JOURNAL OF RESEARCH of the National Bureau of Standards- D. Radio Propagatior. Vol. 65D, No. 5, September- Odober 1961 VHF and UHF Signal Characteristics Observed on a Long Knife-Edge Diffraction Path A. P. Barsis and R. S. Kirby (R eceived April 6, 1961) Contribution from Central Radio Propagation Laboratory, National Bureau of Standards, Boulder, Colo.
During 1959 and 1960 long-term transmission loss measurements were performed over a 223 kilom eter path in Eastern Colorado using frequencies of 100 and 751 megacycles per second. This path intersects P ikes P eak which forms a knife-edge type obstacle visible from both terminals. The transmission loss measurements have been analyzed in terms of diurnal a nd seaso nal variations in hourly medians and in instan taneous levels. As expected, results show t hat the long-term fading range is substantially less t han expected for t ropospheric seatter paths of comparable length. T ransmission loss levels were in general agreem ent wi t h predicted k ni fe-edge d iffraction propagation when a ll owance is made for rounding of t he knife edge. A technique for est imatin g long-term fading ra nges is presented a nd t he res ults are in good agreement with observations. Short-term variations in some case resemble t he space-wave fadeo uts commonly observed on within- the-horizon paths, a lthough other phe nomena may contribute to t he fadin g. Since t he foreground telTain was rough there was no evidence of direct a nd grou nd-refl ected lobe structure. ' In most cases comparative ly 11igh co rrelation exists bct\\'ec ll signals received simultane ously on t wo a ntenn as with 8.3 and 14 meters vertical separation . These separations were chosen as being representative fo r practical space diversity systcms designed for eli minating t he eff ects of fading a ri sin g from direct and grou nd-rcf1ected phasc interFerencc phc nomena. The co mparatively hi gh correlation observed suggests t hat space diversity wi ll be relativcly less successfu l in mountain obstacle paths with rough terrai n ncar t he termina ls than on tropospheri e seatter paths or on line-of-sight paths over smootll terrain. The enha ncement of fi eld strength associated with propagatioll over mountain ridges ITIay causc concern ill applications where Illountain are being co unted on to hi elel uJlwantod radio waves. Somc radio astronomy installations have becn locateel in llloullLain vall eys fOf t his reason, and it is possible that obstacle-gai ll eii'ects ma.\" aggravatc rather t ha n a ll eviaLe Interference.
1. Introduction diurnal trencls and cOlTebtion with meLeorological phenomena as well as with sim.ultlm eo us ducting on It)s gen.erally known t hat moun tain ridges can act paths just beyond the horizon. as diffractmg knife-edges in point-to-point communi To obtain a better understandin g of the long-term cation circy its on VHF and UHF frequencies. The sig nal and Jading chamcteristics, N BS established a mathematical approach has been developed using the test path in Colorado to study kni fe-edge type di f Fresnel-Kirchhofl' diffraction t heory [Schelleng, Bur fra.ction phenomena. This path extends in a roughly rows, and F errell, 1933] . Substantial reductions in north-south direction along the Front R ange of the t~'al~sm i ssion loss as co mpared to scatter circuits of Colorado Rockies, and uses Pikes Peak as t he dif 811mlar length, and relatively fading-free signals were Jracting knife-edge. An outline map of the path with reported by several workers, both in this country and the locations of the terminftls at Beulah and Table a broad [Dickson, Egli, H erbstreit and Wickizer Mesa, Colo., is shown on figure 1. Pertinent path 1953; Kirby, Dougherty, and McQ~ate 1955 ' Kon~ and equipment data are given in table I , below. The and .Hirai, 1955; Crysdale, D ay, Cook,' Psut l ~a, and path profile is shown on fig ure 2, based on an equiva Robillard, 1957 ; Nishikori, Kurihara, Fukushima, lent earth radius of 9,000 km. TillS is l.41 times the and Ikeda, 1957; Hirai, Fujii and Saito, 1958]. More actual earth's radius and corresponds to a surface re?