"Another problem with the spark transmitter was a result of the shape of the waveform produced by each burst of electromagnetic radiation. These transmitters radiated an extremely 'dirty' wide band signal which could greatly interfere with the reception of other transmissions on nearby frequencies. Receiving sets located relatively close to such a transmitter would have entire sections of a band masked by this wide band noise."
http://en.wikipedia.org/wiki/Spark-gap_transmitter
"Another problem with the spark transmitter was a result of the shape of the waveform produced by each burst of electromagnetic radiation. These transmitters radiated an extremely 'dirty' wide band signal which could greatly interfere with the reception of other transmissions on nearby frequencies. Receiving sets located relatively close to such a transmitter would have entire sections of a band masked by this wide band noise."