Antenna Overview For Aircraft #
Antennas are possibly the most underappreciated component of an avionics system, despite their importance; this antenna overview explains this importance. Except for a few boxes (such as autopilots), avionics communicate with the outside world through this antenna overview. Antenna’s today come in a variety of designs and sizes. The function of each antenna shapes it. An antenna farm is often seen on the belly of a well-equipped airplane, and figuring out what each antenna accomplishes might be confusing. However, if you look at those antennae one by one, they become clearer. Their shape and placement are usually determined by the frequencies at which they operate and their directional properties.
Communication Antennas #
The operation of communication antennae is simple. Each com transmitter has its own antenna for a variety of reasons, including redundancy. They can be installed on the top or bottom of the plane, however either installation is subject to shadowing from the fuselage. Structures in the transmitting path of the antenna, such as the vertical stabilizer or landing gear doors, produce shadowing. Understand where your antennae are located and how shadowing may affect their range and coverage.
UHF Antennas #
Transponders and distance measuring equipment (DME) require UHF antennas, which are always situated on the bottom of the aircraft. They’re about four inches long, and because the transponder frequency is in the middle of the DME frequency band, the same antenna can be used for both systems. Spike and blade antennas are the most frequent varieties. Because the antenna length is set to one frequency—the transponder frequency—the spike should only be utilized for transponders. Because it is tuned for a wide range of DME frequencies, the blade antenna is also known as a wideband antenna. A spike antenna would not be suitable for a DME; instead, blade antennas are chosen because to their superior emission pattern.
The spikes are prone to clogging up with oil, decreasing the range of transmission. Cleaning a spike antenna will often quadruple your transponder range and eliminate intermittent Mode C issues. This is true of all antennas; a clogged antenna will not perform to its full potential. Blade antennas are prone to delamination, which detunes the frequency response and distorts the broadcast signal—hence the importance of the biennial transponder inspection.
Nav Antennas #
The cat whisker, dual blade, and towel bar are the three types of VHF nav antennas that are virtually invariably positioned on the vertical tail. The cat whisker is made up of two rods that jut out at a 45-degree angle from each side of the vertical stabilizer. The cat whisker antenna, on the other hand, is not very good at receiving signals from the side. Two blades, one on each side of the tail, make up the dual blade. One on each side of the tail is a towel bar that looks like a standard bathroom fixture. From all directions, the blade and towel bar antennas have equal receiving sensitivity. Almost always, a single nav antenna feeds multiple nav receivers, as well as the glideslope. As a result, if the nav antenna system fails, additional systems will fail as well.
GPS Antennas #
Because GPS satellites transmit fewer than five watts of power, the signal is quite weak by the time it reaches you. As a result, the GPS antenna includes an amplifier built in to amplify the signal to the receiver. Furthermore, because the GPS frequency is so high (in the gigahertz region), the signals move in a straight path. As a result, receiving the signal is subject to airframe shadowing, necessitating the installation of a GPS antenna at the very top of the fuselage. Because of the proximity of the panel units or their antennae, communications radios might generate a lot of interference with GPS. As a result, the com and GPS antennas should be positioned as widely apart as practicable. A com antenna may need to be relocated to the bottom of the aircraft on occasion.
Marker Beacon Antennas #
Because marker beacon signals are highly directional, they must be received practically directly over the transmitting ground station; consequently, marker beacon antennas must be mounted on the bottom of the aircraft. Marker antennas come in a variety of shapes and sizes; the most popular ones resemble small canoes about 10 inches long. Cessna has employed flush antennas that appear to be flat plates under the empennage in some installations. It has also utilized an antenna made up of a thick wire that protrudes straight down from the empennage before turning toward the tail.
Emergency Locator Transmitter Antennas #
Hopefully, you’ll never need to use an emergency locator transmitter antenna, but if you do, know that they’re built to withstand an “unplanned” landing. They are nearly always constructed of a flexible substance and are found on the upper skin of the empennage. However, there are a few outliers; some are hidden in the vertical tail or resemble little com antennae.
Performance Consideration #
The antennae and their physical condition has a significant impact on its performance. Water may enter the antenna through a fracture and induce delamination (a separation of the composite layers), rendering the antenna ineffective. The antenna will vibrate from the slipstream and cause the skin to wear, eventually producing cracks, if the antenna base is not physically sound To ensure a solid electrical connection, the antenna must be electrically linked (grounded) to the aircraft.
This bond may be disrupted if rust seeps underneath the antenna, and the antenna’s effectiveness may suffer as a result. This can be avoided by using sealant around the base of the antenna. Antennas should never have their original coatings painted over; any paint deposit decreases the antenna’s performance. On an airplane, space is limited, and antennas are sometimes the last thing to go. A sturdy mounting, shadowing, other antenna interference, ground planes, and aerodynamics all play a role in antenna placement.