The heterodyne receiving system, and notes on the recent Arlington-Salem tests

Abstract

Detectors are classified as of the relay or of the transformer type. Relay detectors are arranged so that the energy for the indicator response comes from a local source of energy at the receiving station. Transformer detectors utilize the received energy for producing the indicator response. The need of an amplifying transformer type detector, which selectively neglects static, is mentioned. The basis of the heterodyne receiver, namely, the audible beats produced by the interference of two vibrations of inaudible (radio) frequency received on a nonpolarized receiver, is then fully considered. The development of the heterodyne receiver, its circuit arrangements, and the apparatus employed are described in detail. The use of the heterodyne in the Arlington‐Salem tests and the results obtained are given. Much interest has been shown in the heterodyne receiver since its use in the recent test between the Fessenden stations of the U.S. Navy at Arlington, Virginia, and aboard the cruiser, Salem. These trials mark the first public use of the heterodyne system, which has often been called the greatest of Professor Fessenden’s inventions; but, as a matter of fact, the method has been utilized in the National Electric Signaling Company’s plants for a number of years. It is the purpose of the present paper to explain the heterodyne principle, and to describe the apparatus by means of which it is put into practice. Since the invention involves a number of points which are quite outside the range of observation of the average worker in radio signaling, an introductory consideration of the fundamentals of receiving instruments in general is desirable. Every radio receiver is composed of two main parts; an energy absorber and an indicator. In some special forms of apparatus these two elements may be physically combined, but functionally they remain as distinct as before. The relation between the energy received and the response of the indicator, together with the process whereby the receipt of that energy effects the indication, probably serves as the best basis for classifying receivers in radio signaling. In the receiving instruments originally used (which were mainly various arrangements of coherers with auxiliary