Recently published HF-enhanced plasma line observations using a 46.8-MHz radar show that the upshifted decay line is absent. The upshifted spectrum consisted of a single ''growing line'' within a few hertz of 51.9 MHz (46.8 MHz plus the heating wave frequency of 5.1 MHz). The downshifted line was obscured by interference. The authors explain their observations by considering Langmuir wave propagation in the electron density depression generated by the ponderomotive force of the heating wave near reflection. Another approach, considered here, is to consider Langmuir wave propagation in magnetic field-aligned ionization ducts which are generated by the HF heating wave. This latter approach appears to yield a more satisfactory explanation of the observations. Using the duct model, it is predicted that for a radar frequency near 50 MHz the downshifted decay line will have a higher intensity and occur at a lower altitude than the upshifted decay line. Some published observations showing an ''early rise'' time for the plasma line using a 430-MHz radar can also be explained using the duct model.