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JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 66D, No. 6, November-December 1962 Prolonged Space-Wave Fadeouts in Tropospheric Propagation Albrecht P. Barsis and Mary Ellen Johnson
Contribution From Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, Colo. (R eceived May 3, 1962; revised June 12, 1962) This paper contains the results of studies performed during the last several years on t he short-term variability of t ropospheric signals received over within-the-horizon paths in Colorado and California. Signal variations of the type observed over such paths have been t ermed "prolonged space-wavE' fadeouts." They are analyzed as a function of carrier frequency, path characteristics, and meteorological parameters. The study also includes an evaluation of fadeouts observed over a path using a mountain peak as a diffracting knife-edge obstacle between transmitter and receiver. Principal results show a stronger diurnal trend of fadeout incidence in continental climates than in ma ritirne climates. A significant dependence of t he fadeout characteristics on the refractive index structure has been observed in maritime climates. In general, fadeouts tend to be more frequent but of shorter duration for higher fre quencies. There are also indications that the occurrence of fadeout is well correlated on vertically spaced antennas. Thus, conventional space divers ity t echniques may not be effective to increase the reliability of systems operating over within-the-horizon paths.
1. Introduction contained in the NBS Technical Note [Barsis and Johnson, 1961]. Two of the paths (including the A recent NBS Technical Note [Barsis and Johnson, knife-edge diffraction or obstacle-gain path) were 1961] contained detailed results on measurements oJ located in Eastern Colorado, which represents a short-term fading characteristics observed over continental, dry climate. Two others were along the within-the-horizon paths at frequencies between 100 coast in Southern California, which represents a and 1250 Mc/s. These data have been re-analyzed maritime climate with almost constantly present in part and are presented here. Short-term signal refractive index stratification. This is indicated by variations on within-the-horizon paths have pre the well-known elevated subsidence inversion ob viously been defined and analyzed as "space-wave served in the summer months. fadeouts" [Bean, 1954}, if they were characterized T able 1 summarizes various characteristics of the by a drop in signal levcl to at least five decibels propagation paths and measurements for eaSler below the monthly median signal for at least one reference. minute's duration. The term "space-wave fadeouts" is used in the present analysis, although fo[, some of TABLE 1. Chamcteristics oJ paths and propagation m easure the data improved recording methods permitted the ments used in the study oj prolonged space-wave fadeouts measurement of fadeouts as short as 0.1 min, and in Path deS ignation Location Lel1gth Carrier P atb and climate some instances fadeouts much longer than 1 hI' have frequency characteristics been observed, so that one cannot classify this phe nomenon as strictly "short-tenn." However, the km M cts Cheyenne M t.- Eastern 113 10,16 Linc·o{-sigb t , dry, analysis considers fading characteristics which differ Karval Colorado cont inental Bculah-'I'able Easi.ern 223 751 Obstacle·gain, dry. from those commonly observed over tropospheric Mesa Colorado con tillental scatter paths in the distributions of instantaneous :\1t. Wilsou- Pt. Southern 185 138, Lin e ·of· s i~ht , Lorna California 1250 m aritime signal envelope levels, which are usually not repre San Nicolas Southern 104 100, Line·o[·sigh t, Island- Laguna California 250, m aritime sentable by a Rayleigh distribution, or by combining Peak 400 a steady with a Rayleigh distributed signal. The variations are also much slower than the fading on tropospheric scatter paths. Tropospheric "obstacle-gain" paths which employ 2 . Definition of Prolonged Space-Wave knife-edge diffraction exhibit fadir~g characteristics Fadeouts very similar to two line-oI-sight paths in tandem. R ecently spaee-wave fadeouts and other fading The study of fadeout phenomena requires first a phenomena were observed on such an obstacle-gain definition of the term "space-wave fadeout" and its propagation path which uses a mountain as a dif distinction from the term "fading" commonly em fracting obstacle between the path terminals [Barsis ployed. "Fading" usually refers to the rapid ampli and Kirby, 1961] . tude variations of a radio signal tnmsmitted over rela D etailed descriptions including terrain profiles, tively long distances through the troposphere or the antenna heights, and other data for all propagation ionosphere. In the case of long-distance tropo paths which were used in the fadeout study are spheric scatter propagation, the rapid fading is
