From the time of Hertz and Marconi to the present, one thing has remained constant in wireless communications: radio waves travel, as if by magic, from a transmitting antenna to a receiving antenna (Figure 2.3). Whether the two antennas are across the garden from each other, across continents and oceans, or on the Earth and the Moon, if there is not a transmitting antenna and at least one receiving antenna in the system then no communications can take place.

At one time, physicists believed that there must be some invisible medium for carrying the radio signal. But we now know that no such medium exists, yet radio waves travel even in outer space. Being electromagnetic waves, radio signals need no medium in order to propagate. If radio signals traveled only in the Earth’s atmosphere, then we could make some guesses about a medium for carrying the wave, but space communications demonstrates that the atmosphere is not necessary (although it does affect radio signal propagation).

Although there is no medium in which radio waves travel, it is useful to look at water waves for an analogy (even though imperfect). In Figure 2.4 we see what happens when an object is dropped into a pool of water. A displacement takes place, which forms a leading wave that pushes out in concentric circles from the impact point. The situation in Figure 2.4 represents a single pulse of energy, as if a transmitter fired a single burst of energy. Real transmitters send out wave trains that are analogous to cyclically bobbing the object up and down so that it goes in and out of the water (Figure 2.5). The result is a continuous stream of identical waves propagating out from the ‘transmitter’ impact point. If another object is floating on the surface, say a cork or toy boat, then it will be perturbed as the wave passes. This is analogous to the receiver antenna.

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