Practical_Antenna_Handbook_0071639586
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Practical_Antenna_Handbook_0071639586

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C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 57 In general, all three layers are affected, but the F2 layer is only minimally involved. Ion density in the F2 layer tends to be highest in winter and less in summer. During the summer, the distinction between F1 and F2 layers is less obvious. 11-Year and 22-Year Cycles The number of sunspots, statistically averaged or smoothed, varies on an approximately 11-year cycle. As a result, the ionospheric characteristics that affect radio propagation also vary on an 11-year cycle. Skip propagation on the HF and lower VHF bands is most prevalent when the smoothed sunspot number is at its highest. During the peak years (1957–61) of the strongest sunspot cycle of the radio era to date, true F-layer skip propagation was frequently enjoyed on 6 m (50–54 MHz) and higher, affecting even some VHF TV channels and possibly even the low end of the FM broadcast band! By comparison, during the latest minimum (2007–10), many populated areas of the midlatitudes went weeks or months with no evidence of skip activity above 20 MHz. Sunspots are regions of high magnetization, and their polarity on either side of the sun’s equator switches with every sunspot cycle. Thus, the true cycle of the sun’s activity is a 22-year cycle, but the effect of sunspots on ionospheric propagation is unrelated to the magnetic polarity of the spots, so, for all practical purposes, we experience only the 11-year cycle. Ionospheric Disturbances Disturbances in the ionosphere can have a profound effect on radio communications— and most of them (but not all) are bad. This section briefly examines some of the more commonly encountered types. Sporadic E Layer A reflective cloud of ionization sometimes appears in the E layer of the ionosphere; this layer is sometimes called the sporadic E, or E S , layer. Theories abound regarding the origins of E S, and at this writing many experts believe the term E S is a catchall for multiple unusual causes of E-layer ionization. In addition to the solar particle bombardment theory mentioned earlier, some believe E S episodes originate in the effects of wind shear between masses of air moving in opposite directions. This action appears to redistribute ions into a thin layer that is radio-reflective. Sporadic E propagation is normally thought of as a VHF phenomenon, with most activity between 30 and 100 MHz, and decreasing activity up to about 200 MHz. However, at times sporadic E propagation is observed on frequencies as low as 10 or 15 MHz. Reception over paths of 1400 to 2600 mi is possible in the 50-MHz region when sporadic E is present. In the northern hemisphere, the months of June and July are the most prevalent sporadic E months. When sporadic E is present, it typically lasts only a few hours. Hint: Amateur radio experimenters who specialize in E S activity carefully track the progress of weather fronts in their part of the world. Sudden Ionospheric Disturbances (SIDs) The SID, or Dellinger fade, mechanism occurs suddenly and rarely gives any warning. The SID can last from a few minutes to many hours. SIDs often occur in conjunction with solar flares, or bright solar eruptions, that emit an immense amount of ultraviolet radiation over a short period. When this UV radiation reaches the earth, the SID causes a tremendous increase in D-layer ionization, and abnormally high levels of absorption

58 p a r t I I : F u n d a m e n t a l s result. The ionization is so intense that all MF/HF receivers on the earth’s sunlit hemisphere experience profound loss of signal strength above about 1 MHz. Received signal strengths can drop by 60 to 90 dB in a few minutes at the onset of the SID, and it is not uncommon for amateurs, shortwave listeners, and others monitoring the HF bands to think the cause is a malfunction in their receivers or antennas! Many times during the record-setting sunspot maximum of 1957–61 the author was fooled into thinking his normally highly sensitive Hallicrafters receiver had “bit the dust”. SIDs are often accompanied by variations in terrestrial electrical currents and magnetism levels. Thus, there is good correlation between the occurrence of an SID and visual or radio aurora borealis (or aurora australis in the southern hemisphere) for a few hours or days thereafter. Because solar flare activity is far higher during periods of high sunspot activity, SIDs are most commonly observed during the peak years of the strongest sunspot cycles. Ionospheric Storms The ionospheric storm, which may last from several hours to a week or more, appears to be produced by an abnormally large rain of atomic particles in the upper atmosphere. These storms are often preceded by SIDs 18 to 24 hours earlier and by an unusually large collection of sunspots crossing the solar disk 48 to 72 hours earlier. The storms occur most frequently and with greatest severity in the higher latitudes, decreasing toward the equator. When such a storm commences, shortwave radio signals may begin to flutter rapidly and then drop out altogether. The upper ionosphere becomes chaotic; turbulence increases, and the normal stratification into layers, or zones, is disrupted. Radio propagation may come and go over the course of the storm, but it is mostly “dead”. The ionospheric storm, unlike the SID, which affects only the sunlit side of the earth, is global in its impact. Observers frequently note that both the MUF and f C , the critical frequency, tend to drop rapidly as the storm commences. An ionospheric disturbance observed on November 12, 1960, was preceded by about 30 minutes of extremely good, but abnormal, propagation. At 2000 hours Greenwich mean time (GMT), European stations were heard in the United States with S9+ signal strengths in the 7000- to 7100-kHz region of the spectrum—an extremely rare midafternoon occurrence when sunspot levels are high (as they were then). After about 30 minutes, the bottom dropped out and even AM broadcast band skip later that evening was nonexistent. At the time, the National Bureau of Standards radio station, WWV, was broadcasting a W2 propagation prediction (which is terrible; back then, “W” stood for “Warning!”) each hour. It was difficult to hear even the 5-MHz WWV frequency on the east coast of North America in the early hours of the disturbance, and it disappeared altogether for the next 48 hours. Of course, in retrospect it is obvious that the sequence observed on 40 m is explained by the MUF rapidly sliding through the band (the 30-minute period of abnormally good DX propagation would correspond to the MUF being in the 7.5- to 8-MHz range) and ultimately dropping to 1 MHz or below by evening, as evidenced by the total lack of broadcast band skip. It is interesting that this particular disturbance occurred on the weekend of the annual ARRL “Sweepstakes” contest, a competitive operating event that brings out thousands of amateurs in the United States and Canada. The ARRL’s summary of results in the May 1961 issue of QST led off with the editorial comment, “Universal lament: [propagation] conditions that first weekend!”