648645- 62-4 681 presumably caused by the vector addition of a large tion, is useful in studies of diurnal and seasonal number of field components arriving at the receiving variations, and their dependence on meteorological location with random phase. The amplitude dis phenomena. Fadeout duration is tbe length of time tribution of such a signal tends to have the form the, signal drops below a given level (or the trans of a Rayleigh distribution , and the fading rate with mission loss exceeds a given level) during an indi reference to a short-term average or median value vidual fadeout, and the cumulative distribution of varies with carrier frequency, atmospheric condi fadeout duration at various levels below a median tions, and path length approximately within the reference level is of interest. limits of 0.1 to 10 cis. As implied by its name, the term "prolonged 3. Comparison of Fadeout Characteristics space-wave fadeout" is restricted to paths where the for Continental and Maritime Climates geometric-optics space-wave is predominant; it may also be applied to knife-edge diffraction paths. The In this comparison, the 113-km Cheyenne Moun fading rate is extremely slow relative to fading rates tain- Karval path in Eastern Colorado and the 185- observed, for inst:mce, on tropospheric scatter paths, km Mount Wilson-Point Loma path in Southern and the amplitude dietribution of the received signal cannot usually be represented by a Rayleigh, California are used, as data on similar frequencies or any other simple distribution function. Fade (1046 and 1250 Mc/s) are available for them. Ad outs of this type may be caused by vector addition ditionally, data at 138 Mc/s are available for the of a few specular components caused by reflection Southern California path. Both paths are com from layers, or a defocusing of energy by layers and pletely lin e-of-sight, i.e., within the radio horizon, similar phenomena, which tend to shift with the and a minimum fadeout duration of 1 min was used. variable refractive index structure of the lower at Fadeout studies for the Colorado path are based on mosphere, thus causing the variations in signal level. data obtained in 1952 and 1953, with important Possible emall contributions to the received field results based on the first year of operation already due to scatter are neglected in this analysis. Figure reported by Bean [1954]. He concluded from an 1 shows a typical space-wave fadeout. The depth analysis of the dependence of these fadeouts on D of the fadeout is measured with reference to an meteorological characteristics that fadeouts are arbitrary signal level, which is usually a long-term more likely to occur whenever a ground-modified median of hourly median values. The same re[er·· refractive index profile was observed. Fadeouts enee level is R decibels above the R \1S receiver are not observed as frequently whenever linear re noise level. The quantity fractive index profiles characteristic of a well-mixed atmosphere are present. RD = R - D (1) Simultaneous measurements on 100 and 200 Mc/s is then the instantaneous carrier envelope signal-to over the Colorado path showed very few fadeouts on RMS noise ratio. It may, as an example, be used to these lower frequencies during both years- not estimate performance characteristics for communica enough to perform an analysis. This is the first tion links using paths of this type. significant result of this comparative study, as the The definition of space-wave fadeouts was re maritime climate of the Southern California path stricted to those fadeouts which extend to at least produced a substantial number of fadeouts on 138 5 db below the reference level. In Bean's original Mc/s, and will be further discussed in section 6 below. work: [1954] and some of the earlier analyses, fade outs of less than 1 min duration at the 5-db level HOURLY MEDIAN CARRIER LEVEL were neglected due to the time-scale of the available records. This restriction was removed in later work where it was possible to measure fadeouts of shorter duration more accurately, and, at the same time, to eliminate scintillation-type fades ascribed to a scatter compo11en t. INSTANTANE shown on figure 2 for both paths. T he abscissa 100~------~ scale indicates the individual bours of the day as well as the "Lime blocks," as defined by Norton, ....(/) Rice, and Vogler [1955] . Whereas the data for the ~ o 50 continental eastern Colorado path show a minimum w of fadeouts during the daylight hours, especially o ~ around noon, the southern California data representing n a maritime climate show less diurnal variation, witb '0 a maximum fadeout incidence during the early ""lJ.. afternoon hours. There are also more fadeouts on o~O r------, a:: w MOUNT WILSON - POINT LOMA PATH the maritime path than on the continental path, if en the normalized fadeout incidence of the frequencies ::;;; 1950-1951 ~ --1250 Mel . in the 1000 Me/s range is compared. It will be seen z 150 o later that the fadeouts on the California path arc w N i generally shorter in duration. Comparing diurnal ...J trends at 138 and 1250 1\1 cis for the California path , _~ 1..-, maximum during the summer months. There arc ,- .--., not enough data available for the California path to LJ L establish a sertso nal trend from the appearance of the graphs on figure 3, either at 1250 or at 138 Mc/s. Durations of fadeouts are compared on figure 4 o0 2 4 6 for the two paths. Cumulative distributions arc shown here of fadeouts to the 5, 10, and 15 db level TI ME BLOCKS I below the monthly transmission loss medians for the I 8 AND 7 frequencies in the 1000 M e/s range. It is seen that the fadeouts are significantly shorter for the Cali FIG VRE 2. Di1lrnaLvariations oJ prolon(led space-waveJadeouts. fornia path. The total number of fadeouts used for the analysis is indica ted for each curve shown. A similar comparison is given on figure 5 for 138 and 1250 Mc/s, both for the California path, showing 1500 that fadeouts are more numerous and shorter at the CHEYENNE MOUNTAIN-KARVAL PATH higher frequency. 