Editing Reflective array antenna (section)

[[File:Eight bay bowtie TV antenna.jpg|thumb|This reflective array television antenna consists of eight "bowtie" dipole [[driven element]]s mounted in front of a wire screen reflector. The X-shaped dipoles give it a wide bandwidth to cover both the VHF (174&ndash;216&nbsp;MHz) and UHF (470&ndash;700&nbsp;MHz) bands. It has a gain of 5&nbsp;dB VHF and 12&nbsp;dB UHF and an 18&nbsp;dB front-to-back ratio. The example shown is horizontally polarised.]] 
[[Image:SCR-270-set-up.jpg|thumb|Reflective array 'billboard' antenna of the [[SCR-270]] radar, an early US Army radar system. It consists of 32 horizontal [[half wave dipole]]s mounted in front of a {{convert|55|ft|m|abbr=on|order=flip}} high screen reflector.  With an operating frequency of 106&nbsp;MHz and a wavelength of {{convert|3|m|ft|0|abbr=on}} this large antenna was required to generate a sufficiently narrow beamwidth to locate enemy aircraft.]]

In [[telecommunications]] and [[radar]], a '''reflective array antenna''' is a class of [[directional antenna|directive]] [[antenna (electronics)|antenna]]s  in which multiple [[driven element]]s are mounted in front of a flat surface designed to reflect the [[radio waves]] in a desired direction. They are a type of [[array antenna]].  They are often used in the [[Very high frequency|VHF]] and [[Ultra high frequency|UHF]] frequency bands. VHF examples are generally large and resemble a highway [[billboard]], so they are sometimes called '''billboard antennas'''.  Other names are '''bedspring array'''<ref>{{cite book | url={{GBurl|41vUCgAAQBAJ|q="bedspring array" antenna}} | title=NAVEDTRA 14183 - Navy Electricity and Electronics Training Series | publisher=Lulu Press | date=September 1998 | author=US Navy | volume=Module 11 - Microwave principles | pages=236 | isbn=1-329-66770-0}}</ref> and '''bowtie array''' depending on the type of elements making up the antenna.   The [[curtain array]] is a larger version used by [[shortwave]] radio broadcasting stations.

Reflective array antennas usually have a number of identical driven elements, fed [[in phase]], in front of a flat, electrically large [[reflector (antenna)|reflecting surface]] to produce a [[directional antenna|unidirectional]] beam of radio waves,  increasing [[antenna gain]] and reducing  [[radiation]] in unwanted directions.  The larger the number of elements used, the higher the gain; the narrower the beam is and the smaller the [[sidelobe]]s are.   The individual elements are most commonly [[half wave dipole]]s, although they sometimes contain [[parasitic element]]s as well as driven elements.  The reflector may be a metal sheet or more commonly a wire screen.  A metal screen reflects radio waves as well as a solid metal sheet as long as the holes in the screen are smaller than about one-tenth of a wavelength, so screens are often used to reduce weight and wind loads on the antenna.  They usually consist of a grill of parallel wires or rods, oriented parallel to the axis of the dipole elements.

The driven elements are fed by a network of [[transmission line]]s, which divide the power from the RF source equally between the elements.  This often has the circuit geometry of a tree structure.