08-22-1994 Council Packet - Laserfiche WebLink

Home Browse Search

Mi 1 1 6C9*»ir 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 ail succeeding hops. And it is the raoiation angle at the transmitter which sets up the trigonometry for the first hop. Solving a propagation triangle is simplified witn 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 m 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. 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. Table 1—Propagation Distance versus Radiation Angle (Data extracted from Fig 3) Raa.a:ion ODtirr.Ltn Ang:e.P'ODagaticn Degrees Distance. .Mnes 2 2250 4 2100 8 1650 to 1500 15 1200 20 1000 30 650 40 450 Scientists and engineers recognize that moo* other than signal hopping account for the propagatic of radio waves over thousands of miles Howeve studies of actual radio propagation in which the writi has participated have displayed signals with as many c 5 hops, so the hopping mode is one distinct possibility Whatever the propagation mode, there is unanimoc agreement that the most effective communications at H most often accompany the lowest radiation angle. Horizontal Antennas A simple antenna that is commonly used for H communications is the horizontal half-wave dipole. Th dipole IS a straight length of wire (or tubing) into whic radio-frequency energy is fed at the center. Because c its simplicity, the dipole may be easily subjected t theoretical performance analyses. Further^he result of proper analyses are borne out in practice. For thes reasons, the half-wave dipole becomes a convenier performance standard against which other antenn. systems can be compared. The dipole antenna, whei viewed from one end. radiates an equal amount of powe in all directions. Because the earth acts as a reflector for HF radi waves, the directive properties of any antenna ar- modified considerably by the earth underneath it. If ; dipole antenna is placed horizontally above the eartl" most of the energy radiated downward from the dipoF is reflected upward The reflected v^aves combine wit,' the direct waves (those radiated at angles above th* horizontal) in various ways, depending on the height c 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 direc 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 thii ’From The ARRL Antenna Book, 14th ed i'on. Chapter 1. Fig 12 2HF backscatler studies by Raytheon Company under contract with Rome Air Development Center. Griffiss AFB. NY. A par of this study was done at Raytheon's South Dartmouth. Mass, field site. Jan-Jun. 1960.