1046 Mc/s,1952-1953 Table 2 summarizes data read from figures 4 and5. 1000 (f) f- ::::> TABLE 2. CompaTison of median fadeout dumlions and 0 w durations exceeded by 10 percent oJ all fadeouts 0 500 <[ lJ... Fadeout durations exceeded, in minutes .D '0 l{) California Path lJ... Color~do Path -- 1046 Me/s o 2000 1250 Me/s 138 Me/s a:: r- w MOUNT WILSON - POI NT LOMA PATH en 1950-1951 Median fadeout durations: :2 - 5 db level 5.8 2.3 6.2 ::::> 10 db level 5.0 1.6 5.7 z 1500 r-- ______:~~OM~J:s - - 15 d.b level 2.9 1.1 3.5 0 w Duration of 10% of all fadeouts: r::::! 5 db level ~8 10. 2 40 ...J 10 db level 26 0.8 38 <[ 1000 - 15 db level 15.5 3.9 :2 20 a:: 0 z The reeClvmg antenna used for the eastern 500 - Colorado path during 1952 and 1953 was mounted -- 13 m above ground. Data obtained during Decem r--1 -- ber 1953 included simul taneous measurements using 0 rl antennas at 1.5 and 4.3 m above ground in addition JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC to the one at 13 m . Thus, ver tical spacings between FrG URE 3. Nf onth-to-month variation of prolonged space-wave approximately 10 ancl40 wavelengths are represented. fadeouts. 683 TABLE 3. Fadeout statistics, Eastern Colorado Path 1046 NIc /s, December 1953 5-db fadeouts lO·db fadeo uts Antenua height above Total Total Total Total ground number fa deout number fadeout time time ------7n min min 1.5 36 771 7 flll 4.3 30 488 5 20 13.0 75 ]528 14 182 antenna height, but may also depend on the ge0I?-et rical characteristics of the foreground and possIbly the thickness of ground-modified layers. For the purpose of estimating the effectiveness of 0102 05 I 2 3 5 10 20 30 40 50 60 70 80 90 95 9B 995 999 vertical diversity, a study was made of the number FADEOUTS,% of instances when 5-db and 10-db fadeouts occured FIGURE 4. Geographical variation of fadeout duration disl1'i butions (Pacific Coast, 1950- 1951, and Eastern Colorado, simultaneously on pairs of antennas or on all three 1952-1953) . of them. Simultaneous fadeouts mean that any portion of the fadeouts observed on the ar::tenn:;s considered coincided in time. Results are gIven m table 4. T ABLE 4. Statistics of simultaneous fadeouts for December 1953 :::-;rumber of fadeouts observed simultaneously on the an tenna combinations listed at left Antenna combina- 5 db 10 db tions: 1.5 and 4.3 m 25 5 1.5 and 13 m 27 6 4.:Jand 13m 24 5 all antennas 19 5 Additional analyses of the data g~ven above are 101 02 0.5 I 2 3 5 10 20 30 40 50 60 70 BO 90 95 98 99.5 99.9 contained in the Technical Note [BarsIs and Johnson, FADEOUTS,% 1961] and lead to qualitative estimates of the effec FIGURE 5. Frequency dependence of fadeout duration distri tiven~ss of vertical space diversity. One may con butions for Mount Wilson- Point Loma path, 1950- 1951 . clude that for installations of this type there is at least a 30 percent probability that lO-db fadeouts Figure 6 shows a ~ample of rec~:n'ding; charts for occur simultaneously on three vertically spaced. an the three antenna heights at the tIme of a fadeout. tennas. It is also seen that there are substantIally The fadeout characteristics appear to be extremely more fadeouts on a high antenna, but with greater well correlated in time. The "transmitter-off" correlation between fadeouts occurring simultane- I period in the record provides an accurate ch.eck on ously on the two lower antennas. . the synchronization of the records for u s~ m. cor Cumulative distributions of the duratIOns of 5-db relating fadeouts. The small, more. rapId sIgnal and 10-db fadeouts for all three antenna heights, variations superimposed on the recordmg traces are usinO' all data available for December 1953, were also not considered in this analysis. obtained and plotted. From these graphs the me DurinO' this December 1953 measurement period, dian fadeout duration, and the duration of 10 percent 322 hI' ;f data were recorded simultaneously using of all fadeouts are tabulated below. all three receiving antennas. Table 3 summarizes the 5-db and 10-db fadeout statistics for these hours. TABLE 5. Fadeout dumtions f or December 1953 The total fadeout time in table 3 is the number of Cheyenne Mountain-Karval Path 1046 Mc/s minutes the signal remained at or below the 5-db or 10-db level respectively, during the 322 hr recorded. Duration of median Duration of 10% of Antenna fadeout all fadeouts It is seen that there are about twice as many fadeouts height on the hiO'hest antenna than on each of the two 5 db 10 db 5 db 10 db lower one~ and that the total fadeout time is also ------significantly larger on the highest (13 m) of the three 7n min min min min 1.5 10.6 3.5 39 20.5 antennas but the minimum fadeout time appears to 4.3 10.4 5. 7 45 18.5 be on th~ 4.3-m antenna. Thus fadeout incidence 13.0 6.1 5.0 60 56. 