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Tuning the Radio Aerial System
By JOHN V. L. HOGAN
Consulting Engineer; Past President and Fellow, Institute of Radio Engineers
IT HAS been pointed out* that interference cannot be eliminated, even with the best radio receivers. Very intense interfering waves from powerful or nearby transmitters are likely to breakthrough the barriers erected by the most highly selective receivers; if the interference is from a poorly designed or poorly adjusted sending plant, to exclude it from the receiver is found well-nigh impossible.
This situation does not argue against the adoption and use of sharply tuned receiving instruments, however. One would be greatly in error if he were to conclude that, since no known receiver (however nearly perfect) will prevent all interference difficulties, he might as well use a non-selective apparatus. A receiver which has the capability of close and exact adjustment to desired wave frequencies (or wave lengths) will invariably aid in minimizing interference effects; with it one will be able to receive clearly under many conditions where a broadly adjusted receiver would be helpless to discriminate between desired and undesired signals.
Now, what is it that makes one receiver "sharp tuned" and another "broad tuned?" How does it happen that a sharply adjusted or selective receiver will distinguish between arriving radio waves of only slightly different frequencies? Why does a broadly tuned instrument accept with almost equal ease signals whose frequencies are entirely different? The replies to these questions include nearly the whole subject of tuning at radio receiving stations. As a first step toward answering them, let us consider what happens when radio waves reach an intercepting aerial and the associated instruments.
Figure i is a diagram of a simple tuned receiving system. The aerial wires, which may be any of the familiar forms now seen throughout the country, are represented by the pitchfork symbol at the top of the drawing; a connection leads from the aerial downward to the inductance or tuning coil within the radio station. As indicated, this coil may be wholly or partly connected into the circuit by means of
* " Interference in Radio Signaling," by John V. L. Hogan, Radio Broadcast, May, 1922, p. 5.
movable tap reaching successive turns. From this variable connection a wire extends farther downward through a sensitive current measuring instrument and thence to the ground connection.
When radio waves pass any receiving aerial wire system, they automatically and inevitably generate in that system a series of rapid alternating electric voltages (or electric pressures tending to cause a flow of electric current). If the aerial wire is connected to the ground, as in Fig. I, the high frequency alternating voltages will produce a series of small but measurable
I Aerial
Tuning Coil or Inductance
Variable Connection
Earth
Sensitive Current Meter
Fig. 1