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INTERFERENCE IN RADIO SIGNALLING
By JOHN V. L. HOGAN
Fellow and Past President, Institute of Radio Engineers. iWember American Institute of Electrical Engineers
In the following article Mr. Hogan's discussion is based entirely upon the modern and scientifically useful relation of wave frequencies. The more familiar concept of wave length as a basis of tuning is equivalent to the idea of wave frequency, and radio frequencies can be converted into meters wave length simply hy dividing into the factor 300,000,000. Thus a frequency of $00,000 cycles corresponds to a wave length of 600 meters or 300,000,000 divided by $00,000. Similarly, the present broadcasting wave length of 360 meters corresponds to a frequency of 300,000,000 divided by 360 or about 833,000 cycles per second.
IN RADIO communication systems tlie word "interference" has been used for many years to describe what happens when a receiving operator hears, in his instrument, signals from stations other than that from which he desires to take messages. The signals from his own communicating station.
JOHN V. L. HOGAN
radio engineer, an early worker with Dr. Lee de Forest. In 1920, President of the Institute of Radio Engineers
that is, the transmitter to which he desires to listen, may be comparatively loud and clear. In this instance, the sounds heard from the other stations will be relatively feeble and only slight "interference" will be experienced; in other words, the receiver will be troubled only slightly, if at all, by the interfering or disturbing sounds and with little difficulty will be able to concentrate upon and decipher the signals he desires to receive. On the other hand, he may be listening to relatively faint sounds from a distant transmitter, anxious to record every word that is sent out, and without warning a powerful near by sending station may commence operations. In this instance the strong interfering waves produced at so short a distance from the receiving station may blot out completely the signals from the far-away sending plant, thus producing insuperably strong " interference." Between these two extremes all shades and colors of interference may be experienced, from a misty background of buzzing against which the desired signals stand forth strongly and distinctly, to interference between several stations of so nearly equal intensity that (when all are sending) it is impossible to distinguish a word from any one of them. Such a gamut of interference has existed from the earliest days of commercial radio signalling, and its minimization or elimination has long been one of the outstanding problems of radio. The natural consequence of such a problem is that the most skilled scientists in the field have applied themselves to finding the solution; the consequence of such intent study is that vast and indeed gratifying progress has been made in overcoming the difficulties created by interference.
The keynote of the most successful normal systems which have been developed to reduce