~ntly, additional work in Japan [Kakita, Iwai, and refractivity of 316 N -units. I ell'l, 1959], and so far unpublished data from Alas- Operation on 751 M c/s commenced in D ecember I 1\:an knife-edge diffraction paths 1 have shown that 1959, and terminated in September 1960. Data 80me pftths exllibit considerable fading at carrier f1'e were recorded continuously during 5-day periods at quencie~ in the 900 Mc/s r ange. Much of the time the rate of one each month with more frequent opera t he fadmg encountered resembles the "space-wave tion in June and August. Operation on 100 Mc/s fadeout." phenomena observed commonly on within started in August 1960, on the same basis. Durin~ the-honzon paths, at comparable frequencies [B ean two 5-day periods in August 1960, an additional 1954]. It consists of relatively deep and slow fades; site was operated on top of Pikes Peak, using hori and the occurrence of these fades may show marked zontal half-wave dipole receiving antennas on both frequencies. These antennas were mounted about 6 m above ground. The length of this auxiliary
1 Private communication from "~cste rn Electric Co. line-of-sight path was 77 km. 437 The receiver outputs were recorded on strip charts 106· 104· and multichannel magnetic tape. Transmitter power output values were also recorded on strip charts, so that a continuous record of the trans mitter power is available for both frequencies. The r eceivers were calibrated by signal generators 40·f---+---=-=-=-;e=-=::.:..cp""!""'\-f------,{------f---- 40· checked against laboratory standards. 2. Cliaracteristics of the Received Signal and Variation of Hourly Median Values 2 ,1. Signal Characteristics Figure 3 shows sample recordings of both fre quencies at the two sites taken on the morning of August 10, 1960. The 751 Mc/s record at the Table 39" Mesa receiving site shows more rapid small signal variations superimposed on a much slower fading pattern, which includes a fade in excess of 15 db lasting several minutes. There appears to be sub stantial correlation between the signal recorded simultaneously on the two antennas on 751 mc/s. The slow fading pattern is absent on the Table M esa 100 Mc/s record; only the more rapid varia tions are present, and they are superimposed on a virtually constant signal. It is believed that these - 10 0 lO 20 30 "8"··...-.. E""3 rapid signal variations of small amplitude are due to KI 0 10 20 JO III e ...... Fi ..----. E-"""'9 a component of scattered field superimposed on the more slowly varying diffracted field . For the line of-sight path with the terminal on the top of Pikes FIG eRE l. Outline map J01' Colorado obslacle gain palh. Peak this scattered component is not noticeable, as
TABI"E 1. Path and eq1tipment data Pikes Pea k obstacle gain peth 751 Mels TAB LE MESA, 3m DISH AT 8.2 m Path distance._. __ . ______. ___. ______223 kill. 190 Angular distance ______64.4 milliradians. 20 0 , Terminal elevations above mean sea 210 le vel: Transmitter. ______. ______(Beulah) 1905 Ill. 22 0 ReceiveL. ______(T able Mesa) 1666 m . N !U 751 Mels TA BL E MESA, COR. RE FL. AT16.5 m .: Height of obstruction above mean sea level: 160 (Pikes Peak) ___ . ______. __ ___ . _____ 4292 m.
Height of obstruction above mean terminal elc\7atioll ______250i Ill.
Antenna heights above ground: 'l'ransmitteL ___ ._. ______._._._._. ___ 7.3m for 751 Me/s, 12.4 m for 100 Me/s. ~VI ; - R eeeiveL ______. ______8.2, 16.5 and 22.3 ill for 751 Me/s, 13.7 m g I OO Me/s TA BLE MESA. YAGI AT 13.7 m for 100 Me/s. Polarization ______Horiz on tal. Ante'mas: 751 jllc/s 100 M e!s Transmitting. ______. ______._. ____ _ 4.3 m Dish Yagi Rpceiving ______3 In Dish Yagi Modulation ______co ntinuous wave. ~~ IY_ a: > u 5.000 VI "OJ 4.000
E. 1000 z o 100 Me Is PIKES PEAK, DIPOLE ~ > ~ :: liIG(~(illlr. 1000 1100 1200 DISTANCE. km 240 223.3 km MST
FIGU RE 2. TelTain~profileoJ Colorado obstacle gain p1"Opagation FIGU RE 3. Sinwllane01ls chm·t recordings for Colm'ado obslacle pllh. gain and line-of-s'ight propagation paths, A Ug1lSl 10, 1960.
