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Reception of Skywave Signals Near a Coastline

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Reception of Skywave Signals Near a Coastline JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 67D, No.3, May- June 1963 Reception of Skywave Signals Near a Coastline J. Bach Andersen Contribution from Laboratory of Electromagnetic Theory, The Technical University of Denmark, Copenhagen, Denmark (Recei\'ed November 5, 1962) An experimental investigation has been made on t he influence of gt'ou nd inhomoge­ neities on the reception of skywave sig nals, especiall y t he in fluence of the conductivity contrast ncar a coastline. This gives rise to a r apid decrease in fi eld strength ncar til(' coastline as is "'ell known from g roundwave mixed path theory. Comparison with Lheory is given. Infl uC'llce of dilhts(' reflection from t he ionosplwre is also cOIl :; idereri . 1. Introduction conditions, i. e., a s Lraight boundary lin e, n,lt <l IH1 homogeneo us sectiollS , it was co nsidered worthwhile ;,lany factors influence the radiation 01' reception to measure tbe field in tensity at a n actual reception of skywave signals in the shortw,we ra nge, a nd in site in order to find Lhe deviation s from the simple choosing a proper site for a HF-a ntenna it is impor­ theory. tant to evaluate the influence of the various factors. lnstead of m eftsuring t he verticlll radiation pat­ They include th e electrical parameters (the conduc­ tern aL d iIrerent distances from Lhe coastlin e, the tivity and the p ermi ttiv ity) of the groulld, hills, or fi eld intensity from a distf\nL Lran smitter was m eas­ clift's a nd other irregularities of the sbape of the ured simultaneo usly at two d ifr erent places. 11\ Lhi s ground surface, and th e curvature of t he earth. It wa,y the relaLive fi eld strength co uld be measured as is n ecessary to take into account t hat tbe electrical a fun ction of the di stanee fr olll Lhe coastline, a nd also ~ parameters m ay vary along the surface of t he the eft· ect of t he ionosphere could be f\ccoun ted for. ground- thi s is, 1'01' exa,rnple, the case at f1 coast­ To makc t he study IlS complete as possible th e lin e- a nd also that they may vary witli Lhe depL h. corresponding curve fo r the groundwave mixed pllth This paper deals with the influence of the ground was also m easured , lh e transmiL Ler b ein g siLuated on t he reception or skywave signals at a n actual flt ground level far from Lh e COflstlin e over the sea. site ror a receiver stn,tioll close to the seashore at Finall~T the aLtenuaLion of t.h e groundwave on Lhe R eersf), D enmark. homogeneo us ground was measured in order Lo find Part of the a ntenna station which will be erected t he degree of in homogeneity of the ground a nd to at R eers0 will receive skywave signals from Godthab estim ate . the value or t he ground co nstants. in Greenland. In the d ay tim e t he ionospheric re­ The eff ect of diffuse reflection from t he iO ll osphere fl ections will take pl:tee in t he E -Iayer a nd during and t he effect of t he Clll'vature of the ear th is also ~ t he ni ght in th e F-laycr, a two-hop path via th e mentioned. E-layel' and a one-hop p ath via the F-Iayer; the geometry is such that t h e angle of arrival in both 2 . Theory for a Two Section G round cases is very small, about 4°. It is well known that it is difficult to m ake an tennas which operate well In a previous paper [Andersen , 1962] a solution at such small angles du e to the absorption in the was given for the radiation field from a ver tical dipoJe ground. situated on an inhomogeneo us ground co nsistin g o f One method of obtaining an improvement in the two or three sections with arbitrary electrical pl'Op­ recep tion at low angles is to use an extended ground­ elties.! Numerical calculations were given in the wire system in the direction of propagation (or case where one or two sections wero perfectly CO Il ­ reception) [A. C. Wilson, 1961]. Another method ducting. The result for a t wo section ground is would b e to choose the a n tenna site n ear a coast- given here for reference where a t ime factor I lin e in order to utilize the high conductivity of sea exp (-iwt) is employed. A(f , 1'0) is