WGU-20

Operating 24 hours a day on a longwave frequency of 179 kHz from Chase, Maryland, WGU-20's programming consisted of pre-recorded announcements (including weather reports) and time checks.

The broadcast had the mechanical sound of early speech synthesis systems but the message could be clearly understood, with time ticks in the background and a continuous announcement akin to the speaking clock: Good evening.

Afterwards, reception reports (some from as far away as Texas) were sent a special QSL card featuring Paul Revere on a horse, raising the alarm.

As originally envisioned, many home devices, including radios, televisions, and even smoke detectors, would have inexpensive longwave receivers built into them, ensuring the attack message would get out.

A longwave frequency was chosen because the extended groundwave signal it produced was supposed to be relatively immune to the effects of a nuclear detonation.

Besides the Maryland site, candidate sites for distribution stations were Maynard, Massachusetts; Mount Joy, Pennsylvania; Gray, Maine; Morristown, Tennessee; Starke, Florida or Chiefland, Florida; Mazomanie, Wisconsin; Carthage, Texas, Marshall, Texas or Seagoville, Texas; Alcova, Wyoming or Riverton, Wyoming; Mendota, California or Selma, California; Winslow, Arizona; Hermiston, Oregon; and Wallula, Washington.