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In Search Ted Shapas. K9 YOb' 14925 Evers Avenue Dolton. Illinois 604J 9 o a Better Angle Observations and Suggestions Involving IIF Radiation A ngle Manipulation When HF DX-getting tactics are discussed, sooner or later the subject will center around antennas, for the antenna, coupled with its geographic positioning. will ultimately make or break a DXer. A status is eventually achieved, however, when the serious DXer bas put up the largest and tallest antenna he cared or dared to, or a point of "signal strength stagnation" is reached. For most of us this is a frustrating level. for it probably still means an S-unit or three gap between us and the big guns. Even if you feel you're near the top, however, there were undoubtedly times when these extra db would have come in handy. This article is no magic panacea for combatting a W3CRA or WSVA in t he pileups, but knowing a little about radiation angle manipulation may give you something to think about along those lines. Low angle advantages I think many of us have operated enough to realize the importance placed upon an antenna with a very long boom perched upo n a very high tower. The fellows with the biggest combinations of these two, possibly coupled with an elegant location, constitute the big guns, or the big DXers and contest Winners. A maj or reason for their seeming invulnerability is the very low vertical rad iation angles associated with such an antenna combination. For normal F2 propagation paths, t he best sit uatio n is for your signal to reach a distant point in the least number of "hops" o r reflect ions. Up to about 2500 miles or so , o ne hop propagation is po ssible, but after that, an earth reflection is neces- 78 sary. Now the signal is being decreased by a number of losses; most pertinent here are distance losses due to spacial. spreading, and ground losses at each earthly hop. Distance loss is of course a function of distance; a lower reflection angle means less distance travelled and therefore less loss than a higher angle, although the difference may only amount to a db or two. Much more important are the ground lo sses. While sea reflections are less critical of incident angle, ground reflections may result in four or five db differences per hop between low and high angles, depending upon frequency (I). Here is one place where the big guns clean up! One or two less of these lossy earth hops experienced by signals approaching from a high angle and we're ta lking about S-units of difference. Fortunately, all is not this rosy for the people with the low take-off angl es, NBS observations - good news? The good news is that in most cases. these things I just talked about occu r only for "storybook" propagation unde r ideal conditions. What about the real world? W. F. Utlaut presented an interesting report along those lines in our National Bureau of Standards research journal Radio Propagation where he made a detailed study o f radiation angle importance. The results were slightly astounding (2). Using a VOA transmitter in Munich, Germany , and receiving antennas in Boulder, Colorado (a receiver was also located in Slough, England , but results were consistent with those in Boulder), all with carefully calculated radiation patterns, Utlaut attempter' to find out if low angles were that 73 MAGA ZINE
Boulder Antenna 18 28 38 48 58 Table I Height ft 50 135 3 10 485 985 Radiation Angle deg 15.6 5.2 2.3 1.4 0.7 good. From March to June, 1959, on 20 MHz, the Munich station transmitted while the receivers in Boulder carefully recorded daily signal variations. Five receiving antennas were used, varying in vertical radiation angle from .7 to 15.6 degrees. Table I lists the various antennas, heights, and associated radiation angles, while Figs. I A, 1B, I C, a nd 1D are graphs of hourly median signal strength for the Munich to Boulder path from March to June. Transmitting antennas at the Munich end were rhombics and vertical main lobes between 12 and 16 degrees (two antennas used). While receiving data extended through times when the predicted MUF (Maximum Usable Frequencies) were below the operating frequencies, this may be slightly impractical from the amateur sta ndpoin t where relatively low power is usually employed. Close inspection of Figs. I A·D will bring out a number of interesting points: 1. No one radiation angle dominated for an entire average day during the time the band was open. (MUF greater than 20 MHz.) 2. Low angles seemed more of an asset during a summer month (June) than during a spring month ( March ). In additio n, the spread of signal strength between one angle and another was greater during the summer months lhan during the spring. 3. Lower angles were characteristically "band openers" with dominance here noted in excess of three S-units over the highest angle. 4. Higher angles seemed valuable during the midday hours, with so me advantages over the lowest angle in excess of two S·units at these times. (Remember, these were monthly averages; there were undoubtedly times when these differences were greater.) The basic observation by Utlaul in this report was that stat istically, and over an average 24-hour period, best results could be gained by using the lowest radiation angle possible. This invo lved times when the predicted MUF was muc h below 20 MH z; which ., .~ •• 00 . " : .'0 • a " 1 S .1lO · ,. • 4' " i _ • •• .,. ~ , m ., · ., •.. Fig. l A . Munich .. .~ - •• .'0 " .., ,. • ~ -If, "· '" i ·.00 ..., .•. 00 .0> .ec ..~ ~ .~ ., • ."~ ~ • • '" ." -• • 'M " .us •-• •• '" "• - • ~ , ., 00 ., ,.. .' _ __ . I • . . t. _... .._ ) . .. ~.. ~._..-.. " .. .. _ ._.- - t. " ............-.. _..._...-,. " II" " 1$ . rt If" ZO l'lltll lO Hourly median signal strength for to Boulder path, March, 1959. - - -_. ,. - _ . _ .- 1 e - .._ - . I • .......... _... " , : .' . ~m ~ ~ ~~ ~~ O I I ~ 'I "I$ . " ' ''ZO ~ lll 11 ' Fig. lB. Hourly median signal strength for M u nich t o B oulder p at h, A p r il 1959 . Fig. 10. H ourly median signal strength for M u n ic h t o Bou lder p ath , June, 1959 . 'reansm itter in all cases w as o ff t h e a ir from 0600 to 0800 M S T . ( 1) MAY 1969 79 ,
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