08-22-1994 Council Packet - Laserfiche WebLink

Home Browse Search

iOS'ZtZ LAyc«< • • Fig 2 —Behavior of radio waves encountering the ionosphere. Rays entering the ionized region at angles above the critical angle are not bent enough to be returned to earth, and are lost to space. Waves entering at angles below the critical angle reach the earth at increasingly greater distances as the angle approaches the horizontal. The maximum distance that may normally be covered in a single hop is 2500 miles. Greater distances may be covered with multiple hops. to the earth. At successively lower angles, the wave returns to the earth at increasing distances. If the radio wave leaves the earth at a radiation angle of zero degrees, just toward the horizon (or just tangent to the earth’s surface), the maximum distance that may be reached under usual ionospheric conditions is approximately 2500 rules (4000 kilometers). However, the earth itself acts as a reflector of radio waves. Quite often a radio signal will be reflected from the reception point on the earth into the ionosphere again, reaching the earth a second time at a still more distant point. As in the case of light waves, the angle of reflection is the same as the angle of incidence, so a wave striking the surface of the earth at an angle of, say, 15® is reflected upward from the surface at the same angle. Thus, the distance to the second point of reception will be approximately twice the distance of the first. This effect IS also illustrated in Fig 2, where the signal travels from the transmitter at the left of the drawing via the ionosphere to Point A. in the center of the drawing From Point A the signal travels via the ionosphere again to Point B. at the right. Signal travel from the earth through the ionosphere and back to the earth is called a hop. Under some conditions it is possible for as many as four or five signal hops to occur over a radio path, but no more than two or three hops is the norm. In this way. HF communications can be conducted over thousands of miles. With regard to signal hopping, two important points should be recognized. First, a significant loss of signal occurs with each hop. Lower layers of the ionosphere absorb energy from the signals as they pass through, and the lonospfiere fends to scatter the radio energy in various directions, rather than confining it in a tight bundle. The earth also scatters the energy at a rellection point. Thus, only a small fraction of the transmitted energy reaches a distant receiving point. Again refer to Fig 2. Two radio paths (heavy lines) are shown from the transmitter to Point B. a 1-hop path and a 2-hop path. Measurements indicate that although there can be great variation in the ratio of the two signal strengths in a situation such as this, the signal power received at Point B will generally be from five to ten times greater for the 1-hop wave than (or the 2-hop wave. (The terrain at the mid-path reflection point for the 2-hop wave, the angle at which the wave is reflected from the earth, and the condition of the ionosphere in the vicinity of all the refraction points are the primary factors in determining the signal-strength ratio.) Signal levels are generally compared in decibels, abbreviated dB. The decibel is a logarithmic unit. Three decibels difference in signal strengths is equivalent to a power ratio of 2:1 ; a difference of 10 dB equates to a power ratio of 10:1. Thus, the signal loss for an additional hop is about 7 to 10 dB. The additional loss per hop becomes significant at greater distances. For a simplified example, a distance of 4000 miles can be covered in 2 hops of 2000 miles each or in four hops of 1000 miles each. For illustration, assume the loss for additional hops is 10 dB. or a 1/10 power ratio. Under such conditions, the 4-hop signal will be received with only 1/100 the power or 20 dB below that received in two hops. The reason for this is that only 1/10 of the 2-hop signal is received for the first additional (3rd) hop. and only 1/10 of that 1/10 for the second additional (4th) hop. It is for this reason that no more than four or five propagation hops are useful: the received signal becomes too weak to be heard.