of vhf propagation provide the only reliable means of long-distance radio communication known today. These types will be discussed in detail later in this chapter.

An ionospheric storm is caused by corpuscular radiation emitted from solar flares at the same time flare emits ultraviolet and X-ray radiation which produce the SID. Corpuscular radiation travels at lower velocity than light and arrives at the earth at a later time period. Particles cause long-lasting ionospheric storm which disrupts long distance radio communication.

Atmospheric Noise The usefulness of a radio signal is limited by the total noise in the receiver which may be either unwanted, external noise or the internal noise of the receiver. Atmospheric static is usually the limiting factor in receiver sensitivity at frequencies below 30 MHz, while receiver noise is the primary limitation at higher fre- quencies, especially those above 200 to 500 MHz. In the hf band, the controlling factor depends upon the loca- tion of the receiver, time of day, man-made noise and atmospheric static. Static is caused by lightning and other natural electri- cal disturbances and is propagated worldwide by iono- spheric seflection. Static levels are generally stronger at night than in the daytime and the levels are higher in the warm tropical areas than in the cooler northern regions, which are far removed from most lightning storms. The average static level in the tropics may be as much as 15 decibels higher than for the temperate zones, while in the Arctic regions the static level may be 15 to 25 decibels lower. In all areas, typical summer averages are a few decibels higher than the winter values.

Propagation in the VHF Region

As a result of the tremendous increase in vhf activity since World War II, much has been learned about the different modes of radio propagation at these frequen- cies. The boundary between the hf and the vhf region is variable, falling between 30 MHz and 50 MHz and is generally taken to be the n.tuF, above which normal ionospheric reflection ceases. Deviations from this sim- ple definition are numerous. Interestingly, certain types

lonospheric Scatter Propagation Ionospheric scatter propagation permits communica- tion in the frequency range of about 30 MHz to 300 MHz over distances ranging from 600 miles (1000 km) to nearly 1200 miles (2000 km). It is believed that this type of propagation is due to scattering of the signal from the lower D layer, or possibly the E layer. Because only a small portion of the radiated energy is scattered and returned to earth, such scatter signals are very weak (Figure 31). The lower limit of ionospheric scatter is determined by the masking action of normal iono- spheric skip distance. Regular skywave propagation will create undesirable interference to a scatter signal and produce selective fading on a scatter link circuit.

Figure 31. lonospheric Scatter Signal Level Is Low, Punctuated by Meteor Bursts Because only a small proportion of the radiated energy is scattered and returned to earth, scatter signals are very weak. Lower limit of ionospheric scatter is determined by masking aaion of normal ionospheric skip distances. Regular skywave propagation can create seledive inter· ference on a scatter link circuit.

Ionospheric scatter seems limited to a single-hop dis- tance. Theoretically, it would be possible to communi- cate via double-hop scatter, which could extend the range to 2000 miles (3200 km) or so, but circuit attenua- tion would be extreme.

Meteor-Burst Propagation Meteors have been observed for centuries, but until recently they were assumed to be relatively few in num- ber. Recent studies, however, have shown that the earth is constantly colliding with innumerable particles as it sweeps on its annual journey around the sun. Over ten billion particles are estimated to reach the earth each 24 hour period, with the largest number of these less than 0.016 cm in diameter. Only a very few are large enough to be noticed, and only an extremely small percentage of the latter are large enough to reach the ground before they are burned up by friction with the earth's atmosphere (Figure 32).

Name of Date of Peak Duration Meteors Shower Intensity (Days) Per Hour Quandranids January 3 1 35-40 Lyrids April21 2 12-15 Eta Aquarids May 5 9 12-20 Delta Aquarids July 29 10 20-30 Perseids August 12 5 50 0rionids October 21 4 20-25 Taurids Nov.5; Nov.12 20 12-15 Leonids November 17 4 20-25 Geminids December 13 5 40-50 Ursids December 22 2 15

Figure 32. Major Meteor Showers Li5t of major meteor showers. The spring showers peak between midnight and 0600, the Ursids peak during the early afternoon hours.0thers generally peak during hours of darkness. Seasonally, more meteors occur dur- ing May and luly than at any other time.

When a meteor strikes the earth's atmosphere, a cy- lindrical region of free electrons is formed at about the height of the E layer. This slender, ionized column is quite long, and when first formed is sufficiently dense to reflect radio waves back to the earth. Frequencies in the range of 50 MHz to 80 MHz have been found best for meteor-burst transmission. The effect of a single meteor of medium size (1 cm) shows up as a sudden "burst" of signal of short dura- tion at a point not normally reached by the transmitter. The aggregate effect of many meteors impinging on the earth's atmosphere, while perhaps too weak to provide loag-term ionization, is thought to contribute to the existence of the nighttime E layer.

