4.
Characteristics of the Ionosphere.
Critical frequency. Layer height will determine how far a
radio transmission travels.
In addition, the higher the frequency
the greater the density of ionization that is required to reflect the
signal back to the earth.
The F2 layer will reflect higher
frequencies than the F1 layer.
The same will hold true for the F1
layer as compared to the E layer.
The D layer will reflect
frequencies below approximately 500 kHz.
For each layer there is a
maximum frequency which is refracted, but higher frequencies are not.
This is called the critical frequency.
Frequencies higher than the
critical frequency will pass through the layer and will not be
refracted. As a radio wave passes through a layer, it is partially
absorbed.
Figure 15 shows different frequencies striking different
layers.
Some will be returned, others will pass through.
All
frequencies below the critical frequency are refracted.
Frequencies
above the critical frequency pass through the layers.
Figure 15.
Critical frequencies.
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