During 5.1-MHz HF transmissions below the F region penetration frequency by a nearby ionospheric modification facility, radar echoes coming from the F region heights at a frequency of 46.8+5.1=51.9 MHz were received while a VHF radar using the 305 m spherical reflector at Arecibo was transmitting pulses coherently on 46.8 MHz. The bandwidth of the echoes was less than 10 Hz. The so-called decay line due to the parametric decay instability was expected at a frequency lower by about 300 Hz, but it was not observed. Density variations caused by the ponderomotive force resulting from the standing wave pattern at 5.1 MHz are tentatively invoked to explain the observations. Poorly understood aspects of some previous observations with the 430-MHz radar can be similarly explained.