0 and fadeout time are not necessarily functions of 684 Il. ______DIPOLE AT 13 METERS I-- I--« 3; -.J -.J ~ 50 3; 55 9 w CD ..a "0 DIPOLE AT 4 .3 METERS I-- 50 ~------_T------~------T_------~ I--« 3; -.J :::! ~ 3; 9 w CD ---j ..a !----_i_I_ _ "0 - TRANSMITTER OFF DIPOLE AT 1.5 METERS I-- I--« 3; -.J -.J 50 ~ 55 3; 0 -.J W CD ..a 70 "0 75 80 85 1100 1000 M.5.1. 0900 0800 0700 DECEMBER 9,1953 FIGUR}J 6. Recording chart samples for Cheyenne M~ountain-J{a rv al path; 1046 Jl!Icl s. The median fadeout duration at tho 5-db level, 4. San Nicolas Island-Laguna Peak Path tends to be shortest for the highest antenna. How- (Southern California) l ever, 10 percent of the fadeouts for this an tenna are . quite long--about 1 hr. The median fadeout duration AddiLional data for tho Southern California mari at the 10-db level is relatively short for the lowest time elimate were provided by a joint propagation . antenna. study by the Naval Air Missile 'Test Center, Point 685 Mugu, California, and the National Bureau of Stand adiabatic charts from radiosonde ascents at San ' ards. Meteorological data to support the transmis Nicolas Island and at Point Mugu, as well as surface I sion loss measurements were supplied by the U.S. observations of temperature, barometric pressure, Naval Air Station at Point Mugu. The path pro and relative humidity. From these data the refrac- , files, operfttional details, and analysis method were tivity was computed and plotted in the form of described by Barsis and Johnson [1961] and in an curves of a modified refractive index "A" as a func : earlier paper by Barsis and Capps [1957], which con tion of height as defined by Bean et al. [1959] . I tains important results of this program. These, and Values of the surface refractivity were determined , additional data evaluated since, form the basis for a as well. Further details are given by Barsis and ! study of space-wave fadeouts and their dependence Johnson [196 1]. This paper also contains typical I on path and meteorological parameters. Minimum samples for and a definition of the five types of I fadeout durations as short as 0.1 min were considered refractive index profiles used. ' here, and the following approach was used. Figure 7 is an illustration of the effect of the re (a) Evaluation of two years radio data on 394 fractive index profiles on the character of the received :vIc/s, including correlation of the observed signal. Two chart si1mplcs of the Laguna Peak fadeout phenomena with refractive index pro record representing the within-the-horizon path are . file types. shown. The upper record, taken at a time when a (b) Comparison of fadeout data at the three fre linear refractive index profile was observed at Point quencies (approximately 100, 250, and 400 Mugu, shows a virtually constant signal over a period Mc/s) for typical winter and summer months. of several hours. The lower record corresponds to a I (c) Comparison of fadeout data for two receiving time when ground-based layers were observed at sites within the radio horizon which are widely Point Mugu and shows very substantial space-wave I' separated in elevation. fadeouts. It will be appropriate to discuss first the evaluation Table 6 shows the relative frequency of occurrence . of the meteorological data, and the classification of of the various profile types at both sounding locations : the refractive index profiles. The data supplied by during the period November 1955 to October 1957. i the Point Mugu Naval Air Station included pseudo- It is not possible to distinguish between "ground- (a) Linear Refractive Index Profile, No Space-Wave Fadeouts 'tc = - .n ~:J- 115 - "D ==I- 11~ .n ::;: -:c!- __ ~ "D - Cl;l15 MEDIAN 5 -:- ~- -- =l: ===I== '" 130 § 10 F ==!C -=!= ~ 135 ~ l~'=--= c:.-==- -.J § 140 :;;;: 15 =!-_ = I-C :-=== 2- :E 20 ~ \--t=: =--=+- . ~=t ' -"" ~- 0500 0530 0600 P. ST 0630 January 31, 1956 0700 (b) Refractive Inde x Profile Characteri zed by a Ground Based Layer, Pronounced Space - Wave Fadeouts -- ~-db5 - j:: .n =1' :;;/.- ===t' {l-' 125 IoIJNTHLY "D =-;= 1 o = -130 MEDIAN c I=-~ =- o 5 ;,<:~r=:i t : {:::/ . l' _ ~1135 10 :cc - (f) - -g 0 - 140 :;;;: t-:"-=!--= 1--"=---+-' .cc_ . ~ - -.J >- 15 c L ~ -145 8 (f) :;;;: 20 --4=\----\'- - 0 150 l, \===:::;: _ =-~: . +=". " =';=-1- ~l a:; -----+--- - ~ A :__ ~ · o - 155 \= ='\=::\-~ =-~~~~: - - s'" -- 0800 0900 1000 P.ST 1100 November 28. 1955 1100 1300 FIGURE 7. Recording chart samples for S an Nicolas I sland- Laguna Peak path; 394 ]lIfc/s. 686 TABLE 6. OCCUTT ence of various profile types from November· TABLE 7. Fadeout characteri::,tics versus r·efractive index profile 1955 to October 1957 types for San Nicolas I sland-Laguna P eak (Summit) path on 394 J1fc/s, November 1955 to October 1957 Number of times Percentage of total observed obse rvation s T otal P e rcc n lagf~ of Percentage of Profile type and total number farirou t fadeout l1ll Dlbrr observed Profile type orob· incidence time Pt.l\lugll San N icolas Pt. Mugu San N icolas s(' rve ci l 35 frequency, but for different grazing angles (2.0 and 30 3.6 mr, respectively). For all frequencies, vertical 25 polarization with corner-reflector antennas was used. 1 Very little meteorological data could be obtained 20 for these two recording periods, so that the evalua 15 !:tl 1 J tion of the data will be restricted to studies of the 10 carrier frequency and antenna height (or grazing angle) dependence