spread out in all directions. These two beams will intersect at a certain point, and it is here that the receiving station is located . “The messages are sent from a keying station midway between the two aerials, and the keying circuit is so arranged that neither transmitter sends the entire message. For example, it can be so planned that one of the transmitters sends tho dots of the Continental code and the other sends the dashes. This is accomplished by using a key that sends dots in one position and dashes in the other. •

“The result is that, while the area of possible reception by an unauthorised' listener is limited in each case to the line of the beam, an eavesdropper if placed in one or the other of these two paths will pick up only a part of the message—onlv a series of dots or dashes.’

“The only points where the entire message can be received lie in the area of the intersection of the two beams. The aerial of each transmitter may he mounted ou a. revolving platform, so that the two rays can be converged at any point within the range of the two transmitters, excluding all others. “ As an additional precaution each beam may transmit a continuous carrier wave, upon which the signals are impressed at different super-audible

frequencies. At the receiving station the two “ super-conic ” frequencies are combined to give an audible beat frequency as the ultimate effect. “ Even in the case of telephony secret systems of radio transmitting have been developed, capable of baffling the most determined efforts of an eavesdropper. If speech is used for modulating a carrier wave of fluctuating frequency instead of one of the usual constant frequency, the tuning of the receiving station must be kept constantly in step with the variations of the transmitter. Otherwise only unintelligible fractions of the messages could be picked up on an ordinary receiver. “If the carrier wave is suppressed at the transmitter so that only one of the-side bands is transmitted, it is impossible to pick up an intelligible signal with an ordinary set. This type of reception can only be accomplished by using an oscillating system at the receiving end to supply the missing carrier wave and ‘ beat ’ the side band down to audible frequencies. If in addition the suppressed carrier wave is a. fluctuating instead of a constant frequency, the eavesdropper must keep his circuit oscillating exactly in step with the fluctuations of the transmitter before he can pick up an intelligible message.

“The message can be further garbled by applying the microphone currents to nn intermediate frequency and selecting the lower of the side bands for modulating the main carrier wave. The speech frequencies are thus inverted or converted from their original form into a confused noise. These can be restored to intelligibility only by reversing the modulating process at the receiving end, an operation that necessitates two sets of oscillators.

“ It would seem that these complications would make for secrecy. The original microphone current from the amplifier is first applied to a balanced modulator fed by a source of intermediate frequency current. The resulting side bands are so filtered that only the lower on© reaches the principal modulator. The main carrier wave is generated separately, the tuning condenser being continuously varied by a motor to keep the carrier frequency at a constantly changing value.”