Editing Direction finding (section)

=== 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}}