of the fadeouts without any correlation with meteorological data. l>N2 (N-UNITS/KILOMETER) Fadeout incidence (here the number of fadeouts in a 90-day period) for the summer measurement FIGU RE 8. Percent fadeou t time as a function of profile period mentioned above is shown in figure 9 as a characteristics for San Nicolas I sland- Laguna Peak path (Nov. 1955- 0ct. 1957; 394 NIc ls). function of carrier frequency. The graph shows points for 5-, 10-, 15-, and 20-db fadeouts at the The graphs of figure 8 provide more detailed three frequencies, arbitrarily connected by straight information on the dependence of fadeout time on lines. Data for the "Van" location have also been layer characteristics, whereas the results shown included. There is frequency dependence, expecially previously in table 7 apply only to a general classifi when comparing 250 and 400 Me/s. The grazing cation of the layers. Figure 8(a) shows quite angle dependence is quite pronounced by the separa clearly that the fadeout time percentage is greatest tion of the "Van" points from the solid lines which for ground-based and very low layers (below 200 m denote the "Summit" data. It should also be noted above the surface). The dependence on the height that there were more than one hundred 20-db fade of the top of the layer, shown in figure 8(b), indicates outs in a 90-day summer period at 400 Mc/s, which maximum fadeout time for heights ht + h2 below would correspond to more than one such fadeout 800 m . Both terminals are well below this height, per day if evenly distributed. suggesting that a well-defined discontinuity in the refractive index profile structure favors the occur 1000 I rence of fadeouts if the ray paths penetrate the f-z 700 0 discontinuity. The second maximum on figure 8 (b) ::;; 500 /"" appears to be less significant due to the wide spread 0:: ~ i /" ~A~b, ~ of the data. Ul 300 f- V ::::> /" /' Fadeout time dependence on the refractive index 0 200 w v gradient t:.N2 within the layer is not significant, as 0 ~ observed in each case, and are coded to denote tho , WINTER INOV. 1957- J~N . 1 ~58 ) three different frequencies and two paths used. 100 f-++-I-- '-,,+-+--+-+-+--+--+--+--l---l--+-J-..j The appearance of figure 11 suggests a log-normill 10 P-o-H--+--+""-,-+---+-+-I--+-+--1--l-- I- f- distribution of fadeout durations, just as was ob ~ ~ ~ f--t-l'--p--l.--,-+-+-'>+--+---+--+ ~- ~94 Me/s (SUMMIT) served on fi gures 4 and 5. ~ . , ··244 Mels The median durations of 10-db fadeout read from f-++-+----""..,"-.:~... +-. -1-"-1,,--+--+ - --· 100 Me/s figure 11 are compared with similar values for the "'1'<. I ' - " Mt. Wilson- Point Lorna path (taken from fig. 5 and table 2) in the following tabulation. TABI,E 8. M edian durations of 10-db f adeo uts }"adCO llL d uration in m inutes Carrier P ath fr equency W intcr Summer All data Afcls 14. 0 9. 1 ______100 Point Mugu ______5.7 138 Mt. Wilson 250 P oint Mugu 6.6 i. O ______400 Point Mugu (S ummit) N o data 3.4 ______5.4 3.5 ______400 P oint J\lu J:( u (Van) ______1. 6 1250 Mt. Wilson Although the data from the Mount Wilson Point Loma path were not broken down into the "winter" and "summer" classification, and the Point Mugu data include shorter fadeouts, both sets FADEOUT, % can be used to establish a common trend of frequency FIGU RE 11. Fre quency dependence of cumulative distj'ibul'ions dependence. As expected, fadeout time shows a of lO-db fadeou ts for San Nicolas I sland- Lagu na P eak trend with frequency opposite to the trend of fadeout path. incidence shown on figure 9. In general, more fadeouts are counted in a given time period for higher frequencies, but they are of shorter duration. 5. Fadeouts on a Knife-Edge Diffraction Path Figure 11 also shows that the curves for the "Summit" and the "Van" locations, both for 394 During 1959 and 1960, transmission loss measure Mc/s, are almost coincident. It is concluded that ments were made over a 223-kl11 path in Colorado, no effects of the grazing angle on fadeout duration using continuous wave signals on 751 Mc/s. The have been found in the data analyzed here. top of Pikes Peak, 4292 m above mean sea level, 689 -.; ) / 70 CORNER REFLECTOR AT 15 METERS f- 75 ~ ~ --.J --.J 80 :'! ~ 0 85 --.J mW .D 90 u 95 100 105 65 3 METER DISH f- ~ 70 ~ --.J --.J :'! 75 ~ 80 0 --.J mW .D 90 u 95 100 105 0400 034 5 0330 M.ST 0315 0300 MARCH 24, 1960 FIGURE 12. Recording chart samples for Colorado obstacle gain path; 751 Mc/s. forms the diffracting knife-edge. The transmitting Figure 12 suggests that fadeout phenomena on this installation is located near Beulah (southwest of obstacle-gain path are similar to the ones observed Pueblo), and the receiving installation is on Table on the within-the-horizon paths, and that verti.cal 11esa, north of Boulder. The path profile, a com space diversity would be of little use due to the plete description of the equipment used and the apparent correlation of the fadeouts observed on the measurements performed are contained in a separate two antennas. paper [Barsis and Kirby, 1961]. The data discussed An analysis of fadeouts was made by data recorded here are concerned with prolonged space-wave fade using two spaced receiving antennas; a corner outs observed on this path on two receiving antennas, reflector, mounted 15 m above