438 expected. However, the slower variations are still 180 present on 751 Mc/s, and they do not appear cor D related with the slow fading observed at the Table " 185 Mesa site. '" '"':3 CORNER REFLECTOR The slow signal variations which are similar to CORNER REFLECTOR AT 22.3 METERS LCORNER REFLECTOR 2 190 0 - AT 16.5 r-METERS__ J " AT 16.5 METERS - the" space-wave fadeouts" commonly observed on in -- L _ /' line-of-sight paths will be discussed in more detail '" -- -..-.., later on. "'"2 195 "
210 ~ ______~ ______~ ______~ ______
751 Me ls CORNER REFLECTOR AT 16.5 m :-<0, Months lI ours 18D
L . ______._._ X ovember to 11 pIiL. ___ • ______•• ____ _ 0600 to 1300 o 0 E 0 2_. ______.. _. 010ve mbcr to ApriL ______1300 to 1800 D o 0 o 3 ______.______:-,rovembcr to ApIiL_. ______.______ISOO to 2400 .J 0 L ______.. ___ ._. ______May to Octooo"-_. ______0600 to 1300 0 0 0 5 ______i'v[ay to Oetooo'-_. ______. ______1300 to lS00 6 ______.______May to Octoool'______1800 to 2400 7 ______May to OclOoo'-______. ______0000 to 0600· 19D 8 ______.______November to 11 priL ____ • ______• _____ 0000 to 0600
Cumulative distributions of the hourly basic trans mission loss medians are calculated for each indi 200 vidual time block, or for combinations of time blocks and plotted on log-normal graph paper. 100 Me Is YAGI AT 13.7 m Thus it is possible to obtain an estimate of the range 160 08 0 of observed hourly medians which is ~he long-t ~rm o 0 0 0 0 0 0 0 0 Y'""~"""!!~~~i-l!~-..!02... 0 fading range. This will be compared WIth theoretlCal o 0 0 0 0 0 0 000 estimates or with similar data obtained from other
62 hr paths. ' 170 The cumulative distributions of all measured 0600 1200 1600 2400 hourly median transmi~sion los.s values are s ho,,~n
HOUR OF DAY on fiaure 6 separately for all tlme blocks, and for both °751 11c/s antennas, with the corner reflector FIGU RE 4. Diurnal variations of hourly median transmission loss levels for August 8- 12, 1960. at the 16.5 m elevation. All curves are bunched 439 WINTER SUMMER 180 distance as tbe obstacle gam path. The actually -" ~ measured paths were selected to correspond closely ~ ~ '3 to these assumptions in path geometry and fre z 0 quency. Table 4, below, shows the path constants ~ used: ~z eo'" TABLE 4. Path constants for compamtive sludy u , ~ I j P a th Angular 220 I 0.1 5 10 dista nce distance PERCENT OF HOURS FrG L-UE 6. Distrib1itions of hourly median basic tran~mission kTil J filliradians Loss values fOT CoLorado obstacle gain palh on 751 klcls. IIypothetical path A ______223 24.0 H ypo thetical path B ______566 0,4.4 and nearly parallel, indicating very little diurnal or Actual path.: Pikes Peak obstacle gain (i51 M e/s) ______223 64.4 seasonal variation. Table 3, below, shows a com Cbeyemle M t. to Garden City (1046 M c/s) ______364 26.7 parison of the overall median values and the long Cbeyemle Mt. to Anthony (1046 Mc/s) ______633 59.8 term fadiag range. Here the overall median is the median of all hourly medians within a time block, and the long-term fading range is defined as t he Overall time block medians and the long-term ·difference between hourly median values exceeded fading ranges for the hypothetical paths were deter 1 percent and 99 percent of all hours. mined from a consideration of the expected long Even though the cumulative distributions of hourly term variation of hourly medians. These distri medians plotted by time blocks on figure 6 appear butions are obtained from the empirical V(p, 0) quite similar, table 3 indicates some seasonal effect curves, which were determined from a large amount in t he long-term fading range. The range is larger of experimental data representing a number of differ during summer, and is also larger for the corner re ent geographical and climatic regions within the iiector at the higher elevation than for the dish. United