Ruth Haines
Antennas are an essential component of aircraft and airports, facilitating communication and surveillance. At airports, radar systems like the Airport Surveillance Radar (ASR) employ antennas to detect and display aircraft positions, ensuring safe air traffic control. These antennas are typically large, rotating parabolic dishes mounted on towers or near the airport premises. On aircraft, antennas come in various shapes and sizes, often resembling lightning rods or protrusions, and are strategically placed to support specific functions, such as communication and navigation. Cessna, for instance, utilizes flush antennas that appear as flat plates and wire antennas protruding towards the tail. Understanding the design, placement, and functionality of airport and aircraft antennas is crucial for efficient air operations and safety.
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What You'll Learn
Airport surveillance radar antennas
The primary surveillance radar typically consists of a large rotating parabolic antenna dish, resembling a "dish" shape, that sweeps a vertical fan-shaped beam of microwaves around the airspace. This primary radar operates at a frequency of 2.7–2.9 GHz in the S-band with a peak power of 25 kW and an average power of 2.1 kW. It is designed to detect the presence and position of aircraft within a range of 60 miles and an elevation of up to 25,000 feet. The antenna rotates at 12.5 RPM, scanning the airspace every 4.8 seconds.
The secondary surveillance radar, on the other hand, consists of a second rotating antenna often mounted on the primary antenna. It interrogates aircraft transponders and transmits the received information to air traffic controllers. This radar is capable of detecting up to 700 aircraft simultaneously and displaying weather conditions and aircraft positions on the same screen.
The design of airport surveillance radar antennas is strictly regulated by government agencies due to their critical safety role. The Federal Aviation Administration (FAA) in the United States, for example, is responsible for developing and upgrading these radar systems to ensure compatibility with various aircraft and avionics systems.
The new generation of airport surveillance radar, known as Digital Airport Surveillance Radar (DASR), is currently being developed to replace the existing analog systems. DASR systems will provide improved detection capabilities and compatibility with new terminal automation systems.
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Cessna's modified marker beacon antennas
Cessna has successfully modified the marker beacon antenna in two ways. The first is a flush antenna that is located under the empennage and looks like a flat plate. The second is an antenna that consists of a thick wire protruding straight down from the empennage and then turning towards the tail of the plane.
Marker beacon antennas are highly directional, which means that the aircraft must be almost directly over the transmitting ground station to receive the signal. Therefore, marker beacon antennas are placed on the bottom of the aircraft. Cessna's flush antennas are flat plates located under the empennage, while the other type of antenna has a thick wire protruding straight down from the empennage.
The shape and placement of antennas are determined by their directional qualities and the frequencies they use to operate. Antennas are formed more by their function than anything else. A well-equipped airplane will often have an antenna farm on its belly, with dozens of antennas, each with a different purpose. These antennas are mostly used to help pilots communicate with others.
Communication antennas are basic in operation, with each com transmitter having its own antenna, mostly for redundancy. They can be mounted on either the top or bottom of the aircraft, but each installation is susceptible to shadowing from the fuselage. Shadowing is caused by structures such as the vertical stabilizer or landing gear doors in the transmitting path of the antenna. It is important to know where your antennas are and how shadowing may affect their range and coverage.
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VHF nav antennas
The shape and placement of VHF nav antennas, like other aircraft antennas, are largely determined by their function, the frequencies at which they operate, and their directional qualities. For instance, UHF antennas, commonly used for transponders and distance-measuring equipment (DME), are always found on the bottom of the aircraft and are only around four inches long. In contrast, GPS antennas, due to the high frequency of GPS signals, must be mounted at the very top of the fuselage. Communication antennas, on the other hand, can be mounted on either the top or bottom of the aircraft but are susceptible to "shadowing" from the fuselage, which can affect their range and coverage.
Aircraft antennas serve a variety of purposes, including communication, navigation, and surveillance. They are often referred to as "aerials" by experienced pilots. These antennas assist pilots in communicating with others and navigating the aircraft to its destination. The number and variety of antennas on an aircraft can be confusing, but understanding their individual functions and characteristics can help clarify their purpose and placement.