ground, and a 3-m I spaced vertically by approximately 20 wavelengths. parabolic reflector, 8 m above ground. l The trans Although this path is not "optical" (terminals mitting antenna was a 4.5-m parabolic reflector within each other's radio horizon), the fading mounted 7 m above ground level. Horizontal characteristics may be compared with the ones pre polarization was used for all tests. Continuous viously described, because the received field may be recordings were available during a number of five considered as the vector sum of up to four rays, each day periods between December 1959 and September of which is modified in magnitude and phase by the 1960. diffracting knife-edge [Dickson et al., 1953]. Figure The diurnal trend in the fadeout incidence ob 12 shows a typical sample of field strength records served at various levels below the median for each obtained simultaneously from the two receiving recording period 2 is shown on figure 13, with the antennas. The fadeouts in this case appear to be extremely well correlated, just as in the case of the 1 During two of the 5·day recordin !; periods (in June). the co rner reflector was Cheyenne Mountain- Karval path (fig. 6). Here, atan elevation of 23 m above ground. No material diffc rcnm in the character too, the small, superimposed, more rapid signal istics of the fadeouts at the two elevations was found . , This referen m level was chosen In lieu of monthl y medians, as data for any variations have not been considered in the analysis. full month were not available. 690 ordinat.e indicating the numbeT of fa,deouts per hour 600 ,------, WINTER D CO RNER REFLECTOR for each time block. 5-db fadeouts are much more AT 15 m 5db frequent than expected from a comparison with the 3 METER OIS H other levels. There are only compara tively few 400 5 db AT 8 m 10-db fadeou ts during the winter . I 5 d b While the 10-db fadeouts show an afternoon (time f- 100 z 5db block 5) minimum similar to the Cheyenne Moun 0 ::;: IQdb tain- Karval path (see sec. 2), 5-db fadeout incidence 0:: IOdb 15db w 15db shows a maximum fOT time block 4 corresponding to Cl. oTINE BLOCK 8 TINE BLOCK I (f) 0000-0600 0600 - 1300 the morning hours. Also of interest is the minimum f- :::J for the 5-db fadeouts in time block 8, corresponding 0 w 800 to night and early morning hours during winter. No 0 c;: SUMMER 5 db significan t difference in fadeout incidence exists for lL. the two antenna heights represented. Fadeout 0 0:: 600 w incidence is somewhat greater on the average for the (]) ::;: higher antenna, although the difference is not as :::J db 5db 5 db Z prominent as was seen for the three antenna heights 400 of Karval. The data suggest the possibility of two separate 100 propagation mechanisms, one producing only 5-db IQ db fadeouts, and the other responsible for the deeper 15db IO db 15db fadeouts. Comparison of the diurnal trend wi th O~UJ~~_L~~~=_~~~ __~ ~~~~~ data from the K arval path suggests that the second TINE BLOC K 7 TI NE BLOCK 4 TIME BLOCK 5 TI ME BLOCK 6 mechanism is similar to the one causing fadeouts on 00 00- 0600 0600-1 300 1300-1 800 1800-1400 within-the-horizon paths. FIGURE 13. Diurnal vW'ialions of prolonged space-wave f ade The cumulative distribu tions of fadeou t durations outs fo )' ColoTado obstacle-gain path ( Dec. 1959- S ept. 1960; for this path are shown on figure 14 for both antennas. 751 M c/s) . Here, fadeout dura tions as short as 0.1 min could be measured. T able 9 is a t abulation of median fade out durations taken from figure 14. signal duration of cumulative distribution curves T A BLE 9. Nl edian fadeout dUTa tions for Colorado Obstacle resulting from this analysis, become the distribution Gain Path, 751 Mc/s of instantaneous signal levels for the period of record analyzed . l\1edian fadeout duration, minutes Level, The eff ectiveness of vertical space diversity for d b below this path may also be evaluated by the number of homly Corner refl ector 3·m dish median fadeouts occurring simultaneously on both an tennas "Vinter Summer 'Win ter Summer in relation to th e total number of observed fadeouts. This is the same type of evaluation performed for 5 0.58 0. 46 0. 3t 0. 32 the three ver tically spaced a ntennas at K arval (see 10 . GO . 44 . 23 . 27 sec. 2). R esults are given in table 10 using 5-db 15 . 28 .35 and 10-db Jadeouts. This study includes the two June r ecording periods, with the 15-111 separation It is seen tha t the median fadeout at the 5- and between th e antennas, so th at vertical spacings of 10-db levels is of longer duration for the corner 17.5 and 37. 