States [Rice, Longley, and Norton, 1959] . There is very little variation in the overall median The time block medians and long-term fading ranges values between time blocks, and the difference be given there are measured relative to a calculated tween tbe dish and the corner reflector appears to he reference value of basic transmission loss, which is -slightly higher in summer. the median of all hourly median values III time block 2 (winter afternoon). The curves are assumed to be valtd for all frequencies ill the 70 to 4000 Mc/s TAB u ;; 3. Comparison of winter and summer res1lits range. Overall median Lon g-term fading A comparison of the long-term fading range decibpls ra nge, decibels determined in this manner for the hypothetical paths Difference (1% to 99%) Time block :-io. of overall with the ones measured on the Pikes Peak obstacle medians gain path IS shown as table 5 for all time blocks ;{ m disll CDrner db 3 III dish Corncr refl ector refl ec tor separately, for all hours during summer and winter, I and for all hours of the year. Winler: 8 (0000 to 06(0). 198.4 19:.\. 2 5.2 8. 9 14_8 1 (0600 to 1300)_ 197.6 192. 6 5.0 8.9 10.5 TABLE 5. Comparison of hypothetical tropospheric propagation 2 (I 300 to 1800)_ ]g8.3 192.8 5.5 12.4 14 .0 paths data with meas1lred data f)'om Pikes P eak obstacle 3 (l800 to 2400)_ 197.9 192.7 5.2 11. 5 11. 6 gain path Summer: ======~======7 (0000 to 0600), 198. 1 192. 7 5.4 13 .•J 16. 2 4 (0600 to 1300)_ 197. 6 191. 4 6.2 13. S 14. 4 Long term fading range, db (1% to 99%) .J (1300 to IS00). 197.3 lS9.9 7.4 11. 4 10. 7 6 (1 800 to 2400)_ 19S. 5 192. 3 6.2 13. 0 12. 9 Time block H ypotlletical H ypothetical Pikes P ea k path A, 223 path E, 566 obstacle gain km 8=?4 km 8=64 4 pa th, 223 km, milli~·adi;n s rnill'iradia~ls 0=64.4 millira 2 .3 . Comparative Studies of Median Distributlor s dians (3m dish)
Although tropospheric scatter is not primarily 8 (winter) ______42. 1 21.1 8.9 involved in these experiments, it is of interest to com L ______37.6 13. 9 S.9 2 ______12. 4 3 ______32. 9 14.0 pare first the measured distribu tions with theoreti 39. 2 16. 3 11. 5 cally expected values and with data from tropospheric 7 (summerl. ______42. 2 21. 4 13.5 4- ______scatter paths baving similar path parameters. For 5 ______37. 7 2S. 1 13. S 6 ______25. 1 18.2 11. 4 this purpose, two hypothetical paths and tbe meas 36.5 lU.O 13. 0 ured results obtained from two actual tropospheric All hours, wintf'r ______38. 8 19. 3 11. 7 All hours, SummrL ______41.9 26. 0 13.4 scatter paths were used. The two hypothetical All yea'- ______paths were considered to have tbe same antenna 39.7 26.4 14. 3 height as the actual path. One was assumed to bave an angular distance which a tropospheric scatter path It is seen that the actually observed long-term of the same length would have over a smooth earth, fading ranges were considerably smaller than the ones and the other would have a path length correspond predicted on the basis of scatter propagation, al ing to a smooth earth path having the same angulaI though the fading predicted for a scatter path 440 having Lhe same angular distance is a somewhat are random variables expressed in decibels. The re better e timate. sulting cumulative distribution of hourly medians for In order t o provide a more valid basis for esti- the entire path may be determined by the convolu mat ing long-term fading ranges and the distribution tion of t he dist ributions of 111 and 112 [Davenport I of hourly medians for knife-edge diffraction paths, and Root, 1958]. use is made of a method suggested by K. A. Norton. The cumulative distribu tions of hourly m edians The results of this method as applied t o t he Pikes for the two individual line-of-sight paths m ay be Pea k obstacle gain pat h will be compared wit h t he calculated by empirical formulas as fun