PROBING THE IONOSPHERE WITH AN EXTREMELY NARROW FREQUENCY SEPARATION

An HF propagation link via ionospheric reflections has been establishe d for several years between Platteville, Colorado and Urbana, Illinois at a separation distance of 1,401 km. Although the probe is wideband in nature, covering the frequency range from 5 to 15 MHz, the transmit ter is capable of transmitting two sets of CW waves at a frequency sep aration as narrow as 2 Hz. The signals are coherently received. Since the receiver bandwidth is 50 Hz, a beat is observed if the frequency s eparation is less than 50 Hz. In this case the spectra so computed and plotted would show two clear and separated peaks. The phase informati on can be extracted which is used to investigate the propagation delay and Doppler shift. The variation of propagation delay and Doppler shi ft with time gives delay spread of about 17 mu s and Doppler spread of about 0.1 Hz for this midlatitude path. These spreads are important c haracteristics for a fading channel.