Frequency and Wave-Length

Author: J.B. Hoag

Fig. 8 B. A radiating magnetic field

Fig. 8B shows a magnetic field radiating from a simple conductor in which the current is alternating at a frequency f. An observer at P would find that f complete oscillations or reversals of the magnetic field went past him each second. Hence f is also the frequency of the radiated wave. It is expressed in slow rate by the voice, or by dots and dashes. The wave which goes out is then called a " modulated carrier wave."

In order that a broadcast signal may be intelligible, the field intensity at the receiver must be greater than the " noise level "; otherwise no amount of amplification will make it useful. The background " noise " is due to other radio waves, atmospheric and man-made static, internal circuit and tube noises of the receiver. Field intensities of 100 μv./m. are considered usable for broadcast reception.