5 wavelengths are both r epresen ted. r eflector antenn a, which is at a greater heigh t. For Fadeouts wer e classified as occurring simultaneously the corner reflec tor an tenna, the fadeouts are longer on both antennas when any portion of them coin in winter than in summer; wher eas for the dish no cided in time. seasonal trend is observed . Of interest here is the The absence of simultaneous 10-db fadeouts much shorter duration of the median fadeouts during all winter hours, and during the summer compared to those observed on within-the-horizon afternoon hours, reflects the small total number of paths (see table 2). This is partially due to the such fadeouts observed on either an tenna. Also, inclusion of fadeouts as short as 0.1 min in the small errors in the timing of the recording ch ar ts Pikes P eak data and supports the idea of additional may have an appreciable eff ect on judging whether propagation mechanisms influencing the fadeout short fadeouts are simultaneous. statistics on knife-edge diffraction paths. The data in table 10 show that a greater proba A som ewhat different analysis of fadeout durations bility fo r simultaneous fadeouts exists during the is contained in figure 16 of a paper by Barsis and summer months, with a clear maximum during the Kirby [196 1] . There, the distributions of fadeout night and early morning hours. The percentages durations for time periods of several hours were obtained here are on the same order of magnitude ob tained at a number of arbitrary levels below the as shown for the three K arval an tennas described median, together with the distributions of high in section 2 above, and listed in more detail on signal dura.tions for levels above the median. The table 4 of the original study [Barsis and Johnson , asymptotic extensions towards a zero fade, or high 1961]. 691 10.0. 70. 3 METER DISH WINTER 3 METER DISH -SUMMER 50. '\. - r-- 30. 3.Sdb IlS99) 20. h "- 10. '\ 7 -- 5 10db (2 73)- " ..... I'\.S db (400) , . , '~ r- IS d J( ~1) '-. ~ \ ,""'- I I' 0..7 10 db (9) - , 0. .5 ~ 0..3 '\. ~ 0..2 -- ~ f- - -'~ -~ ':-,.. ~ 0. I 0- -.l ..1 --- -.l ::;; z o ~ 10.0. ~ 70. CORNER REFLECTOR-WINTER CORNER REFLECTOR-SUMMER o 50. '. 30. 1,\Sdb (IS74) 20. 1\ - I- \ - r-- t--, 10 M Sdb (301) 7 -- ., r-- 10 db (239) • 3 "- -. ---':\ r,· . ~ IS db (49) . ., f-- - l- 10db ( 3';}~ • . f'" I -- 0.7 0. 5 . 0. 3 I'\. . "';:"" 0. 2 T'--. -'. - f-- ~ ~" o.I -- ..1 ~ ~ ..1 0.1 0.5 I 2 3 5 10 20 30 40 50 60 70 80 90 95 97 99 0.1 0.5 I 2 3 5 10 20 30 40 50. 60 70. 80. 90. 95 97 99 PERCENT OF FADE OUTS LONGER THAN ORDINATE VA LUE FIGU RE 14. Cumulative distributions oj Jadeout dW'ations Jar Colorado obstacle-gain paths. 6. Conclusions T ABLE 10. Statistics oj Jadeouls observed simultaneously on both antennas Jor the P ikes Peak Ob stacle-Gain Path, 751 A comparison of fadeou t characteristics for differ Mc/s ent climatic areas was given in the discussion of the data obtained . Some noticeable eff ects are the very P ercen t of fades T ime blorks occulTing simultane- definite lack: of fadeouts during daytime hours for the ously Karval path, and the absence of any pronounced sea sonal variations for Pacific Coast data, as far as in Winter: 5 db 10 db vestigated. In contrast to the maximum fadeout 8 (OOO(H)600) 48.3 ------incidence for the K arval path at nigh t, fewer diurnal 1 (0600-1300) ~2. 5 ------2 (1300-1800) 44.5 ------variations appear on the Pikes Peak obstacle-gain 3 (1800· 2400) 27.1 -- --_.- ----. path. All hours, 37.6 ------Diurnal and seasonal trends in the occurrence of winter fadeouts similar to the ones observed at Karval were Summer: 7 (0000-0600) 76. 1 42. 0 reported from measurements at 1300 Mc/s over an 4 (0600-1300) 59. 6 28.6 82-km overland path within the radio horizon , lo 5 (1300-1800) 35.6 6 (1800-2400) 59. 7 ' ·"24.'2" " cated in E ast Germany [Kuehn, 1957) . D espite the All hours, difference in the general climatology, the low-level summer 58.8 24.9 modifications of the refractive index structure which are most likely to produce fadeouts appear to have 692 similar diurnal and season trends in both regions. £adeouts were observed at the higher antennas, at Kuehn's data, like the ones obtained at Karval, show least for the Cheyenne Mountain- Karval path. minimum occmrence of fadeouts around the noon The most important result of these meaS Llrements hour, strong maxima at night, and also substantially is the recognition of space-wave fadeouts as an im more fadeouts during the summer months. portant phenomenon in line-of-sight or related prop The lack of fadeo uts at the lower frequ encies over agation characteristics, especially at frequencies the Colorado paths, as contrasted with the relatively above 400 Mc/s. The phenomenon is observed when large number observed at 138 Mc/s for the Mt. ever the refractive index profile has strong low-level Wilson path, and at 100 Mc/s for the Point Mugu modification, regardless of climatic conditions and path, is an indication of the effects of climatic differ physical causes of the modification. Space-wave ences between the two areas. The nocturnal duct fadeouts have to be taken into account in the design which constitutes a plausible cause for the observed of communication systems, because of the relatively fadeout phenomena at Karval [Bean, 1954] is rela long and large drops in signal I evel. Studies per tively shallow and is therefore less likely to affect the formed so far show that vertical space diversity is lower frequencies. However, ducts and ground not effective in compensating for aU fadeouts be based layers of sufficient thickness to affect lower cause of the observed degree of correlation. Ex frequencies are present in the maritime climate of periments on correlation, using horizontally spaced Southern California. This was indicated by the anal antennas or frequency diversity, are yet to be ysis of the Point Mugu data. The occurrence of performed. such 1avers in Southern California is not restricted to nighttime hours and is more likely a result