Better Wireless Reception

Radio Record, Volume III, Issue 28, 24 January 1930, Page 31

Better Wireless Reception

A. N ow’ Super-Selective -_ _ Receiver HE problem of reducing interference between wireless stations is now so acute in Hurope and America that any proposal for relieving what is often called: the congestion of the ether demands" sérious attention.’ Broadcasting or telephony stations are known ‘to be'the chief offenders in this’ over: crowding, in that, when transmitting spgech or tnusic, they occupy not only Lae nominal frequency put also a range of frequencies both above and below this nominal figure. ° Some years ago a great step forward was.made in solving this diffieulty by Dr. J. R. Carson; of the American Telegraph and Telephone Company, who pointed out that it was not necessary: to transmit the whole of this range of frequencies: in. order to convey the speech: or music. It is only necessary to send a part of it, or what is called technically one of the side-bands. Working in this way it is possible to use more selective receivers, which reduce the amount of interferencé; both natural ‘and artificial, which eritérs the receiving set. This method of communication is known as. single side-band telephony, and is used.in the Trans-Atlantic telephone service between America and England. . Up to. the present, howeyér, its. has not extended to broadcasting because of the more complicated receiving circuits required. The announcement was made recently of the development of another sys_tem which ‘is-designed to assist in the solution of the same.problem. With the Stenode Radiostat system of communication it is claimed that the "spread" of frequencies can be enormously reduced, a reduction of 50 times being mentioned. Such a possibility would be denied by most technical experts, for. it is an axiom of modern wireless theory that the "spread" of a’ telephone station, even with single side-band. working, must occupy at least the ‘range of musical frequencies from, say, 50 to.5,000 cyeles a second. Until full technical details of the methods to bé.ased for transmission and the results of actual tests are disclosed, i therefore difficult to be other than tical. : n the receiving side, however, it is . much more likely that real progress has been made. In a typical form of Stenode’Radiostat receiver, Dr. G. R. —

Robinson, the inventor of the system, has made use of the highly selective properties of the quartz crystal resonator, which up.to now has not been used in ordinary broadcast: receivers. It ise well known that by the use of such a crystal in Igboratory: experiments, an extraordinary degree of selectivity may be obtained. But the difficulties in using such a crystal in an ordinary receiver are twofold. In‘the first place: a crystal of a certain size is tuned to one frequency only; secondly, there is a certain electrical persistence in such a highly selective circuit which renders it sluggish,.and unable to follow such rapidly:-varying signals as are emitted by a broadcasting station. Dr. Robinson: has‘ strmounted the first difficulty by using ‘the super heterodyne principle, so that the frequency ‘of any station that.is received is converted first to the natural frequency of the erystal before being applied to it. To prevent the persistence of signals and so eradicate the electrical sluggishness of the circuit, Dr. Robinson uses a form of inaudible quenching signal in somewhat the same manner as is used in the Armstrong or Flewelling super-regenerative ‘circuits, At the demonstration in London recently, .the extreme seléctivity of the receiver was illustrated by receiving -a broadcast transmission on a frame aerial without a trace of interference from another transmitter a few feet away from’ the ‘set. A photograph of this new receiver appears on page 9 of this issue.