the radiation r wa ter wherever it is possible. However, the antenna pattern for a ver tical dipole placed on section 1 at would still be situa ted on a lossy ground a nd it a distance To from seetion 2, and f is Lhe angle of r would b e of interest to know the influence of the elevation . various sections on the radiation pattern. A theo­ r etical solution to this problem has previously been I A more ge nom! so lution has heen given hy W ait [1961 :for th e case ofa spher· given [A nd ersen , 1962] but due to the idealized ical earth. J lowev('r, it is no t in as con venient fo rm as (1) here. 325 (1) (nth medium) 2(l -cosy.,) to c iui (1-ui) Po to= kl'o(l-cos y., ) (lc= wave number) (numerical distance to boundary) (Fresnel in tegral) (So mm erfeld's attenufLtion function) R. is the Fresnel reflection coeffi.cient for vertical The horizon tal profiles of the paths are shown on polariza tion. fi gur es 4 and 2 respectively for the skywave and the vVhen medium 2 is perfectly conducting, '/)2 = 0 groundwave measurement. UnJortunately, the two and R v(2) = 1 and paths could no t b e made the same. The paths are nearly perpendicular to the coastline. For both the skywave path and the groundwave path there is a -<J section of marshy ground extending from 20 to 50 l1l from the coastline; this section has a very high con­ ductivity (<T~1 0 - 1 mho/m). From 90 to 150 m from the coastline there is a field with caraway seed, representing a very low con­ In this particular case (U2 = 0) the solution is valid ductivity. For the groundwave mixed path there is a also for the groundwave mixed path where it reduces small hill about 100 l1l [rom the coastlin e. to It. is seen that a full theoretical description should take into account the three or [our difl' erent sections of ground including the variations o[ the profile A = -=~ ( I - G(po)). (3) and the different layers in the vertical direction of -Y7fpo which nothing is known. Only the two-section problem, the sea-land case, is treated here numeri- I This result is identical to that obtained by other cally and the sea is considered infinitely conductin g. authors [Po C. Clemmow, 1953; E. L . Feinberg, 1959]. In order to investigate in more detail the proper­ From the numerical calculations [Alldersen, 19 62] ties of the ground, two groundwave measuremen ts it is seen that for small angles of y., the r ate of decay were made. The results arc given in fi gure 1. The of the field away [rom the boundary is nearly the independent variable is the distance from the trans­ same for the groundwave (3) and the skywave (2). mitter which is placed 42 0 m from the coastline This relationship is true approximately up to the and the ordinate is the field strength in a db scale. end o[ the first Fresnel zone. One of the reasons for The most marked features of the curves are the undertaking this study was to verify this connection great, attenuation of the caraway fi eld and the between the skywave and the groundwave for an recovery effect following it; this recovery eff ect is actual inegular ground. reinforced by t he m arshy ground close to the coast. The 15 M c/s curve is smoother than the 25 M c/s " 3 . Investigated Terrain curve and a small r ecovery effect about 60 m from the transmi tter seems to indicate a better soil. The terrain under investigation is situated at T his is in agreement with an inspection of the R eer·so on the west coast of Zealand, Denmark. ground. 326 I I I I II MA RSH Y GROU NO CA RAWAY SEED \ '-...11 1111111 LAN D ISEA --<D 0 0 I- 0 f- r----- -1 ~ 0 -10 ~ - 4 1\ f- ~ 0 I\f 0 25.21 8 MelS ~\ 0 -20 -6 f- db ~ -8 ~ -3 0 ~ I- 0 0 ""'- 0 f- -10 - 4 0 ~ I- "'" 0 ·1 1 ~ f- , 0 -50 ~ -14 --u I I I I I I 1 db f- Va~ o 10 100 100 300 40 0 -60 ro , meter - FIG UH E '2 .- GI· ollndwave mixed path, sea-land. '( - 7 0 0 , l\!.ca surcd fi eld intensi t y - , Th eoretical cun'c . E= lO ,u= 2.10- 3mho/m f- ~ ~ f = 25.218 _\ I Cis -80 r---.r-. r- "- f 0 15.663 MelS - 90 i't" high conductivity soil is entered. The small hill I- seems also to have some eff ect on the received fi eld ~ ~.n -1 00 strength but the unknown ver ticftl distribution of I- different layers in the ground nny be tbe true cause for the r apid variation.
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