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2-27 Classification of the earth s atmosphere for radio propagation
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EM wave propagation phenomena 43 100 to 150 miles above the earth s surface, and the F2 layer is 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 because of recombination of positive and negative ions At some time after local sunset, the F1 and F2 layers have effectively merged to become a single reduced layer beginning at about 175 mi The height and degree of ionization of the F2 layer varies over the course of the day, with the season of the year, and with the 27-day sunspot cycle The F2 layer begins to form shortly after local sunrise and reaches maximum shortly before 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 There is some evidence that ionization in the F layer does not completely disappear, but its importance to HF radio communication does disappear
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Measures of ionospheric propagation
There are several different measures by which the ionosphere is characterized at any given time These measures are used in making predictions of radio activity and long-distance propagation The critical frequency 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 and international communications usually begin within 30 minutes Critical frequency Fc The critical frequency, designated by Fc, is the highest frequency that can be reflected when a signal strikes the ionosphere as a vertical (90 with respect to the surface) incident wave The critical frequency is determined from an ionogram, which is a cathode-ray 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 2-28) at the ionosphere from the transmitting station The critical frequency is that frequency that is just suffficient to be reflected back to the transmitter site Values of Fc can be as low as 3 MHz during the nighttime hours, and as high as 10 to 15 MHz during the day Virtual height Radio waves are refracted in the ionosphere, and those above a certain critical frequency are refracted so much that they return to earth Such waves appear to have been reflected from an invisible radio mirror An observer on the earth s surface could easily assume the existence of such a mirror by noting the return of the reflected signal The height of this apparent mirror is called the virtual height of the ionosphere Figure 2-29 shows the refraction phenomenon by which a radio wave is bent sufficiently to return to earth 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 receiving station (Fig 2-30) A radio signal travels at a velocity of 300,000,000 m/s (the speed of light) By observing the time between transmitting the pulse and receiving it, you can calculate the virtual height of the ionosphere Maximum usable frequency (MUF) The maximum usable frequency is the highest frequency at which communications can take place via the ionosphere over a given path The MUF between a fixed transmitter site and two different, widely separated, receivers need not be the same Generally, however, the MUF is approximately
44 Radio-wave propagation
Layer 2
Ionospheric layers
Layer 1
c = speed of light t = time (seconds)
Waves at critical frequencies
Waves above critical frequency (no reflection back to surface)
Hmeters = 1/2 ct Earth
2-28 Finding critical height of the ionosphere
Lower edge of the ionosphere
2-29 Finding virtual height of the ionosphere
EM wave propagation phenomena 45
Ionosphere
Signal refracts
time for round trip 3 108 m/s 2 T
Virtual height (H)
h's Eart
surfa
Transmitter
2-30 Finding virtual height of ionosphere
3 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 It is a general rule that the best propagation occurs at frequencies just below the MUF In fact, there is a so-called frequency of optimum traffic (FOT) that is approximately 85 percent of the MUF Both noise levels and signal strengths are improved at frequencies near the FOT Lowest usable frequency (LUF) At certain low frequencies, the combination of ionospheric absorption, atmospheric noise, miscellaneous static, and/or receiver signal-to-noise ratio requirements conspire to reduce radio communications The lowest frequency that can be used for communications, despite these factors, is the lowest usable frequency Unlike the MUF, the LUF is not totally dependent on atmospheric physics The LUF of a system can be varied by controlling the signal-to-noise ratio (SNR) Although certain factors that contribute to SNR are beyond our control, the effective radiated power (ERP) of the transmitter can be changed; a 2-MHz decrease in LUF is available for every 10-dB increase in the ERP of the transmitter
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