The simple answer to your question is that we have no modulator, it is pure CW we are sending out. There is however no prefect square wave but it has a short rise- and decay time (about 12 ms).
However, Arne Siko here at the Alexander association had a data simulation made on a rough theoretical model of the antenna, and in the curves of that simulation one can discern an overlay that would be perceived as a 1100 Hz tone. We have never thought of this, as we see an overshoot in the amplitude at approximately 15 ms, that we thought little of, but not found any explanation. All this can be seen on the attached report.
/Fredrik Wiklund @AlexanderSAQ with help from Ola Hernvall