of the almost always present subsidence inversion. In dealing with an extensive measurement pro Anderson and Gossard [1953 a and b] have studied gram of this type it is not possible to aclmowledge the the effects of nocturnal and oceanic ducts on UHF con tributions of individuals. D ata collection and propagation, using largely unpublished data from evaluation was performed by the Cheyenne Mountain the Cheyenne Mountain program and data obtained Field Station (partially with the support of the from experiments in the Irish Sea [McPetrie and Air Navigation Developmen t Board), the Tropo Stal'llecki,1948]. Their studies were principally con spheric M easuremen t Section, and the Propagation cerned with the enhancement of fi eld str ength on Terrain Effects Section of the National Bureau of beyond-the-horizon paths produced by ducts, and Standards. Also acknowledged is the assistan ce of with the analysis of the degree of enhancement as a the Navy Electronics Laboratory, San Diego, CaliL, function of meteorological parameters. Bean [1954] and of the Naval Air Missile T est Cen tel', Po in t has shown that the occurrence of fadeouts on the Mugu, CaliL , in obtaining and supplying data and within-the-horizon Cheyenne Mountain- Karval path equipmen t, and in doing analysis work. Support from the U.S. Army Signal Research and Develop is usuall~r associated with field strength enhancement on the beyond-the-horizon Chevenne Mountain ment Laboratories in Ft. Monmouth, N.J., facili Haswell path. Data from t he latter path are used in tated compilation of the original publication [Barsis one of the papers by Anderson and Gossard [19 53a] . and Johnson, 1961] and of this paper. The authors thank B. R. Bean, E. J. Dutton, R . S. The results shown h ere confirm the principal de Kirby, and K. A. Norton for their review and sugges pendence of fadeout phenomena on ground-based tions, and J . B. Smyth fo r drawing their attention to ducts. Observed differences between continental the work by Anderson and Gossard. and ll1.aritime areas are due to the difference in the thickness of layers, their height above ground, and the diurnal and seasonal trends in their occurrence. 7. References Thus fadeout phenomena are, at least indirectly, a function of cl imate. Anderson, L. J., and E. E . Gossard, Prediction of the nocturnal duct and its effect on UHF, Proc. IRE 41, No. 1, 136- 139 The effects of vertical space diversity have been (Jan. 1953) . discussed for both Colorado paths in sections 2 and 5. Anderson, L. J., and E. E . Gossard, Oceanic duct and its It appears that fadeouts are reasonably well corre effect on microwave propagation, Nature 172, 298- 300 (Aug. 1953). lated at antenna spacings which might be practical Barsis, A. P., and F. M. Capps, Effect of super-refractive for diversity arrangements, and thus impair the effec layers on tropospheric signal characteristics in t he Pacific tiveness of vertical space diversity for 1ine-of-sight coast region, IRE 1957 WESCON Convention Record, paths as well as for obstacle-gain paths. P a rt 1, 116- 133 (1957). Barsis, A. P ., J. IV. H erbstreit, a nd K . O. H ornberg, Cheyenne The two antennas on Laguna Peak have too large Mountain tropospheric propagation experiments, NBS a vertical separation to consider such an arrangement Circ. 554 (Jan. 1955). a practical diversity system. The experiment was Barsis, A. P., and M. E. Johnso n, Prolonged space-wave designed in this case to study the eff ect of the grazing fadeouts in tropospheric propagation, NBS Tech. Note 88 (Feb. 1961). angle on fadeouts. Although results are not very Barsis, A. P., and R. S. K irby, VHF and UHF signal charac pronounced, there is a tendency for more fadeouts on teristics observed on a long knife-edge diffraction path, the summit of Laguna Peak, representing a greater J . Research NBS 65D (Radio Prop.) No.5, 437-488 (Sept. elevation and a greater grazing angle (see fig . 9). Oct. 1961). Bean, B. R., Prolonged space-wave fadeouts at 1046 Mc This res ult is also somewhat supported by the Colo observed in Cheyenne Mountain propagation program, rado heigh t-gain stud ies, where substantially more Proc. IRE 42, No. 5, 848- 853 (May 1954). 693 Bean, B. R., L. P. Riggs, and J. D. Horn, Synoptic study of Uehertl'agungsstrecke mit optischcr Sicht, Tech. Mitt. t he vertical distribution of the radio refractive index, J . (B.R.F.) 1, No. 1, 4- 10 (Oct. 1957), Berlin-Adlershof (East Research NBS 63D (Radio Prop.) No.1, 249- 254 (Sept. Germany). Oct. 1959). McPetrie, J. S., and B. Stal'lleckie, Low-level atmospheric Dickson, F . H., J . J . Egli, J. W. H erbstreit, and G. S. Wiclci ducts, Nature 162, 8 18 (Nov. 1948). zer, Large reductions of VHF transmission loss and fading Norton, K . A.,P. L. Rice, and L. E. Voglel', The use of angular by t he presence of a mountain obstacle in beyond-line-of distance in estimating transmission loss and fading range sight paths, Proc. IRE 41, No.8, 967-969 (Aug. 1953). A for propagation t hrough a t urbulell t atmosphere over discussion of this paper is given by J . H . Crysdale in Proc. irregular terraill, Proc. IRE 43, No. 10, 1488- 1526 (Oct. IRE 43, No. 5, 627 (May 1955). 1955). Kuehn, U., Ein Beitrag zu r K ent ni ss del' Ausbreitungs bedingungen bei 1.3 GHz nach Messungen an einel' (Ptlper 66D6- 227) 694