01-18-1994 Planning Packet - Laserfiche WebLink

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r li y > ’ ic* • Fig 3 —Distance plottea against wave angle (one-hop trans.nission) for the nominal range of virtual heights for the E and F2 layers, and for the F1 layer. The second important point to be recognized in mult'hop propagation is that the tngcnometry of the first hop generally establishes the tfgonometrv for all succeeding hops. Ano't is the raoiation 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 smqie-hop distance for the effective layer height along the bottom. Table 1 shows the optimum propagation distance for various radiat'on angles. The data for this taoie was read from the graph of Fig 3, based on an assumed F-layer height of I3i miles From Table 1, if the radiation angle from a given transmitting antenna is concentrated at 30®. the first and succeeding heps m radio propagation will span about 650 miles each With a usaoie mdAimurr, limit of f ve hops under the best of conditions, one can pr-jiect ina« the greatest distance for ootimum communications for a 30® wave angle is five times 650 miles, or 3250 miles. For effective communications ove' greater distances, it wduld be necessary to lower the radiation angle at the transmirer antenna site As will be oiscussec shoniy, this can be done only py increasing the height of the hori zontal transmitting antenna. Although the discussion m the preceding para graphs has been m 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 The point of these several paragraphs may be summarized briefly; The vertical angle of radiation is the key factor in d®termining effective communications distances beyond line of sight. Table 1—Propagation Distance versus Radiation Angle (Data extracted from Fig 3) RaAation Ootimum Ang‘6.f'ooagaior Degrees Distance, \t-ies 2 2250 i 2100 3 1650 10 1500 15 1200 20 1000 30 650 40 450 Scientists and engineers recognize that modes other than signal hoppmg account for the prcpagafior of radio waves over fhousanas of miles However studies of actual radio propagation m which the write' 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 Furtherjhe results of prooer 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 in all directions. Because the earth acts as a reflector for HF radio waves, the directive properties of any antenna are modified considerably by the ea.ih underneath it If a dipole antenna is placed horizontally above the earth, most of the energy rad ateo downward from the dipole IS reflected upward The reflected waves combine with the di.'ect waves (those raaiated at angles above the horizomari m various ways, depending on the height of the antenna, tne 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 instant at some distant point. In this 'From The ARRL Antenna Boox. 1-itned tion. Cnaoter 1. Fig 12. -HF backscaner studies by Rayineon (Company under contract with Rome Air Development Center. Gritfiss AFB. NY. A part of this study was done at Raylheon s South Dartmouth, Mass, field site. Jan-Jun, i960.