ctions of the actual measurem ents in what follows. path distan ce, the eff ective antenna heights, the For the purpose of studying variations in hourly carrier frequency, the horizon elevation angles, and media n field str ength or transmission loss values a season, summer or winter . The m ethods used are knife-edge diffraction path is considered to consist unpublish ed empirical m ethods communicated to of t wo line-of-sight paths in tandem. The diffract the authors by P . L . Rice . This material will be ing knife-edge constitutes a common terminal for described in detail in forthcoming N BS T echnical the t wo paths. If tbe hourly median signal for N otes and in a U .S. Air Force T echnical Order each of tile t wo line-of-sigh t paths is subject t o dealing with design procedures for tropospheri c r andom variations, VI and V2, t hen t he random communication circuits. R esults are shown on variation V expected on t he mountain obstacle figure 7, where the empirical distributions nr o I path would be 11= VI + V 2, where V, VI, and 112, compared with the actually measured distribut ions . Comparison is made for the data on 751 M e/s. 160 I I I I I obtained u.sing the 3-m dish seDtLrately for all hour 751 Me / s, WINTER during the summer and during the winte r m on ths ,. ltll d for the 100 NI c/s data for the summer m onths . Calcul ation or the emnirieal d istr ibution s of 190 1- ... hourly m ed ians sJl own on figur e 7 in clu es detcrJlli -t-~- ' ...... nHtiOIl of the ovemll medi lL n reference vltlu c by r- ~ ... 275 hr / r-- ...... F resnel diffraction lw d rnx on tics methods wh ie]l ._- r-- _ will be discll ssed in more de tail in sect ion 3 bclolV . 200 " - - r-- --- VVh 40~~.~r---~--_+------~~.,~~--4_ ""km. " ., m,] "",m1046 Me ." -"1646"" h r ott ?"i Is - 18 0 \ JUNE - OC T. 1952 t--__'-+- . ·-',,;:-+-l~~ypOT H ET ICAL PATH '\~, \ ~ . .0 35 1/' 223km ,24mr \ / \ 1 I I ", PIKES PEAK OBSTACL E GA IN PATH 30 FOR \. / 190 km ,64.4 mr ) - i ----(~ TED .0 "0 k223 ~ i"- 751 Me/s- 613 hr en , MAY- SE PT. 1960 en / o " _l,"~''' ''''n," / ...J ~ I--- r---... I: 2200 Q , en , r- I \ en I-- \ ::;: " - ~ \ en ~ " , ~ 15 \~--_r~~~------/L---~---+--~ '----~ :i 210 tx: ' I'. f- ",..... ~ '" 10 J-----h"L--+--~~ MEASURED VALUES en - ---.... PREDICTED FOR PIKES PEAK Figure 10 shows a comparison of all measured This tabulation shows a substantial frequency .(lata on 100 Mc/s and 751 Mc/s during the t wo dependence of the fading range for the obstacle gain weekly periods when the receiver site on Pikes Peak path- more than doubled when comparing 100 Mc/s was operated. These are overall summer-time dis with either one of t he 751 M c/s antennas_ There is t nbutions; they have .not been separated into time not nearly that much frequency dependence on the bl o~ks because of t he comparatively few hours line-of -sight path. It li ftS ttl ready been shown that ~vaJ l a bl e . As expected, t lte long-term fadin g ra nge the V(p, 6) d istributions are not appropriate for use IS smaller for the line-oI-sight path to the top of the in predi cting t he performance of kn iIe-edge diffrac tion paths, fwd that m uch better agreement is obtained by t he usc of a different empirical method 120 I 1 1 I I I Ltl king inLo acco unt carri er frequency and horizo n I-- -~ -k_ BEULAH - PIKES PEAK , 100 Me / s elevation a ngles. -r--t--.. ------~ - f-- 13 0 3_ Height Gain Studies f-- - - t-- -- '7--i-- Previous measurements made of obstacle diffrac BEUL AH - PIKE S PEAK, 7 5 1 M e/s ./ tion over P ikes Peak [Kirby, D ougherty, amI 14 0 YIcQu aLe, 1955] showed marked agreement between t he predicted and observed vftriation of transmission loss with height . 