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and direction usually undergo change. Speed may either increase or decrease; the change of direction usually depends upon what happens to the speed. If the refracting medium has characteristics which change gradually wit hin the material, the speed and direction of the refracted wave will also change gradually as the wave proceeds in the material. The ionized layer is such a medium; its makeup changes - both from minute to minute (and other periodic changes) and at various points within the layer at the same time. Thus a wave transmitted from the earth will be bent or refracted only sligh tly as it enters the ionized layer, but the deeper it penetrates into the layer the more its direction is changed. When the original direction has been changed enough to turn it around a corner, the wave is moving out of the material rather than in, and then the change in direction becomes less the farther it travels. Eventually the wave will co me back out of the layer, provided that the refraction doesn't just happen to trap it completely within the layer and bend it only enough to keep it trapped. Even if this should happen at some spot, there arc enough irregularities in the layers that the energy would escape elswhere - and such an action may be at least partially responsible for some types of fading. As Fig. 2 shows, when the wave emerges from the layer there is no way at all you can _.- --- ---_.~---- "" / , / -, / .> , -- -- , " . .. , / " ,,~- --- --------- ~-- Fig. 2 - Refraction of radio wave in ionosphere IS cause of apparent "reflection" of skip signals as shown here. Since ionization level changes gradually within an ionized layer. angle of refraction is continually changing. This bends wave back in new d irection, making it appear to have been reflected from a su rface at somewhat greater height (dashed line). Wave reaching layer at shallow angle (dotted) does not penetrate so deeply as one hitting at sharp angle (solid); therefore it is bent less and so retu rns to earth at greater range than difference of angles alone would indicate. determine that it wasn't simply reflected from a sharp surface at a somewhat greater height. This fictional reflecting surface's height is what is referred to as the "virtual height" of the skip layers. The reason we know it works by refraction rather than reflection is that the virtual height of a layer appears to change with the angle at which energy hits it. The sha llower the angle, the lower the virtual height. You can see from the dotted-line example in fig. 2 that this would be expected wit h refraction, but not wit h reflection. This mechanism in the ionisphere indicates that the angle at which the signal will be "reflected" depends critically upo n the angle at which the signal arrives, and also upon the condition of the ionized layer at that particular time. High-frequency signals packing more punch per photon, bore right on through much more readily than do those of lower frequency - so that as you keep going u p in frequency. you find a point at which the signal simply doesn't come back down. Instead, it bores on out headed toward outer space. The angle at which the signal hits the layer depends, in turn, upon the act ual angle at which the wave leaves the transmitting antenna. This depends upon the antenna design, its height above electrical ground, and the nature of the ground surface within several wavelengths of the antenna site. The lower the angle at which the signal leaves, the more shallow will be the angle at which it hits the refracting layer, and the greater will be the distance covered before it returns to earth. Any substance which is capable of refracting the wave can cause "reflection" by refraction in this same manner. In addition to the horizontal ionized layers which make up the ionisphere, rf signals are frequently "reflected" from the aurora borealis and from the trails of ionization left behind by meteors. At VHF, similar effects are caused at the boundary between different layers of air in the atmosphere. Ho w Does Reflection Affect the Signal? True reflection has virtually no effect upon the signal, except that its phase changes 180 0 during the process of reflection. " Reflection" by means of the refraction effect, though, can affect a signal in many ways. Reflection of VHF signals from the shimmering veils of ionizati on which are known to science as the aurora and to the general public as " the Northern lights" offers several examples of such effects. The aurora is a rapidly moving affair. Its 106 73 MAGAZINE
exact cause and corn posuron is still not accu ra tely kno wn, but it is believed to be especially intense ionization of the upper atmosphere under influence of solar redialion trapped by the earth's magn etic fie ld. It is visible as curtains, columns, and so metimes horizonta l sheets, and moves in both the horizontal and vertical planes at relatively high speeds. Ofte n, the au rora appears to shim mer wit h a to-and-fro motion. Any radiation reflected (that is, refracted back toward the source) from these clouds has a frequency shift imposed upon it by the motion of the clouds, by Doppler effect. This frequen cy shift is, e ffectively, I' M of th e original signal, in which the modulating signal is the oscillating movement o f the aurora itsel f. The frequency of the aurora's oscilla tion is ofte n so great that the resulting FM completely wipes out any intelligibility of audio upon the signal, and makes a CW signal appear to occupy a wide band rather than the normal ncar-zero ba ndwidth. If aurora- re flected signals are received by the normal CW method, using a product detector or BFO, the FM causes the received signal to appear to warble. But since the reflection is coming to the receiver fro m a wide source - the entire aurora cloud - and part of it is moving toward the receiver while other parts are moving away, it isn't just a ll,BOV( CR,TlCll,L A NGL E CO.. . ...... 31 Blitm.... t..... Ash..,IIe. ,,1[ . 2111" 2 Element QUAD FOR 10 - 15 - 20 T!'. J" i ,A OI) ·\D· ' ., 'atr'l ." I "' 1'1 .\ \ ' ,,"'~ . ...., ,. " , a ~, , ""l. u:> to 100 ... " .. ' I . " g ~ ' ~~ ,~ k ' ".,p. 0' '.... ~ 6" boo'" '"
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WORLD'S LARGEST INDEPENDENT HAM MAG
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MAGAZINE 73 Magazine 11'104 , May 1
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y t hose whom I thought were of hig
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WHAT IS THE BEST ". CO'. TO SET ,,'
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I ,~ - Gel more receiver than you p
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the boom, shorting all loops so tha
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• ,~, • • • ••• 1>00"
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transmission line or the rf voltage
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sumed. A signal generator of suffic
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cuitry developed through the earlie
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The Super SS Clifford Klinert 520 D
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