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 53 Sporadic E propagation is another phenomenon associated with the E layer. This mode is made possible by scattered zones of intense ionization in the E-layer region of the ionosphere. Sporadic E varies seasonally, and some believe it results from solar particle bombardment of the layer. Sporadic E propagation affects the upper HF and lower VHF region. It is observed most frequently in the lower VHF spectrum (30 to 150 MHz), but it is also sometimes observed at higher frequencies. Skip distances on VHF can reach 500 to 1500 mi on one hop—especially in the lower VHF region (including the 6-m band). F Layer The F layer of the ionosphere is the primary support for long-distance shortwave communications. This layer is located between 100 and 300 mi above the earth’s surface. Unlike the lower layers, the air density in the F layer is low enough that ionization levels remain high all day and decay slowly after local sunset. Minimum levels are reached just prior to local sunrise. Because of its height, the F layer can support skip distances up to 2500 mi on a single hop. During the day, the F layer often splits into two identifiable and distinct sublayers, designated the Fl and F2 layers. The F1 layer is found approximately 100 to 150 mi above the earth’s surface, with the F2 layer above the F1, extending up to the 270- to 300-mi limit. Beginning at local sundown, however, the lower regions of the F1 layer begin to deionize as positive and negative ions recombine. Sometime after local sunset, the F1 and F2 layers effectively merge to become a single reduced layer beginning at about 175 mi. The height and degree of ionization of the F2 layer varies with local sun time, with the season of the year, and with the 27-day and 11-year sunspot cycles. The F2 layer begins to form shortly after local sunrise and reaches maximum shortly after noon. During the afternoon, the F2-layer ionization begins to decay in an exponential manner until, for purposes of radio propagation, it disappears sometime after local sunset. During sunspot maxima, F-layer ionization does not completely disappear overnight, and the 20-m amateur band may be open worldwide 24/7. Measures of Ionospheric Propagation At any given time, several different characteristics of the ionosphere can be measured and used to make predictions of radio activity and long-distance propagation. The critical frequency (f C ) and maximum usable frequency (MUF) are indices that tell us something of the state of ionization and communications ability. These frequencies increase rapidly after sunrise, enabling international communications at higher and higher frequencies as the MUF rises with the sun. Critical Frequency (f C ) The critical frequency, f C , is the highest frequency that can be reflected when a signal strikes the ionosphere as a vertical (90 degrees with respect to the reflecting surface) incident wave. The critical frequency is determined from an ionogram, which is a cathoderay tube (CRT) oscilloscope display of the height of the ionosphere as a function of frequency. The ionogram is made by firing a pulse vertically (Fig. 2.30) at the ionosphere from the transmitting station. The critical frequency, f C , is then the highest frequency for which a reflected signal is received back at the transmitter site. Values of f C can be as low as 3 MHz during the nighttime hours and as high as 10 to 15 MHz during the day.
54 p a r t I I : F u n d a m e n t a l s Layer 2 Ionospheric layers Layer 1 Waves at Waves above critical critical c = speed of light frequencies frequency t = time (seconds) (no reflection back to surface) H meters = 1 /2 ct Earth Figure 2.30 Finding the critical frequency of the ionosphere. Virtual Height Radio waves are refracted in the ionosphere, and those below the critical frequency are refracted so much that they return to earth. Such waves appear to have been reflected from an invisible radio “mirror.” The height of this apparent mirror (point C′ in Fig. 2.27) is called the virtual height of the ionosphere. Virtual height is determined by measuring the time interval required for an ionosonde pulse (similar to that used to measure critical frequency) to travel between the transmitting station and a nearby receiving station (Fig. 2.31). By observing the time interval between the transmitted and the received pulses and applying a correction factor for the estimated speed of the wave through the atmosphere, the virtual height of the ionosphere can be calculated. Maximum Useable Frequency (MUF) The maximum useable frequency is the highest frequency at which communications can take place via the ionosphere over a given path. Normally the MUF for a given transmitter site is found to be at the farthest distance(s) the signal can graze the ionosphere in the direction of the sun. As a very rough rule of thumb, the MUF is approximately three times higher than the critical frequency. Both the MUF and the critical frequency vary geographically, and they become higher at latitudes close to the equator. However, the critical frequency is determined by bouncing a signal off the ionosphere directly overhead; the measured MUF, on the
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54 p a r t I I : F u n d a m e n t a l s<br />
Layer 2<br />
Ionospheric layers<br />
Layer 1<br />
Waves at<br />
Waves above<br />
critical<br />
critical<br />
c = speed of light frequencies<br />
frequency<br />
t = time (seconds)<br />
(no reflection<br />
back to<br />
surface)<br />
H meters = 1 /2 ct<br />
Earth<br />
Figure 2.30 Finding the critical frequency of the ionosphere.<br />
Virtual Height<br />
Radio waves are refracted in the ionosphere, and those below the critical frequency are<br />
refracted so much that they return to earth. Such waves appear to have been reflected<br />
from an invisible radio “mirror.” The height of this apparent mirror (point C′ in Fig.<br />
2.27) is called the virtual height of the ionosphere. Virtual height is determined by measuring<br />
the time interval required for an ionosonde pulse (similar to that used to measure<br />
critical frequency) to travel between the transmitting station and a nearby receiving station<br />
(Fig. 2.31). By observing the time interval between the transmitted and the received<br />
pulses and applying a correction factor for the estimated speed of the wave through the<br />
atmosphere, the virtual height of the ionosphere can be calculated.<br />
Maximum Useable Frequency (MUF)<br />
The maximum useable frequency is the highest frequency at which communications can<br />
take place via the ionosphere over a given path. Normally the MUF for a given transmitter<br />
site is found to be at the farthest distance(s) the signal can graze the ionosphere<br />
in the direction of the sun. As a very rough rule of thumb, the MUF is approximately<br />
three times higher than the critical frequency.<br />
Both the MUF and the critical frequency vary geographically, and they become<br />
higher at latitudes close to the equator. However, the critical frequency is determined<br />
by bouncing a signal off the ionosphere directly overhead; the measured MUF, on the
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