Apex was an experimental radio broadcasting system introduced in the United States in 1934 that used high frequencies between roughly 25 and 42 MHz to achieve high fidelity sound with less static and distortion on AM stations. They were called "apex", "skyscraper" or "pinnacle" stations because of the height of the broadcast antennas used. Until the late 1930s, commercially-made radio receivers did not operate within that band of frequencies, so early listeners of apex stations used self-built receivers, or built converters for existing models.