The Adcock antenna is an antenna (British English aerial) array consisting of four equidistant vertical elements which can be used to transmit or receive directional radio waves. It was invented and patented by British engineer Frank Adcock in 1919 as British Patent No. 130,490, and has been used for a variety of applications, both civilian and military, ever since. Although originally conceived for receiving Low Frequency (LF) waves, it has also been used for transmitting, and has since been adapted for use at much higher frequencies, up to Ultra High Frequency (UHF).
In the early 1930s, the Adcock antenna (transmitting in the LF/MF bands) became a key feature of the newly created radio navigation system for aviation. The Low Frequency radio range (LFR) network, which consisted of hundreds of Adcock antenna arrays, defined the airways used by aircraft for instrument flying. The LFR remained as the main aerial navigation technology until it was replaced by the VOR system in the 1950s and 1960s.
The Adcock antenna array has been widely used commercially, and implemented in vertical antenna heights ranging from over 130 feet (40 meters) in the LFR network, to as small as 5 inches (13 cm) in tactical direction finding applications (receiving in the UHF band).
Read more about Adcock Antenna: Radio Direction Finding, Low Frequency Radio Range