Figure 35. Knife-Edge Diffraction A ridge of hills or mountains may exhibit diffradion of a vhf wave traveling over the crest. An obstacle gain as high as 20 decibels may be realized when transmitting and receiving sites are optimized for maximum diffrac- tion.
Me-of-Sight Propagation nder normal propagation conditions, the refractive Idu of the atmosphere decreases with height so that ves travel more slowly near the ground than at Igber altitudes. This variation in velocity with height uults in bending of the wave toward the earth's sur- ce. Under unusual atmospheric conditions, the re- sGve index may increase with height, causing the tve to bend upward, resulting in a decrease in the line- f-sight path. Over most of the time, uniform, downward bending present in the vhf and uhf region and may be repre- ted by straight-line propagation, but with the radius ' the earth modified so that the relative curvature mains unchanged. The new radius is known as the ective earth radius (K). The average value of K in mperate climates is about 1.33. The distance to the radio horizon over smooth earth, en the height h is very small compared with the rth's radius, is given with a good approximation by:
32h
k = height in feet above the earth, d = distance to radio horizon in miles, K = effective earth radius in miles.
The nomograph of Figure 36 gives the radio horizon
h RECEIVING - RADIO ANTENNA h = TRANSM ITTING - HEICHT GEOMETRICAL HORIZON ANTENNA HEIGHT IN FEET "HORIZON" DISTANCE IN FEET , IN MILES IN MILES zooo zooo 120 loo lsoo 1500 110 1600 90 1400 loo lzoo 90 looo loao zo oo 60 TD 600 500 50 60 400 50 40 200 40 200 30 100 30 100 50 2D 20 50 lo lo lo lo
oo o-L Figure 36. Radio Horizon Nomograph Example shown: height of receiving antenna, 60 feet; height of transmitting antenna, 500 feet; maximum ra- dio path length, 41.5 miles. Effedive earth radius is taken as 133.
distance between a transmitter at a height h, and a receiver at height h,.
Forecast of High-Frequency Propagation
From theory and experimentation, constantly advanc- ing hand in hand since the first ionospheric experiments of 1925, techniques have been evolved for applying certain measurable ionospheric data to the solution of propagation and other engineering problems encoun- tered in establishing hf radio circuits. It is possible, therefore, to estimate the tvtuF and Fo'r for a particular smoothed sunspot number for a given communication circuit. A representative propagation analysis chart for the New York to London circuit for a sunspot number of 150 is shown in Figure 37. World maps with overlay frequency contours are available for making frequency estimates manually and lvtuF estimations for months in advance may be made, if a predicted value of smoothed sunspot number is known. The Institute of Telecommunication Sciences of the Environmental Sciences Services Administration (ESSA) issues forecasts which may be used to determine the rvtuF and Fo'r for high-frequency communication paths. A handy source of propagation information is broadcast by the National Bureau of Standards station WWV during part of one l5th minute period of the standard frequencies in the hf range. Finally, the head- 20-21
RADIATION AND PROPAGATION
i I '·.
15
Y
9 b a m
ls =
Y 9 w b 6 " 5
2 2 0000 0200 0400 0600 0970 1000 1200 1400 1600 1800 T000 2200 2400 LOCALTIME
Figure 37. Propagation Analysis Chart for New York to London Path This analysis chart shows the propagation path for a frequency of 14 MHz and an estimated radiated power of 1000 watts. The highest probable frequency (HPF) is that value of muF that will occur on less than 10 percent of the days of the month. The lowest usable frequency (LuF) is dependent upon the local noise level at the re- ceiving site. The path will be closed down when the LuF is greater than the HPF.
quarters station of the American Radio Relay League, WlAW, rebroadcasts Propagation Forecast Bulletins on a regularly, weekty scheduled basis to all radio ama- teurs. In addition to propagation bulletins and broadcasts, monthly propagation charts are carried in QST maga- zine. The charts cover 30 propagation paths and pro- vide an estimation of the average, best and worst conditions that can be expected during the last half of one month and the first half of the next.
The best estimates indicate that the usable hf spec- trum is expected to dwindle to half that space available during 1980 and that between the years 1985 and 2005 the amount of usable hf spectrum may never exceed 70 percent of that available during 1980. On the other hand, the steady use of hf spectrum is expected to continue, even in spite of the transfer of large volumes of traffic to space satellites. Spectrum conservation and improved propagation knowledge are two actions that must be taken to prevent the high-frequency spectrum from becoming less useful for communications as a result of decreasing solar activity.
Figure 38. The Approximate Relationship
Between 2800 MHz 5olar flux
and Sunspot Number
Propagation Bulletins Propagation bulletins are issued by the National Bu- reau of Standards radio stations WWV and WWVH and are updated four times daily, usually at 18 minutes after each hour. A propagation quality forecast is given along with conditions of the geomagnetic field, the K- index and the solar flux index. The K-index is a statement of geomagnetic activity and provides an insight of propagation quality on high latitude communication paths. The solar flux index is a measure of solar radiation and is related to the sunspot number and hence the maximum usable frequency (Fig- ure 38). Both indices tend to follow a 27-day pattern as a result of the period of rotation of the sun. They are used to make short-term propagation forecasts for cir- cuits of interest. Generally speaking, the higher the value of solar flux and the lower the level of the K- index, the better will be the propagation conditions. A complete overview of high-frequency propagation is available in The Shortwave Propagation Nandbook, by Jacobs and Cohen, published by Cowan Publishing Corp., NY.
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