04-18-1994 Planning Packet - Laserfiche WebLink

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■s ive the lelow he vered efleciion nsmitted vy lines) top path although to signal #1 power en times ive. (The sp wave, le earth, lity of ail :tors in vels are dB. The fference 0 of 2:1; of 10:1. )Out 7 to ficant at jistance )0 miles stration. r a 1/10 gnal will B below hat only jditional second ore than eceived Table 1—Propagation Distance versus Radiation Angle (Data extracted from Fig 3) Wi « I Fig 3—Distance plotted against wave angle (one-hop transmission) for the nominal range of virtual heights for the E and F2 layers, and tor 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 ait succeeding hops And it is the radiation angie at the transmitter wnich sets up the trigonometry for the first hop Solving a propagation triangle is simplified with the aid of the graph tn Fig 3 ’ In this graph the radiation angie in degrees is given on the left, and th, ' >gle-hop distance for the effective layer height along ..le bottom. Table i 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-iayer height of 131 miles From Table 1. if the radiation angle from a given transmitiing antenna is concentrated at 30®. the first and succeeding hops m radio propagation will span about 650 miles each With a usable rr.aximum 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 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 m''re 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 determining effective communications distances beyond line of sight. Raa'Stion Ootfm/fT. Angte.Propagation Degrees Distance. KtHes 2 2250 4 2100 8 1650 10 1500 15 1200 20 10C3 30 650 40 450 Scientists and engineers recognize that modes other tnan signal hopping account (or the propagation of radio waves over thousanas of miles However, studies of actual radio propagation in which the writer has participated have displayed signals with as many as 5 hops. 50 the hopping mode is one distinct possibility.^ Whatever the propagation mode, there is unanimous agreement that the .most effective cotrunumcations 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^he results of proper cnaiyses 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 eanh 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 var'ous 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 ana 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 TheARRL Antenna Book. 14th edition. Chapter 1, Fig 12 ^Hr backscatier studies by Raytheon Company under contract with Rome Air Development Center. Griftiss AFB. NY A pan of this study was done at Raytheon's South Dartmouth, Mass, field site. Jan-Jun. 1960. 3