C h a p t e r 2 : r a d i o - W a v e P r o p a g a t i o n 57<br />

In general, all three layers are affected, but the F2 layer is only minimally involved. Ion<br />

density in the F2 layer tends to be highest in winter and less in summer. During the<br />

summer, the distinction between F1 and F2 layers is less obvious.<br />

11-Year and 22-Year Cycles<br />

The number of sunspots, statistically averaged or smoothed, varies on an approximately<br />

11-year cycle. As a result, the ionospheric characteristics that affect radio propagation<br />

also vary on an 11-year cycle. Skip propagation on the HF and lower VHF bands is most<br />

prevalent when the smoothed sunspot number is at its highest. During the peak years<br />

(1957–61) of the strongest sunspot cycle of the radio era to date, true F-layer skip propagation<br />

was frequently enjoyed on 6 m (50–54 MHz) and higher, affecting even some<br />

VHF TV channels and possibly even the low end of the FM broadcast band! By comparison,<br />

during the latest minimum (2007–10), many populated areas of the midlatitudes<br />

went weeks or months with no evidence of skip activity above 20 MHz.<br />

Sunspots are regions of high magnetization, and their polarity on either side of the<br />

sun’s equator switches with every sunspot cycle. Thus, the true cycle of the sun’s activity<br />

is a 22-year cycle, but the effect of sunspots on ionospheric propagation is unrelated<br />

to the magnetic polarity of the spots, so, for all practical purposes, we experience only<br />

the 11-year cycle.<br />

Ionospheric Disturbances<br />

Disturbances in the ionosphere can have a profound effect on radio communications—<br />

and most of them (but not all) are bad. This section briefly examines some of the more<br />

commonly encountered types.<br />

Sporadic E Layer<br />

A reflective cloud of ionization sometimes appears in the E layer of the ionosphere; this<br />

layer is sometimes called the sporadic E, or E S , layer. Theories abound regarding the origins<br />

of E S, and at this writing many experts believe the term E S is a catchall for multiple<br />

unusual causes of E-layer ionization. In addition to the solar particle bombardment<br />

theory mentioned earlier, some believe E S episodes originate in the effects of wind shear<br />

between masses of air moving in opposite directions. This action appears to redistribute<br />

ions into a thin layer that is radio-reflective.<br />

Sporadic E propagation is normally thought of as a VHF phenomenon, with most<br />

activity between 30 and 100 MHz, and decreasing activity up to about 200 MHz. However,<br />

at times sporadic E propagation is observed on frequencies as low as 10 or 15<br />

MHz. Reception over paths of 1400 to 2600 mi is possible in the 50-MHz region when<br />

sporadic E is present. In the northern hemisphere, the months of June and July are the<br />

most prevalent sporadic E months. When sporadic E is present, it typically lasts only a<br />

few hours. Hint: Amateur radio experimenters who specialize in E S activity carefully<br />

track the progress of weather fronts in their part of the world.<br />

Sudden Ionospheric Disturbances (SIDs)<br />

The SID, or Dellinger fade, mechanism occurs suddenly and rarely gives any warning.<br />

The SID can last from a few minutes to many hours. SIDs often occur in conjunction<br />

with solar flares, or bright solar eruptions, that emit an immense amount of ultraviolet<br />

radiation over a short period. When this UV radiation reaches the earth, the SID causes<br />

a tremendous increase in D-layer ionization, and abnormally high levels of absorption

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