Simple Antennas

Author: J.B. Hoag

Fig. 8 C. A Marconi type antenna

In the center of Fig. 8 C, the vertical wire is the antenna, the circle is the h.f. generator, and the shaded area is the earth. When the generator is first connected, an electrical impulse travels up the wire to its end. Here the magnetic field collapses because the current impulse stops at the end of its conductor. The collapse of the magnetic field sets up an electrical field which adds to the existing electric field and increases the voltage at the end of the wire. This starts an impulse back down the wire. As the electric field starts back, it begins to produce a magnetic field, and continues to do so until the cycles per second (Hz, Hertz), kilocycles per second (kHz, kilohertz) or in megacycles per second (MHz, megahertz).

The distance in space occupied by each oscillation is called the wave-length. It is indicated by λ (lambda) in Fig. 8 B and is usually measured in meters.

The relationship between the free space velocity c, the frequency f and the wave-length is: fλ = c. If A is in meters and f is in megacycles per second, we have,

A low-frequency radio wave of frequency 15 kHz has a wave-length of 20,000 meters; a broadcast frequency of 1,000 kHz corresponds to a 300-meter wave-length; while an ultra-high frequency of 60 MHz corresponds to a 5-meter wave-length.