Editing Direction finding (section)

=== Microwave receivers ===
==== Early receivers ====
Early microwave receivers were usually simple "crystal-video" receivers,<ref name = Wiley>Wiley R. G., ''Electronic Intelligence: The Interception of Radar Signals'', Artech House, 1985</ref>{{rp|169}}<ref name=Lipsky/>{{rp|172}}<ref>Lipkin H.J., "Crystal-Video Receivers", MIT Radiation Series Vol 23, Microwave Receivers, Chapter 19 pp.504-506.  Find at:  https://archive.org/details/MITRadiationLaboratorySeries23MicrowaveReceivers</ref> which use a crystal detector followed by a  video amplifier with a compressive characteristic to extend the dynamic range.  Such a receiver was wideband but not very sensitive.  However, this lack of sensitivity could be tolerated because of the "range advantage" enjoyed by the DF receiver (see below).

==== Klystron and TWT preamplifiers ====
The klystron and [[Traveling-wave tube|TWT]] are linear devices and so, in principle, could be used as receiver preamplifiers. However, the klystron was quite unsuitable as it was a narrow-band device and extremely noisy<ref name = Beck />{{rp|392}} and the TWT, although potentially more suitable,<ref name = Beck />{{rp|548}} has poor matching characteristics and large bulk, which made it unsuitable for multi-channel systems using a preamplifier per antenna. However, a system has been demonstrated, in which a single TWT preamplifier selectively selects signals from an antenna array.<ref>Kiely D.G., "Advances in microwave direction finding", Proc. IEE, Vol. 113, No.11, Nov 1964, pp. 1967–1711</ref>

==== Transistor preamplifiers ====
Transistors suitable for microwave frequencies became available towards the end of the 1950s. The first of these was the [[metal oxide semiconductor field effect transistor]] (MOSFET). Others followed, for example, the [[metal-semiconductor field-effect transistor]] and the [[high electron mobility transistor]] (HEMT). Initially, discrete transistors were embedded in [[stripline]] or [[microstrip]] circuits, but [[microwave integrated circuit]]s followed. With these new devices, low-noise receiver preamplifiers became possible, which greatly increased the sensitivity, and hence the detection range, of DF systems.

==== 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]].