'fh ese measurements were made using n, shorter path fw d with a relatively smooth 15 0 terrain arca in t he immediate foreground of the .0 a ntenna whose height was varied. -u 170 r-----r----,-----,-----,---,--,--- 170 --- R = - 0.9 {CALCULATED HEIGHT - GAIN CURVES I C~LCULATE6 HEIGHT ~ GAIN CUR ~ E ----- R = -0.5 FOR IDEAL KNIFE EDGE FOR IDEAL KNIFE ED~E I ,...... ----.V ----. .r.. ~~~~~_r~~•._ ~~T_~+;L~'.--T~·~-- ~ " . ' / ! \ ' " ! .I \ /' . / \ .I r,\ 1/,-, 1\ / j j -.l II -\1\ / I ~ \\ L/-", I .D 180 I---H--/-I--t---tj------n~-t--___t_-';-__t_-- 180 \\ ,/~ h it- I~'- \\ '/ I D \{ \ \! \ i,t' \~ ~I 11 f (f) \ i \ 'j ~\" (f) V> ~ \ il jJ V>g 9 z z Q Q \! \ \i 'i V> '!? -il ...... ~ 190~-~-J~---t --~~~-___t_- -- ~~~ -1~ T_-__1 190 Ii ~ H :. V> z - ~ ", /X ,"-\ /~rf--" a:" > ::'i i'.\ / 1\ li u cr: iii " f- 1 I / \' '" ',2(f) \fI'I \I' I" I I I TIME BTOCK 1 I FIGU RE 11. Comparison oj calculated and measured height-gain FIG UHE 12. Comparison 0/ calculated and measured height-gain curves Jor Colorado obstacle ga'in path. wrves jor Colorado obstacle gain path. 444 /. FIGURE 13. 1'mnsverse profile through Pikes P eak as seen f rom tmnsmitter site. measured beight gain eurve. For the Lwo different illuminated by Lhe l:1. ntenna bCH.ms. IL Ims so far heights u ed for the high er antenll il. (co m er refl ector no t been poss ible Lo ma ke a quantitativc study of at 16 .5 and 22.3 m) there is little difrerence in the t hese co ntributions in order to usc t hem for fi eld values of transmission loss observed. This tends to computations. The eff ect of a t ransverse triangular confirm t he similarity of long-term transmission loss mountain profile was studied by Matsuo in [1950]. values observed at the two heights as shown in Numerical results for a specifi c example were given fi gure 5. but no compiLriso n was made with iLctu~tl measured It is quite clear from the above discussion that res ults. the basic transmission loss dependence on iLntenna height cannot be represented in this case by the 4. Vertical Space Diversity and Fading simple four-ray knife-edge model, even if the knife Characteristics edge is assumed to be rounded. The reason becomes evident if the triLnsverse profile through the obstacle A detailed study of the short-term fading char is studied, which is a terrain profile drawn through iLcteristics observed on t his path is contained in the obstacle iLt righ t iLngles to the propagation path. another paper [Bal's is and Johnson, 1961 ]. A num Figure 13 is a photograph of the visuiLl horizon as b er of possible mechanisms appeiLr responsible for seen from the transmitting iL ntenna with various the fading. These include those associated with visible ridge lines emphasized. A grid is super prolonged space-wave fadeouts, which resemble imposed which shows the angles sub tended from phenomena observed on within-the-horizon paths, the transmitting site. It is seen that the trans as well as those associated with scattering com mitting antenna beam , with a 7.7 ° half-power beam ponents. Other mechanisms appeal' to be due to width sees a substantial portion of the ridge line the terrain configuration iLnd may be the cause of on either side of the principal obstacle. Similar an intermediate type of fading, characterized by conditions exist if the transverse profile is viewed fadeouts on the order of 5 db which last less than a from the receiving terminal. Thus fi eld contribu few minutes. tions may exist from the ridges and other terrain In view of the apparent positive correlation features between the terminals and Pikes Peak between the 751 Mc/s signals recorded simulta- 445 neously on the two re.ceiving antenna~, studies ~ere JUNE 15, 1960; 1200-1210 performed to determme the correlatlOn coeffic~ent P =0.03 between instantaneous envelope values for variOUS 191.5 TABLE 9. R esults of envelope c01"Telation studies fOT P ikes SEPT 16, 1960; 1015 -1025 P eak obstacle gain path 207 P =0.78 206 Length Antenna Correlation 205 Date and time of sample (1960) 448