In [Barr et al. 1997> we presented ELF signals received near the Troms¿ ionospheric heater facility when the heater radiated HF signals amplitude modulated by a 505 Hz square wave. Square wave modulation enabled simultaneous multiple frequency measurements to be made of the signals radiated from the ionosphere as the heater was switched to radiate both 'X' and 'O' mode HF waves. We found that whereas the first 3 odd harmonics of the square wave (≥1515 Hz) all showed the accepted behaviour of reduced wave excitation on switching from 'X' to 'O' mode HF heating, the fundamental frequency showed enhanced excitation of more than 10 dB with 'O' mode heating. These observations were given an ionospheric explanation in terms of a change from Hall to Pedersen current modulation with the change from 'X' to 'O' mode HF heating. We present here an alternative analysis, which has both instrumental and ionospheric components and is also able to explain the observations. ¿ 1998 American Geophysical Union