Wireless Communication

SOME notes on the present state of wifeless telegraphy were communicated bv Dr. M. I. Pupin in the course of a lecture which he delivered before the National Academy of Science in New York, on “Aerial Transmission Problems.” None of the points raised by Dr. Pupin were entirely new, as they have been frequently discussed in both continents without being appreciably helped towards solution. But the manner in which they were presented and illustrated assisted towards a better understanding of the formidable character of the obstacles in the way of extending the distance of wireless communication. These obstacles are due mainly to the interference produced by electrical waves, which are passing through the terrestrial atmosphere continuously, and it is not until we attempt to magnify the minute signals coming from a distant transmitting station that we are really aware of their presence. An engineer who took part in the recent long-distance wireless telephony trials and listened for the famous telephone message from Arlington, reports that at times “it was drowned completely in a roar of musketry,” due, of course, to the action of the electrical waves produced by the incessant electrical discharges in the atmosphere. All attempts up to the present which the oopractical wireless engineer has made in the direction of overcoming these disturbances have consisted in increasing the power applied at the transmitting station so as to make the incoming signals at the receiving station stronger than the signals made by the “static.” Ordinary electrical tuning is not sufficient for the purpose, because every system which is highly selective through ordinary tuning is also highly sonorous; every tap of the static will cause it to vibrate, and it will vibrate in the same way as when it is under the action of the signalling waves. The method advocated by Dr. Pupin involves the use of a sectional wave conductor between the antenna and the receiving apparatus, which will not transmit electrical waves of a frequency higher than a given range of frequencies. By this means, he states, “the station becomes an ear, which is quite sensitive for frequencies which are in the vicinity of the signalling frequency, which is deaf to frequencies which are considerably beyond this range, as most static disturbances are.” “Similarly,” he adds, “a sectional wave conductor can be constructed which is quite responsive to frequencies in the vicinity of the signalling1 frequency, but absorb almost completely everything below this range.”