Editing Direction finding (section)

== 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),<ref>Tsui J.B., "Microwave Receivers with Electronic Warfare Applications", Kreiber, 1992</ref>{{rp|6}}<ref name = Lipsky>Lipsky S.E., "Microwave Passive Direction Finding", Wiley 1987</ref>{{rp|126}}<ref>Richardson D, "Techniques and Equipment of Electronic Warfare", Arco Publishing N.Y., 1985</ref>{{rp|70}} 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 radar]]s and covert [[Data link]]s.

=== 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.<ref name = Gilmour>Gilmour  jnr. A.S., "Microwave Tubes", Artech House, 1986</ref>{{rp|192}}<ref name = Beck>Beck, A. H. W., "Thermionic Valves", Cambridge University Press, 1953</ref>{{rp|394}}<ref>Baden Fuller A. J., "Microwaves"Pergamon Press, 1979</ref>{{rp|206}}  Even with special processes to reduce lead lengths,<ref>Hooijmans P., "Philip's tuner history". Find at http://maximus-randd.com/tv-tuner-history-pt1.html</ref> such as frame grid construction, as used in the [[EF50]], and planar construction,<ref name = Gilmour />{{rp|192}} 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]]<ref name = Gupta>Gupta K.C., "Microwaves", New Age Intnl. Pub., 2012</ref><ref name = Gilmour />{{rp|201}}  the [[cavity magnetron]]<ref name = Gilmour />{{rp|347}} <ref name = Gupta />{{rp|45}}  and the [[travelling wave tube]] (TWT).<ref name = Gilmour />{{rp|241}}<ref name = Gupta />{{rp|48}} 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 to [[RF]] systems. This permits better identification of multiple targets and, also, gives improved directional accuracy.<ref>Tutorial, "Advantages of Microwaves", Microwave Engineering Introduction article</ref> Also, the antennas are small so they can be assembled into compact arrays and, in addition, they can achieve well defined beam patterns which can provide the narrow beams with high gain favoured by [[radar]]s and [[Data link]]s.

Other advantages of the newly available microwave band were the absence of fading (often a problem in the [[Shortwave radio]] (SW) band) and great increase in signal spectrum, compared to the congested RF bands already in use. In addition to being able to accommodate many more signals, the ability to use [[Spread spectrum]] and [[frequency hopping]] techniques now became possible.

Once microwave techniques had become established, there was rapid expansion into the band by both military and commercial users.

=== Antennas for DF ===
Antennas for DF have to meet different requirements from those for a radar or communication link, where an antenna with a narrow beam and high gain is usually an advantage. However, when carrying out direction finding, the bearing of the source may be unknown, so antennas with wide [[beamwidth]]s are usually chosen, even though they have lower [[antenna boresight]] gain. In addition, the antennas are required to cover a wide band of frequencies.

The figure shows the normalized [[polar plot]] of a typical antenna gain characteristic, in the horizontal plane. The half-power beamwidth of the [[main beam]] is 2 × Ψ<sub>0</sub>. Preferably, when using amplitude comparison methods for direction finding, the main lobe should approximate to a Gaussian characteristic. Although the figure also shows the presence of [[sidelobe]]s, these are not a major concern when antennas are used in a DF array.

Typically, the boresight gain of an antenna is related to the beam width.<ref name = Stutzman>Stutzman W.L. & Thiele G.A., "Antenna Theory and Design", 2nd Ed., Wiley 1998.</ref>{{rp|257}} For a rectangular horn, Gain ≈ 30000/BW<sub>h</sub>.BW<sub>v</sub>, where BW<sub>h</sub> and BW<sub>v</sub> are the horizontal and vertical antenna beamwidths, respectively, in degrees. For a circular aperture, with beamwidth BW<sub>c</sub>, it is Gain ≈ 30000/BW<sub>c</sub><sup>2</sup>.

Two antenna types, popular for DF, are cavity-backed [[spiral]]s and  [[horn antenna]]s.

<gallery widths="200px" heights="200px">
File:Antenna_polar_plot.png|Antenna polar plot
File:Antenna_log_plot.png|Antenna log plot
File:Cavity_Backed_Spiral.png|Cavity backed spiral
File:Pyramidal_Horn.png|Pyramidal horn
</gallery>

[[Spiral antenna]]s are capable of very wide bandwidths <ref name = Stutzman />{{rp|252}}<ref>Morgan T.E., "Spiral Horns for ESM", IEE proc., Vol. 132, Pt. F., No. 4, July 1985, pp. 245 - 251</ref> and have a nominal  half-power beamwidth of about 70deg, making them very suitable for antenna arrays containing 4, 5 or 6 antennas.<ref name = Lipsky />{{rp|41}}

For larger arrays, needing narrower [[beamwidth]]s, horns may be used.  The bandwidths of horn antennas may be increased by using double-ridged waveguide feeds<ref>Milligan T.A., "Modern Antenna Design", 2nd Ed., Wiley 2005</ref><ref name = Lipsky />{{rp|72}} and by using horns with internal ridges.<ref name = Kingsley>Kingsley S. and Quegan S., "Understanding Radar Systems", McGraw -Hill 1992, SciTech Publishing, 1999</ref>{{rp|267}}<ref>[[Peter Clarricoats|Clarricoats P.J.B]] and Olver A.D., "Corrugated horns for microwave antennas", Peter Perigrinus 1984</ref>{{rp|181}}

=== 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]].

=== The new demands on DF systems ===
The move to microwave frequencies meant a reappraisal of the requirements of a DF system.<ref>Woolier D.F., "System considerations for naval ESM", IEE Proc. Vol. 132, Pt. F, No. 4, July 1985.</ref> Now, the receiver could no longer rely on a continuous signal stream on which to carry out measurements. Radars with their narrow beams would only illuminate the antennas of the DF system infrequently. Furthermore, some radars wishing to avoid detection (those of smugglers, hostile ships and missiles) would radiate their signals infrequently and often at low power.<ref>Wise J.C., "A Perspective on EW Receiver Design", Tech. Report APA-TR-2009-1102, J.C. Wise and Associates, Nov. 2009,  Find at:. https://ausairpower.net/APA-Maritime-ESM.html</ref> Such a system is referred to as a [[low-probability-of-intercept radar]].<ref>Davidson K., "Low Probability of Intercept", find at: http://radar-engineer.com/files/Lecture_LPI_Radar.pdf</ref><ref>Stove A.G. Hume A.L. and Baker C.j., "Low probability of intercept radar strategies", IEE Proc. Sonar Navig., Vol. 151, No. 5, October 2004</ref>  In other applications, such as microwave links, the transmitter's antenna may never point at the DF receiver at all, so reception is only possible by means of the signal leakage from antenna [[side lobe]]s.  In addition, covert [[Data link]]s<ref>Mills R.F. and Prescott G.E., "Detectability Models for Multiple Access Low-Probability-of-Intercept Networks", IEEE Trans on Aerospace and Electronic System, Vol.36, No.3, July 2000, pp> 848-858.</ref> may only radiate a high data rate sequence very occasionally.

In general, in order to cater for modern circumstances, a broadband microwave DF system is required to have high sensitivity and have 360° coverage in order to have the ability to detect single pulses  (often called [[amplitude monopulse]]) and achieve a high "Probability of Intercept" (PoI).<ref name = Hatcher>Hatcher B.R., [https://www.rfcafe.com/references/articles/wj-tech-notes/ew-acquisition-systems-probability-intercept-time-v3-3.pdf "EW Acquisition Systems - probability of intercept and intercept times"], Watkins-Johnson Tech-notes Vol. 3, No. 3, May/June 1976</ref>