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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.
The Federal Communications Commission thought initially that very high frequency (VHF) radio waves would have a small, discrete range, and would allow stations to broadcast in duplicate frequencies without interfering with each other. But as wattage increased, the opposite was realized: because of the short wavelengths associated with high frequencies, apex station signals could sometimes be heard on the other side of the planet at night. In October 1937, the FCC made public its allocation plan for VHF radio broadcasting: 75 channels with 40 kHz separation on 41.02 to 43.98 MHz for apex stations and 16 channels in 30-40 MHz for relay stations. Twenty-five of the 75 channels were reallocated for educational use in 1938. By 1939, apex stations were operating in 34 cities in 22 states.
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.
Most apex stations operated under experimental licenses, and were affiliated with and subsidized by commercially licensed stations. Apex radio broadcasting's goal of high fidelity sound was later realized by frequency modulation (FM), which operated at 42–50 MHz (later 88–106 MHz, 88–108 MHz later still, and currently 87.8–108 MHz), immediately above the apex band of frequencies. The FCC in 1939 began encouraging apex stations broadcasting in AM to consider the change to the technically superior FM system.