NVIS Communications During the Solar Minimum

A popular technique for providing non line-of-sight coverage in mountainous or urban terrain is high frequency (HF) radio near-vertical incidence skywave (NVIS) mode. In NVIS operation, HF energy is directed vertically, and is refracted from the ionosphere to return nearly vertically to the area surrounding the transmitter; this nicely avoids nearly all obstacles. Direction finding versus a NVIS transmitter is also made difficult to the extent that the energy is directed vertically and groundwave is suppressed. However, NVIS operation requires the presence of substantial ionization in the ionosphere directly overhead the transmitter, so some concern is warranted about the effectiveness of NVIS operation during a solar minimum. In this paper, we present measurements and analysis of a multi-year NVIS experiment conducted during the current minimum of the sunspot cycle, including the effectiveness of the NVIS mode of operation, and evaluation of the accuracy of standard HF propagation programs in predicting usable operating frequencies.