locate one or more radio transmitters with radio direction finding techniques. Stations ― international regulation A radio direction-finding station is – according to article 1.91 of the International Telecommunication Union's (ITU) ITU Radio Regulations (RR) – defined as "a radiodetermination station using radio direction-finding." Each radiodetermiantion station shall be classified by the radiocommunication service in which it operates permanently or temporarily. If this station operates in a safety-of-life service, it will be protected for interferences. In accordance with ITU Radio Regulations (article 1) this type of radio station might be classified as follows: Radiodetermination station (article 1.86) of the radiodetermination service (article 1.40 ) Radionavigation mobile station (article 1.87) of the radionavigation service (article 1.42) Radionavigation land station (article 1.88) of the radionavigation service Radiolocation mobile station (article 1.89) of the radiolocation service (article 1.48) Radiolocation land station (article 1.90) of the radiolocation service Radio direction-finding station Radiobeacon station (article 1.92) of the radionavigation service Selection radio direction-finding stations Direction finding at microwave frequencies DF techniques for microwave frequencies were developed in the 1940s, in response to the growing numbers of transmitters operating at these higher frequencies. This required the design of new antennas and receivers for the DF systems. In Naval systems, the DF capability became part of the Electronic Support Measures suite (ESM), where the directional information obtained augments other signal identification processes. In aircraft, a DF system provides additional information for the Radar Warning Receiver (RWR). Over time, it became necessary to improve the performance of microwave DF systems in order to counter the evasive tactics being employed by some operators, such as low-probability-of-intercept radars and covert Data links. Brief history of microwave development Earlier in the century, vacuum tubes (thermionic valves) were used extensively in transmitters and receivers, but their high frequency performance was limited by transit time effects. Even with special processes to reduce lead lengths, such as frame grid construction, as used in the EF50, and planar construction, very few tubes could operate above UHF. Intensive research work was carried out in the 1930s in order to develop transmitting tubes specifically for the microwave band which included, in particular, the klystron the cavity magnetron and the travelling wave tube (TWT. Following the successful development of these tubes, large scale production occurred in the following decade. The advantages of microwave operation Microwave signals have short wavelengths, which results in greatly improved target resolution when compared