Time Dispersion

Since the ionosphere is a dispersive medium, separated frequencies travel with different velocities. When a pulse is represented in the frequency domain, we find that the spectral width is inversely proportional to pulse length. As a result, short pulses suffer a larger relative distortion following ionospheric transit than longer pulses. Millman and Olsen [1980] have examined ionospheric dispersion effects in wideband transmissions. Figure 417 shows the pulse distortion impact (i.e., group time delay difference) as a function of frequency with pulse width as parameter. We see for a 100 MHz channel, and a pulse width of 1 [xsec, that the time delay difference is ~ 0.1 (usee. This is a 10% distortion effect. Of course the situation will get worse if the pulse length is reduced. On the other hand, at 900 MHz, the time delay difference is only 0.0002 ¬°isec.

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