Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located.
Early systems tended to use omnidirectional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance, the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other showed a minimum. One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.
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 utilize 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.
With the end of the Cold War and the collapse of the Soviet Union, the RAF's focus has returned to delivering expeditionary air power. Since 1990 the RAF has been involved in several large-scale operations, including: the 1991 Gulf War, the 1999 Kosovo War, the 2001 War in Afghanistan, the 2003 invasion and war in Iraq and the 2011 intervention in Libya.
The RAF's 90th anniversary was commemorated on 1 April 2008 by a flypast of 9 Red Arrows and four Typhoons along the Thames, in a straight line from just south of London City Airport Tower Bridge, the London Eye, the RAF Memorial and (at 13.00) the Ministry of Defence building.
Four major defence reviews have been conducted since the end of the Cold War: the 1990 Options for Change, the 1998 Strategic Defence Review, the 2003 Delivering Security in a Changing World and the 2010 Strategic Defence and Security Review. All four defence reviews have resulted in steady reductions in manpower and numbers of aircraft, especially combat aircraft such as fast-jets. As part of the latest 2010 Strategic Defence and Security Review, the BAE Systems Nimrod MRA4 maritime patrol aircraft was cancelled due to over spending and missing deadlines. Other reductions saw total RAF manpower reduced by 5,000 personnel to a trained strength of 33,000 and the early retirement of the Joint Force Harrier aircraft, the Harrier GR7/GR9.
In recent years fighter aircraft on Quick Reaction Alert (QRA) have been increasingly required to scramble in response to efforts made by the Russian Air Force to approach British airspace. On 24 January 2014 in the Houses of Parliament, Conservative MP and Minister of State for the Armed Forces, Andrew Robathan, announced that the RAF's QRA force had been scrambled almost thirty times in the last three years: eleven times during 2010, ten times during 2011 and eight times during 2012.
RAF Coningsby in Lincolnshire and RAF Lossiemouth in Moray both provide Quick Reaction Alert, or QRA, and scramble their fighter jets within minutes to meet or intercept aircraft which give cause for concern. Lossiemouth generally covers the northern sector, while Coningsby provides QRA in the south. Typhoon pilot Flight Lieutenant Noel Rees describes how QRA duty works. "At the start of the scaled QRA response, civilian air traffic controllers might see on their screens an aircraft behaving erratically, not responding to their radio calls, or note that its transmitting a distress signal through its transponder. Rather than scramble Typhoons at the first hint of something abnormal, a controller has the option to put them on a higher level of alert, a call to cockpit. In this scenario the pilot races to the hardened aircraft shelter and does everything short of starting his engines".
On 4 October 2015, a final stand-down saw the end of more than 70 years of RAF Search and Rescue provision in the UK. The RAF and Royal Navy's Westland Sea King fleets, after over 30 years of service, were retired. A civilian contractor, Bristow Helicopters, has taken over responsibility for UK Search and Rescue, under a Private Finance Initiative with newly purchased Sikorsky S-92 and AgustaWestland AW189 aircraft. While the Royal Navy's SAR force will continue to operate until the end of the year, the new contract means that by 2016, all UK SAR coverage will be provided by Bristow's.