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1-1 This AM/FM broadcast antenna tower bristles with two-way antennas
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THE PROPAGATION OF RADIO SIGNALS IS NOT THE SIMPLE MATTER THAT IT SEEMS AT
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first glance Intuitively, radio signal propagation seems similar to light propagation; after all, light and radio signals are both electromagnetic waves But simple inverse square law predictions, based on the optics of visible light, fall down radically at radio frequencies because other factors come into play In the microwave region of the spectrum, the differences are more profound because atmospheric pressure and water vapor content become more important than for light For similar reasons, the properties of microwave propagation differ from lower VHF and HF propagation In the HF region, solar ionization of the upper reaches of the atmosphere causes the kind of effects that lead to long-distance skip communications and intercontinental broadcasting This chapter examines radio propagation phenomena so that you have a better understanding of what an antenna is used for and what parameters are important to ensure the propagation results that you desire
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Although today it is well recognized that radio signals travel in a wave-like manner, that fact was not always so clear It was well known in the first half of the nineteenth century that wires carrying electrical currents produced an induction field surrounding the wire, which is capable of causing action over short distances It was also known that this induction field is a magnetic field, and that knowledge formed the basis for electrical motors In 1887, physicist Heinrich Hertz demonstrated that radio signals were electromagnetic waves, like light Like the induction field, the electromagnetic wave is created by an electrical current moving in a conductor (eg, a wire) Unlike the induction field, however, the radiated field leaves the conductor and propagates through space as an electromagnetic wave
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6 Radio-wave propagation The propagation of waves is easily seen in the water analogy Although not a perfect match to radio waves, it serves to illustrate the point Figure 2-1 shows a body of water into which a ball is dropped (Fig 2-1A) When the ball hits the water (Fig 2-1B), it displaces water at its point of impact, and pushes a leading wall of water away from itself The ball continues to sink and the wave propagates away from it until the energy is dissipated Although Fig 2-1 shows the action in only one dimension (a side view), the actual waves propagate outward in all directions, forming concentric circles when viewed from above The wave produced by a dropped ball is not continuous, but rather is damped (ie, it will reduce in amplitude on successive crests until the energy is dissipated and the wave ceases to exist) But to make the analogy to radio waves more realistic, the wave must exist in a continuous fashion Figure 2-2 shows how this is done: a ball is dipped up and down in a rhythmic, or cyclic manner, successively rein-
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Leading wave forms at instant object strikes water
Spray Leading wave moves radially outward
Point of original disturbance
A B Notes: A Amplitude of leading wave B Corresponds to 1 cycle of oscillation
2-1 A ball dropped into water generates a wavefront that spreads out from the point of original disturbances
Radio waves 7 forcing new wave crests on each dip The waves continue to radiate outward as long as the ball continues to oscillate up and down The result is a continuous wave train There are two related properties of all waves that are important to radio waves as well: frequency (f) and wavelength ( ) The frequency is the number of oscillations (or cycles) per unit of time In radio waves, the unit of time is the second, so frequency is an expression of the number of cycles per second (cps) If the period of time required for the leading wave to travel from point A to B is one second (1 s), and there are two complete wave cycles in that space, then the frequency of the wave created by the oscillating ball is 2 cps At one time, radio frequencies (along with the frequencies of other electrical and acoustical waves) were expressed in cps, but in honor of Heinrich Hertz, the unit was renamed the hertz (Hz) many years ago Because the units are equal (1 Hz = 1 cps), the wave in Fig 2-2 has a frequency of 2 Hz Because radio frequencies are so high, the frequency is usually expressed in kilohertz (kHz 1000s of Hz) and megahertz (MHz 1,000,000s of Hz) Thus, the frequency of a station operating in the middle of the AM broadcasting band can be properly expressed as 1,000,000 Hz, or 1000 kHz, or 1 MHz, all of which are equivalent to each other Radio dials in North America are usually calibrated in kHz or MHz In Europe and the rest of the world, on the other hand, it is not uncommon to find radio dials calibrated in meters, the unit of wavelength, as well as in frequency In most equations used in radio antenna design, the proper units are hertz, kilohertz, and megahertz
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