It is important to note that antennas are a critical yet often overlooked part of an avionics system. Their design and placement are crucial for effective communication and navigation. Antenna maintenance is also essential, as dirty or damaged antennas can negatively impact their performance. Regular cleaning and inspections are necessary to ensure optimal functionality and prevent issues such as intermittent Mode C problems.
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UHF aircraft antennas
UHF stands for ultra-high frequency, and these antennas are used for long-range communication. They are also used in television broadcasting, mobile phone networks, and satellite communications. The shape and placement of UHF aircraft antennas are determined by their directional qualities and the frequencies at which they operate.
Modern antennas come in a variety of shapes and sizes, and each is designed for a specific function. Antenna farms, which consist of dozens of antennas, can be found on the belly of a well-equipped plane. These antennas are used for communication, with each com transmitter having its own antenna. Communication antennas can be mounted on either the top or bottom of the aircraft but are susceptible to shadowing from the fuselage, which can affect their range and coverage.
Marker beacon antennas, which are used to transmit highly directional signals, are typically found on the bottom of the aircraft. They can look like small canoes or have thick wires protruding straight down from the empennage that turn towards the tail. Cessna has also used flush antennas that appear as flat plates under the empennage.
Nav antennas, on the other hand, are almost always found on the vertical tail and come in three types: cat whisker, dual blade, and towel bar. Cat whisker antennas have rods jutting out at 45-degree angles and are good for low-altitude flying as they cannot receive signals from the side. Dual-blade antennas have two blades, one on each side of the tail, while towel bar antennas are balanced loops that can easily receive signals from all directions.
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Loran antennas
Long-range navigation, or Loran antennas, may resemble communication antennas, but they have distinct internal features. Loran antennas typically contain an amplifier, either built into their base to enhance the signal or located just under the skin. They are designed to be placed on either the top or bottom of the plane, and proper configuration of the receiver is essential for their functionality. Loran systems are susceptible to P-static, which occurs due to electrical charge buildup when the plane navigates through heavy dust or rain.
Loran, an acronym for Long Range Navigation, originated as a hyperbolic radio navigation system developed in the United States during World War II. It offered an improved range of up to 1,500 miles (2,400 km) with an accuracy of tens of miles, making it valuable for ship convoys crossing the Atlantic Ocean and long-range patrol aircraft. The original Loran system, now referred to as Loran-A, utilised two frequency bands at 1.85 and 1.95 MHz.
The evolution of Loran-A led to the development of Loran-C, which provided even longer ranges and improved accuracy. Loran-C transmitters operate at peak powers ranging from 100 to 4,000 kilowatts, comparable to longwave broadcasting stations. The antennas used by Loran-C stations vary in design, including mast radiators, tower radiators, and T-antennas. The height of these antennas can range from 190 to 412 metres.
Loran-C's introduction in 1957 was initially limited to military use due to the expense of the equipment required to interpret the signals. However, the advent of solid-state electronics and microcontrollers in the 1970s significantly reduced the cost, weight, and size of the necessary technology. This transformation led to the widespread adoption of Loran-C and the eventual discontinuation of the original Loran system.
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Frequently asked questions
Airport antennas vary in appearance. The primary surveillance radar (PSR) consists of a large parabolic "dish" antenna mounted on a tower. The secondary surveillance radar consists of a second rotating antenna, often mounted on the primary antenna. Antennas on airplanes often look like lightning rods or other interesting protrusions.
Airport antennas are used to detect and display the presence and position of aircraft in the terminal area and the airspace around airports.
Some types of airport antennas include the primary and secondary surveillance radar, UHF aircraft antennas, VHF nav antennas, and marker beacon antennas.
Marker beacon antennas are highly directional and are used to transmit signals to aircraft. They are often located on the bottom of the aircraft and can look like small canoes or thick wires protruding from the empennage.
To improve antenna reception near an airport, select an antenna with a narrow beam width in the vertical axis, such as the DB4 or DB8 antenna.
