A sweep frequency ionosphere recorder for the low frequencies
James C. Blair, J. N. Brown, James M. Watts · Transactions of the IRE Professional Group on Antennas and Propagation · 1952
Application of the techniques heretofore used in high frequency ionosphere recorders is described. The beat frequency method of generating wide frequency sweeps is used, covering frequencies from 50 kc to 1,000 kc in a short time without band switching. Advantage is gained by the use of transformers containing ferromagnetic cores in the wide band transmitter amplifiers, but the antenna system, for practical reasons, is very inefficient. Sample records are shown.