Frequency variations due to over-the-horizon tropospheric propagation

An experiment was performed over a 161-mile path between Alpine, N. J., and Round Hill, Mass., to determine the frequency fluctuations produced by the propagation mechanism on a highly stable signal in an over-the-horizon tropospheric circuit. A signal at 388.0 Mc was transmitted from Alpine using a 10-kw transmitter and a beamwidth antenna. These transmissions were received at Round Hill with a beamwidth antenna and heterodyned to 416.7 Mc using a highly stable local oscillator and retransmitted to Alpine. Using coherent reception techniques, the retransmitted signal was received at Alpine and heterodyned with the signal originally transmitted. The difference frequency was fed to a bank of narrow-bandwidth crystal filters. An analysis of the data obtained from these filters indicated that the standard deviation of the frequency fluctuations of the signal was approximately 0.6 cps when CW transmission was employed. An additional feature of the experiment was an attempt to measure the variations of the propagation path length as a function of time. It appears that the standard deviation of the path length variations was less than 55 meters when the average path length in -second intervals was measured.