Aurora Propagation At the earth's poles, where the atmosphere is more rarefied than elsewhere, radiation from the sun not only causes ionization, but often causes the air mole- cules to ignite. This phenomenon is called an aurora (or "northern" or "southern" lights). The action is similar to that which takes place in a neon tube. The aurora is a spectacular observance, with lights arcing across the night sky as yellowish-green dancing ribbons, or cur- tains, or great draperies which appear to fold and un- fold. They occur at E layer height in the ionosphere and can be seen on the horizon as far as 600 miles (960 km) from the zenith point. In the northern hemisphere, the zone of maximum occurrence (aurora! zone) swings across northern Nor- way, Greenland and central Canada, and back across Alaska, Siberia, and northern European USSR (Figure 33). Both north and south of this belt the occurrence of auroras decreases. Auroras play havoc with high frequency radio com- munication and cause severe absorption of any hf wave that passes near or through the auroral zone. Besides ebsorption, the aurora superimposes an auroral flutter on hf signals. Aurorol propagation of vhf signals is common at

Aurora can be seen on occasion as far south as Mexico City. The average number of nights per year having aurora displays are shown in this polar chart. Auroral propagation of vhf signals is common at frequencies between 100 and 450 MHz, but aurora disrupts hf radio communication at the same time.

frequencies between 100 MHz and 450 MHz. The prop- agation involves reflection of the wave from the auroral display. The reflection properties of the aurora vary quite rapidly, with the result that the reflected vhf sig- nal is badly distorted by multipath effects. Voice modu- lation becomes very rough and cw telegraphy is usually employed for auroral communication in the vhf ama- teur bands. Since aurora is caused by emission of charged parti- cles from the sun, it is natural to find that aurora propagation follows the sunspot cycle and reaches a peak at the same time as the cycle. In addition, auroras follow a seasonal pattern, peaking around March and September, although they may occur at any time. Because of the shallow nature of the aurora belt, east-west transmission paths are usually favored. At times it is possible to communicate up to 2000 miles (3200 km) or more, via aurora propagation, but ranges of a few hundred miles are more common. Aurora propagation seems to reach a peak around sundown or early evening, and again around 0200, local time. The farther north a station is situated, the more frequently it will encounter aurora propagation, but during rare occasions it may be possible to employ this mode of transmission in the southernmost portions of the United States. Vhf aurora propagation may be predicted by moni- toring signals in the 2-MHz to 5-MHz range for the characteristic aurora distortion. This is evidence that vhf propagation may soon be possible.

Tropospheric Scatter Propagation Tropospheric scatter (troposcatter) is thought to be caused by random irregularities in the atmosphere in which the refractive index differs from the mean value of surrounding areas. The scattering effect seems to take place by partial reflection where there is a rapid change of reflective index over a small range in height associated with temperature and humidity changes. The result of scatter refraction is a faint signal illumination of the ground well beyond the horizon (Figure 34).

Figure 34. Geometry of Tropospheric Scatter System Forward scatter mechanism involves a large transmission loss and requires high gain, narrow beam antennas at both ends of the circuit. The scatter angle is kept as small as possible by proper choice of transmitting and receiving sites.

The forward-scattering mechanism involves a large transmission loss and it becomes necessary to use high gain, narrow beam antennas for both transmission and reception. The effect of the scatter angle between the receiving and transmitting beam antennas is significant and is kept as small as possible by choosing transmit- ting and receiving sites so as to have an unobstructed view of the horizon. The received scatter signal fluctuates continuously due to the large number of randomly varying compo- nents; hourly, daily and monthly variations may reach 10 to 20 decibels or more. However, consistently usable signals are obtainable at ranges exceeding 400 miles (700 km). The scattering mechanism may be compared to the scattering of a light beam in a heavy fog, or mist, which results in a heavy glare of light caused by miniature water droplets, leaving the background weakly illumi- nated. No critical frequency is involved in the scattering mechanism, though the intensity of the scattered reflec- tions decreases with increasing frequency.

Trans-Equatorial Scatter Propagation Trans-equatorial scatter (T E scatter) has been observed on the 50-MHz amateur band during periods of moder- ate and high solar activity, over long north-south paths spanning the magnetic equator at times when the ex- pected rvtUF is considerably lower for the paths in- volved. T-E scatter is believed to be due to a highly ionized distortion known to exist in the ionosphere over the

magnetic equator. Waves entering this area at a favorable angle are reflected considerable distances be- tween the sides of the bulge, resulting in a long, single- hop opening, without intermediate ground reflection, of up to 5000 miles (8000 km). T-E scatter is a nighttime propagation phenomenon, with most openings occurring between 2000 and 2300 hours, local time at the path midpoint. The signals must cross the magnetic equator in a north-south direc- tion or propagation will not take place. The T-E maxi- mum usable frequency is approximately 1.5 times greater than the daylight rvtUF observed on the same path. To date, no T-E scatter propagation has been observed over 100 MHz.

Sporadic E Propagation Sporadic E propagation has been mentioned earlier in this chapter. It is a popular form of communication for radio amateurs on the hf and vhf frequencies as it calls for no special station equipment. Sporadic E openings on the higher frequency bands may often be predicted by observing the characteristics of the 28-MHz band. The geometry of propagation is such that as the skip distance decreases on the 28-MHz band, the highest frequency that will be reflected by a sporadic E cloud is increasing. Experience has shown that when skip sig- nals are heard less than 500 miles (800 km) away on 10 meters, the chances are very good that sporadic E prop- agation will be noted on the 50-MHz band over the same general direction.

Tropospheric Ducting Tropospheric ducting of vhf signals is quite common and is the result of change in the refractive index of the atmosphere at the boundary between air masses of dif- fering temperatures and humidities. Using a simplified analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rarer upper air, imparting a downward curve to the wave travel. Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion and the boundary between the two air masses may extend for 1000 miles (1800 km) or more along a stationary weather front. Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper air layers. Tropospheric communication as a result of ducting is rare below 144 MHz, but occurs commonly in the 144- MHz to 450-MHz range. Less spectacular communica- tions are possible as a result of simple temperature inversion, where ducting is not believed possible. Duct- ing over water, particularly between California and Ha- waü, and Brazil and Africa, has produced vhf communication in excess of 3000 miles (4500 km).

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