Field Intensity

Author: J.B. Hoag

The strength or electric field intensity of a radio wave is measured in terms of the e.m.f. (in microvolts, μv.) which it produces between two points 1 meter apart. For example, if 20 micro-volts are generated between the ends of a conductor 1 meter long when the radio wave cuts across it, the field intensity is said to be 20 microvolts per meter (abbreviated μv./m.). If the conductor were 5 meters long, the induced potential would be 100 microvolts.

When the fluctuating magnetic field of a radio wave moves across a wire, it sets up a fluctuating e.m.f. or voltage between the ends of the wire and this in turn drives a current through any electrical circuit connected to the ends of the wire. The amount of this current is exceedingly small unless the circuit is " tuned " to the same frequency of fluctuation as that of the radio wave. When you tune your radio receiver you are adjusting its frequency to that of the radio waves from the desired transmitting station. But even with a tuned circuit, the currents are usually small because the energy is sent out in all directions from the transmitter, and the receiving wire or antenna catches only a small part of it. Therefore, part of the receiving set is built to amplify the signal until it is strong enough to operate a loudspeaker or a meter. A message or signal can be impressed at the transmitter on the radio " carrier " wave just described. In the " amplitude-modulated " system, the carrier wave is varied in strength at a comparatively energy is once more divided equally between them. If a second impulse from the generator starts up the wire just when the first impulse has returned and is ready to start its second trip, the two impulses will aid each other and a condition of resonance will exist between the antenna and the generator. It is also possible that the time for the impulse to travel to the open end and back again should be more than that just described; such that the second trip starts just when the generator starts its third rather than its second pulse.

In Fig. 8 C, is shown the idealized case of current i and voltage e along the antenna when it is oscillating at its fundamental frequency or " first harmonic." In figure 8 D, the voltage and current curves are shown for the second mode of vibration. The fundamental wave-length of the antenna is approximately λ= 4 l.

Fig. 8 D. A Hertz type antenna

In Fig. 8 D, the h.f. oscillator is located in the center of a straight wire, with the result that the fundamental wave-length is approximately two times its physical length. The Marconi type is more suitable for the longer wave-lengths because it need be only one-half as long as the Hertz type for a given frequency. For higher frequencies (shorter wave-lengths) the Hertz type becomes sufficiently short to be constructed. In addition, it can be mounted far above the earth or other absorbing bodies, whereas the Marconi type uses a ground connection.