06-16-1997 Planning Packet - Laserfiche WebLink

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Table 1—Propagation Distance versus Radiation Angle (Data extracted from Fig 3) 500 1000 1500 2000 SlUGLt HOP QlSTANCe I Ml 1 1 »09«>»» I 2500 miles Fig 3—Distance plotted against wave angle (one*hop transmission) for the nominal range of virtual heights for the E and F2 layers, and for the FI layer. The second important point to be recognized in multihop propagation is that the trigonometry of the first hop generally establishes the trigonometry for all succeeding hops. And it is the radiation angle at the transmitter which sets up the trigonometry for the first hop. Solving a propagation triangle is simplified with the aid of the graph in Fig 3.' In this graph the radiation angle in degrees is given on the left, and the single-hop distance for the effective layer height along the bottom. Table 1 shows the optimum propagation distance for various radiation angles. The data for this table was read from the graph of Fig 3, based on an assumed F-layer height of 131 miles. From Table 1, if the radiation angle from a given transmitting antenna is concentrated at 30", the first and succeeding hops in radio propagation will span about 650 miles each. With a usable maximum limit of five hops under the best of conditions, one can project that the greatest distance for optimum communications for a 30" wave angle is five times 650 miles, or 3250 miles. For effective communications over greater distances, it would be necessary to lower the radiation angle at the transmitter antenna site. As will be discussed shortly, this can be done only by increasing the height of the hori zontal transmitting antenna. Although the discussion in the preceding para graphs has been in terms of a transmitting antenna, the same principles apply when the antenna is used for reception. A high antenna will receive low-angle signals more effectively than will a low antenna. Thp point of these several paragraphs may be summarized briefly: The vertical angle of radiation is the key factor in determining effective communications distances beyond line of sight. Radiation Optimum Angle.Propagation Degrees Distance, .Miles 2 2250 4 2100 8 1650 10 1500 15 1200 20 1000 30 650 40 450 Scientists and engineers recognize that modes other than signal hopping account for the propagation of radio waves over thousands of miles. However, studies of actual radio propagation in which the writer has participated have displayed signals with as many as 5 hops, so the hopping mode is one distinct possibility.^ Whatever the propagation mode, there is unanimous agreement that the most effective communications at HF most often accompany the lowest radiation angle. Horizontal Antennas A simple antenna that is commonly used for HF communications is the horizontal half-wave dipole. The dipole is a straight length of wire (or tubing) into which radio-frequency energy is fed at the center. Because of its simplicity, the dipole may be easily subjected to theoretical performance analyses. Further«4he results of proper analyses are borne out in practice. For these reasons, the half-wave dipole becomes a convenient performance standard against which other antenna systems can be compared. The dipole antenna, when^ viewed from one end, radiates an equal amount of power . h all directions. Because the earth acts as a reflector for HF radio waves, the directive properties of any antenna are modified considerably by the earth underneath it. If a dipole antenna is placed horizontally above the earth, most of the energy radiated downward from the dipole is reflected upward. The reflected waves combine with the direct waves (those radiated at angles above the horizontal) in various ways, depending on the height of the antenna, the frequency, and the electrical character istics of the ground under and around the antenna. At some vertical angles above the horizon, the direct and reflected waves may be exactly in phase —that is, the maximum signal or field strengths of both waves are reached at the same i.nstant at some distant point. In this ’From The ARRL Antenna Book, 14lh edition. Chapter 1, Fig 12. *HF backscatter studies by Ra^heon Company under contract with Rome Air Development Center, Griffiss AFB, NY. A part of this study was done at Raytheon's South Dartmouth, Mass, field site, Jan-Jun, 1960.