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» TaM* 1 —Propagation Oistanct vartus Radiation Anglo (Data oxtraetad from Fig 3) 500 5n»8i.t «*0* 0<lT*».Ct ~<M ItMt***' tad tranamitaiaa) tor tfio nomina l rango of virtual ttoighta for tlio f and P2 loyora, and for tha FI layar. Tha sacond important point to ba racognizad in multihop propagation ia that tha trigortomatry of tha first hop ganaraHy astablishas tha trigonometry for all succaading hops. And it is tha radiation angla at the transmitter which sets up tha trigonometry tor tha first hop. Solving a propagation triangle is simplified with tha aid of tha graph in Fig 3.' In this graph the radiation angle in degrees is given on tha left, and tha singla-hop distance foi tha affective layar height along tha bottom. Tabia t shows tha optimum propagation distance tor various radiation anj^. Tha data tor this table was read from the graph of Fig 3. based on an assumed F-layar height of 131 miles. From Table 1. if the radiation angle from a given transmitting antenna is concentrated at 30**. tha first and succaading 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 protect that the greatest distance tor optimum communications for a 30* wave angle is five times 650 miles, or 3250 miles. For affective communications over greater distances, it would be necessary to lower the radiation angle at the tranamittar antenna site. As will be discussed s.honty, this can ba dona only by increasing the height of tha horn zontal transmitting antenna. Although the discussion in the preceding para* graphs has bean in terms of a transmitting antenna, the same principles apply whan tha antenna is used for recaption. A high antenna will receive low-artgle signals more effactivoly than will a low antenna. The point of these several paragraphs may be summarized briefly: Tha vertical angla of radiation is tha key factor in determining affective communications distances beyond line of sight. fisastion Angie. Dtignn 2 4 8 10 IS 20 30 40 Optimum PfOppgMon Distanet. Mips 22SO 2100 1650 1S00 1200 1000 650 450 Scientists and engineers recognize that mode other the" signal hopping account for the propagatic of radio waves over thousands of miles. Howeve studies of actual radio propagation in which the write has participated have dieplayed signals with as many e 5 hops, so the hopping mode is one distinct possibility Whatever the propagation mode, there is unanimou agreement that the most effective communications at H most often accompany the lowest radiation angle. Horlaental 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 i its simplicity, the dip^ may be easily subjected t theoretical performance analyses. Further«4he 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, whe: 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 art modified considerably by the earth unMrneath it. if dipole antenna is placed horizontally above the earth most of the energy radiated downward from the dipou is reflected upward The reflected waves combine witr the direct waves (those radiated at angles above th^ honzontal) in various ways, depending on the height o the antenna, the frequency, and the electrical character istics of the ground under and around the antenna. At some vertical angles above tne horizon, the direc and reflected waves may be exactly in phase-~that is the maximum signal or field strengths of both waves arr reached at the same instant at some distant point. In thr. Trom The APRL AnUnna Book, 14th edition. Chapter 1 . Fig 12 >HP baexscatier studies by Raytheon Company under contrac with Rome Air Oevekipment Center. Gnffiss AFB. NY. A par of this study was done at Raytheon's South Dartmouth Mass, field site. Jan*Jun. 1960.