Practical_Antenna_Handbook_0071639586

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 73
a rarely heard station in a remote part of the world. Because of the gray line, many serious
DXers make sure they are carefully monitoring the HF bands during the twin twilight
periods of their local dawn and dusk. It is not unusual for a gray line opening
between two specific locations to be only 10 or 15 minutes in duration!
Because the tilt of the terminator changes over the course of a year, the paths that
can benefit from it change as well. For instance, in Fig. 2.39, which is a gray line projection
for early June, a station near Vancouver, British Columbia, Canada, might observe
enhanced signal levels from stations in Bangladesh, Myanmar, or interior China. Three
months earlier, in early March, that same Vancouver station might be more apt to hear
surprisingly strong signals from Turkey, Saudi Arabia, and the horn of Africa.
Although our understanding of how gray line enhancement works is imperfect,
some experts believe the effect is related to the D layer of the ionosphere. The D layer
responds directly and rapidly to sunshine, disappearing very shortly after local sundown
and returning shortly after the sun rises in the morning. Along the evening half
of the gray line (Southeast Asia in Fig. 2.39), the D layer is rapidly decaying, but it has
not yet built up along the morning half of the line (Vancouver). Thus, a fleeting period
of less-than-normal absorption and enhanced signal levels may exist in a narrow
“trough” along the terminator on any given day. Note that, for these long-distance
paths, typically one station is near sunrise and the other near sunset.
Long Path
The great circle connecting any two points on the earth has two segments; for radio
communications purposes we refer to them as the long path (major arc) and the short
path (minor arc). If both paths are open, probably 99 percent of the time signal propagation
between the two points is substantially more efficient along the short path—precisely
because it is shorter! Not only is the incoming waveform at the receiving site
greater when the transmitted power spreads out over a radiation envelope of smaller
radius, but attenuation caused by extra hops and varying MUFs is less on the shorter
path. However, despite greater path length, when the short path is not open at all there
are many times when the long path can deliver an adequate signal to a specific location.
Figure 2.40 is a great circle map centered on New York, with night and day for 0200
Zulu (GMT) on April 1 shaded in. Suppose an amateur in the northeastern United
States with a rotatable directional antenna had hoped to work a 5R8 station in Madagascar
(off the east coast of Africa) on 15 m via short path (beam heading 80 degrees
from New York) at that time. Assuming a solar flux level of about 100, most likely prolonged
darkness over the Atlantic has caused the short-path MUFs between the two
locations to fall below 21 MHz. But there is still a chance that the New York operator
might be able to contact the 5R8 by turning his directional antenna 180 degrees to compass
heading 260 degrees and testing the long path. Remember that the nearest important
MUFs to the two stations on very long paths are about 1250 mi or so from each end
of the path. So the first MUF of importance is about 1250 mi west of New York, in the
southwestern United States, where the sun has just set. With any luck, the MUF there
may still be above 21 MHz.
Similarly, at the other end of the link, the 5R8 has daylight an equal distance east of
his station, directly opposite his short-path bearing to New York. Further, the entire
path between his first MUF control point and the U.S. operator’s control point is in
daylight. Even though peak long-path signal strengths will likely be lower than shortpath
strengths (because the long path requires roughly twice as many hops between