Radar
Phased-array interferometry or aperture synthesis techniques, using an array of separate dishes that are phased into a single effective aperture, are not typical for radar applications, although they are widely used in radio astronomy. Because of the thinned array curse, such multiple aperture arrays, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques could increase spatial resolution, but the lower power means that this is generally not effective.
Aperture synthesis by post-processing motion data from a single moving source, on the other hand, is widely used in space and airborne radar systems.

Frequency bands: Antennas generally have to be sized similar to the wavelength of the operational frequency, normally within an order of magnitude. This provides a strong incentive to use shorter wavelengths as this will result in smaller antennas. Shorter wavelengths also result in higher resolution due to diffraction, meaning the shaped reflector seen on most radars can also be made smaller for any desired beamwidth.
Opposing the move to smaller wavelengths are a number of practical issues. For one, the electronics needed to produce high power very short wavelengths were generally more complex and expensive than the electronics needed for longer wavelengths or didn't exist at all. Another issue is that the radar equation's effective aperture figure means that for any given antenna (or reflector) size will be more efficient at longer wavelengths. Additionally, shorter wavelengths may interact with molecules or raindrops in the air, scattering the signal. Very long wavelengths also have additional diffraction effects that make them suitable for over the horizon radars. For this reason, a wide variety of wavelengths are used in different roles.
The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world. They have been adopted in the United States by the Institute of Electrical and Electronics Engineers and internationally by the International Telecommunication Union. Most countries have additional regulations to control which parts of each band are available for civilian or military use.
Other users of the radio spectrum, such as the broadcasting and electronic countermeasures industries, have replaced the traditional military designations with their own systems.

Modulators: Modulators act to provide the waveform of the RF-pulse. There are two different radar modulator designs:

High voltage switch for non-coherent keyed power-oscillators These modulators consist of a high voltage pulse generator formed from a high voltage supply, a pulse forming network, and a high voltage switch such as a thyratron. They generate short pulses of power to feed, e.g., the magnetron, a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as pulsed power. In this way, the transmitted pulse of RF radiation is kept to a defined and usually very short duration.
Hybrid mixers, fed by a waveform generator and an exciter for a complex but coherent waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g., a klystron or a solid state transmitter. In this way, the transmitted pulse is intrapulse-modulated and the radar receiver must use pulse compression techniques.

Coolant: Coherent microwave amplifiers operating above 1,000 watts microwave output, like travelling wave tubes and klystrons, require liquid coolant. The electron beam must contain 5 to 10 times more power than the microwave output, which can produce enough heat to generate plasma. This plasma flows from the collector toward the cathode. The same magnetic focusing that guides the electron beam forces the plasma into the path of the electron beam but flowing in the opposite direction. This introduces FM modulation which degrades Doppler performance. To prevent this, liquid coolant with minimum pressure and flow rate is required, and deionized water is normally used in most high power surface radar systems that use Doppler processing.
Coolanol (silicate ester) was used in several military radars in the 1970s. However, it is hygroscopic, leading to hydrolysis and formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire. Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named Pollution Prevention (P2) to eliminate or reduce the volume and toxicity of waste, air emissions, and effluent discharges. Because of this, Coolanol is used less often today.

Regulations: Radar (also: RADAR) is defined by article 1.100 of the International Telecommunication Union's (ITU) ITU Radio Regulations (RR) as:

A radiodetermination system based on the comparison of reference signals with radio signals reflected, or retransmitted, from the position to be determined. Each radiodetermination system shall be classified by the radiocommunication service in which it operates permanently or temporarily. Typical radar utilizations are primary radar and secondary radar, these might operate in the radiolocation service or the radiolocation-satellite service.

Configurations: Radar come in a variety of configurations in the emitter, the receiver, the antenna, wavelength, scan strategies, etc.

Bistatic radar
Continuous-wave radar
Doppler radar
Fm-cw radar
Monopulse radar
Passive radar
Planar array radar
Pulse-doppler
Synthetic-aperture radar
Synthetically thinned aperture radar
Over-the-horizon radar with Chirp transmitter

See also: Terrain-following radar
Radar imaging
Radar navigation
Inverse-square law
Wave radar
Radar signal characteristics
Pulse doppler radar
Mmwave sensing
Acronyms and abbreviations in avionics
Definitions
Amplitude-comparison monopulse
Constant false alarm rate
Sensitivity time control
Application
Proximity fuze
Hardware
Cavity magnetron
Crossed-field amplifier
Gallium arsenide
Klystron
Omniview technology
Radar engineering details
Radar tower
Radio
Travelling-wave tube
Similar detection and ranging methods
Acoustic location
Lidar
LORAN
Sonar
Historical radars
List of radars
Chain Home and Chain Home Low
Würzburg radar
Hohentwiel radar
H2S radar
SCR-270 radar

Notes and references: 

Bibliography: 

References: Barrett, Dick, "All you ever wanted to know about British air defence radar". The Radar Pages. (History and details of various British radar systems)
Buderi, "Telephone History: Radar History". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.)
Ekco Radar WW2 Shadow Factory Archived 12 December 2005 at the Wayback Machine The secret development of British radar.
ES310 "Introduction to Naval Weapons Engineering.". (Radar fundamentals section)
Hollmann, Martin, "Radar Family Tree". Radar World.
Penley, Bill, and Jonathan Penley, "Early Radar History—an Introduction". 2002.
Pub 1310 Radar Navigation and Maneuvering Board Manual, National Imagery and Mapping Agency, Bethesda, MD 2001 (US govt publication '...intended to be used primarily as a manual of instruction in navigation schools and by naval and merchant marine personnel.')
Wesley Stout, 1946 "Radar – The Great Detective" Archived 28 July 2020 at the Wayback Machine Early development and production by Chrysler Corp. during WWII.
Swords, Seán S., "Technical History of the Beginnings of Radar", IEE History of Technology Series, Vol. 6, London: Peter Peregrinus, 1986

General: Reg Batt (1991). The radar army: winning the war of the airwaves. R. Hale. ISBN 978-0-7090-4508-3.
E.G. Bowen (1 January 1998). Radar Days. Taylor & Francis. ISBN 978-0-7503-0586-0.
Michael Bragg (1 May 2002). RDF1: The Location of Aircraft by Radio Methods 1935–1945. Twayne Publishers. ISBN 978-0-9531544-0-1.
Louis Brown (1999). A radar history of World War II: technical and military imperatives. Taylor & Francis. ISBN 978-0-7503-0659-1.
Robert Buderi (1996). The invention that changed the world: how a small group of radar pioneers won the Second World War and launched a technological revolution. Simon & Schuster. ISBN 978-0-684-81021-8.
Burch, David F., Radar For Mariners, McGraw Hill, 2005, ISBN 978-0-07-139867-1.
Ian Goult (2011). Secret Location: A witness to the Birth of Radar and its Postwar Influence. History Press. ISBN 978-0-7524-5776-5.
Peter S. Hall (March 1991). Radar. Potomac Books Inc. ISBN 978-0-08-037711-7.
Derek Howse; Naval Radar Trust (February 1993). Radar at sea: the royal Navy in World War 2. Naval Institute Press.