What frequency did wireless telegraphs operate at?
Early radio transmission used telegraphy for communication using spark transmitters. Telegraphy is still used today, but does not use spark transmission (it is banned).

Spark transmitters were inefficient partly because they spread noise energy over a fairly broad bandwidth. However, energy was confined to a small part of the electromagnetic spectrum by resonance of coils and capacitors tuned to a desired frequency. In addition, antennas helped by radiating some frequencies more efficiently than others. Efforts to confine the broadband spark signal source to a narrower transmitted band not only improved efficiency, but allowed more users to communicate on nearby frequencies with less interference.
The frequency 500 kHz. was reserved for distress calls. Other frequencies were used for general communication traffic.
When vacuum-tube oscillators were introduced, it was possible to confine transmitted telegraphy signals to a single frequency. Telegraphy still requires some bandwidth to transmit information (see Nyquist rate), but beyond this bandwidth, very little noise is broadcast.
Your query is still of interest today, where much effort goes into generating coherent radio-frequency carriers free of adjacent noise sidebands.