INVERSE SQUARE LAW

When radio waves travel they become weaker by a relationship called the inverse square law. This means that the strength is inversely proportional to the square of the distance traveled (1=D2). Figure 2.6 shows how this works using the analogy of a candle. If the candle projects a distance r, all of the light energy falls onto square ‘A’. At twice the distance ð2rÞ the light spreads out and covers four times the area (square ‘B’). The total amount of light energy is the same, but the energy per unit of area is reduced to one-fourth of the energy that was measured at ‘A.’ This means that a radio signal gets weaker very rapidly as the distance from the transmitter increases, requiring ever more sensitive receivers and better antennas.

THE ELECTROMAGNETIC WAVE

The electromagnetic (EM) wave propagating in space is what we know as a ‘radio signal.’ The EM wave is launched when an electrical current oscillates in the transmitting antenna (Figure 2.7). Because moving electrical currents possess both electrical (E) and magnetic (H) fields, the electromagnetic wave launched into space has alternating E-field and H-field components. These fields are transverse (meaning they travel in the same direction) and orthogonal (meaning the E- and H-fields are at right angles to each other). When the EM wave intercepts the receiver antenna, it sets up a copy of the original oscillating currents in the antenna, and these currents are what the receiver circuitry senses.

The orthogonal E- and H-fields are important to the antenna designer. If you could look directly at an oncoming EM wave, you would see a plane front advancing from the transmitting antenna. If you had some magical dye that would render the E-field and H-field line of force vectors visible to the naked eye, then you would see the E-field pointing in one direction, and the H-field in a direction 90° away (Figure 2.8).

The polarization of the signal is the direction of the E-field vector. In Figure 2.8 the polarization is vertical because the electric field vector is up and down. If the E-field vector were side-to-side, then the polarization would be horizontal. One way to tell which polarization an antenna produces when it transmits, or is most sensitive to when it receives, is to note the direction of the radiator element. If the radiator element is vertical, i.e. perpendicular to the Earth’s surface, then it is vertically polarized. But if the radiator element is horizontal with respect to the Earth’s surface, then it is horizontally polarized. Figure 2.9 shows these relationships. In Figure 2.9, two dipole receiver antennas are shown, one is vertically polarized (VD) and the other is horizontally polarized (HD). In Figure 2.9A, the arriving signal is vertically polarized. Because the E-field vectors lines are vertical, they cut across more of the VD antenna than the HD, producing a considerably larger signal level. The opposite is seen in Figure 2.9B. Here the E-field is horizontally polarized, so it is the HD antenna that receives the most signal. The signal level difference can be as much as 20 dB, which represents a 10-fold decrease in signal strength if the wrong antenna is used.

 

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