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  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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