Editing Direction finding (section)

==== Range advantage ====
''Source:''<ref>East P.W., "ESM Range Advantage",  IEE Proceedings F - Communications, Radar and Signal Processing, Vol.132, No.4, Jul 1985, pp. 223 - 225</ref>

The DF receiver enjoys a detection range advantage<ref>Davidson K., "Electronic Support Sensors".  Find at: https://radar-engineer.com/files/Lecture_ES_Sensors.pdf</ref>  over that of the radar receiver.  This is because the signal strength at the DF receiver, due to a radar transmission, is proportional to 1/R<sup>2</sup> whereas that at the radar receiver from the reflected return is proportional to σ/R<sup>4</sup>, where R is the range and σ is the [[radar cross-section]] of the DF system.<ref>Connor F.R., "Antennas", Edward Arnold, 1972, p.8.</ref>  This results in  the signal strength at the radar receiver being very much smaller than that at the DF receiver.  Consequently, in spite of its poor sensitivity, a simple crystal-video DF receiver is, usually, able to detect the signal transmission from a radar at a greater range than that at which the Radar's own receiver is able to detect the presence of the DF system.<ref name = Lipsky />{{rp|8}}

In practice, the advantage is reduced by the ratio of antenna gains (typically they are 36&nbsp;dB and 10&nbsp;dB for the Radar and ESM, respectively) and the use of [[Spread spectrum]] techniques, such as [[Chirp compression]],  by the Radar, to increase the processing gain of its receiver.  On the other hand, the DF system can regain some advantage by using sensitive, low-noise, receivers and by using Stealth practices to reduce its [[radar cross-section]],<ref name = Kingsley />{{rp|292}} as with [[Stealth aircraft]] and [[Stealth ships]].