How Antennae Work

Antennae work by sending electrical charges back and forth across a conductor called an antenna. The electric field induces a magnetic field, which in turn induces another electric field, which in turn induces another magnetic field, and so on, as the wave propagates across a medium (such as air). This is called an electromagnetic field. How do you determine the appropriate size of the antenna? Because radio waves travel at the speed of light (a constant), the optimum antenna size depends on the frequency of the wave. Let's say that you wanted to build an antenna to broadcast signals for a radio station operating at 1000 kHz (a.k.a., 1 MHz) on the AM dial. 1 MHz means that the radio is operating at a frequency of 1 million cycles per second.

For each full cycle of the sine wave, the transmitter will move electrons in the antenna in one direction, then switch back in the other direction, then back again in the original direction, and finally back once more in the reverse direction. Therefore, each cycle results in the electrons flowing through the conductor four times. If our system is operating at 1 million cycles per second, the time for each individual cycle to complete the period is 0.000001 seconds (1/1,000,000). One fourth of that is 0.00000025 seconds. Since the speed of light (and electric currents) is 186,000 miles per second, then electrons can travel .0465 miles in 0.00000025 seconds. There are 5,280 feet in a mile so the ideal height for an antenna operating at 1 MHz is 245.52 feet. Compare that with a PCS cell phone operating at 1900 MHz. At that frequency, the optimum antenna size is about an inch and a half.