File:Spark gap transmitter bandwidth - A.png

English: "Characteristic of a commercial transmitter on shipboard showing broad range of waves sent out."

A graph showing the broad band of frequencies radiated by a spark gap transmitter, the type of radio transmitter used until about 1920. The vertical axis is the energy of the received signal at the test receiver, and the horizontal axis is wavelength. Since they generated radio waves using an impulsive spark, spark transmitters did not transmit on a single frequency but a band of frequencies, and tended to interfere with other transmitters on neighboring frequencies. From the graph, this transmitter's center wavelength was 800 meters (a frequency of 370 kHz) but it blanketed the band between 500 and 1200 meters (600 - 250 kHz) with its signal.

This graph was submitted by the US Bureau of Standards (the predecessor of the National Institute of Standards and Technology) to the radiotelegraph committee of US Congress 1912 when this book was published, to illustrate the noise problem of spark transmitters, to support legislation to restrict their use. Spark transmitters were superseded by vacuum tube transmitters around 1920, and remaining legacy sets on ships were banned in 1934.