Dme Antenna Aircraft - If you're an airplane fan, chances are you've wondered what all those antennas were for at some point. How do pilots communicate with people on the ground? Where does airplane WiFi come from? What antennas do they use for what? Some of these questions have interesting answers, but none of them are complicated or difficult to understand.
On any airplane, often on its belly, you'll find dozens of antennas, each used for a different purpose. These antennas, called aerials by many pilots who have been in the business for a while, are basically there to help pilots communicate with other people, and most of them look like lightning rods or other interesting protrusions.
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Aircraft antennas can come in many different shapes and sizes, largely determined by the manufacturer. However, antennas are designed more for their function than anything else, and their shape and placement are usually determined by their directivity qualities and the frequencies they use to operate. Essentially, these antennas must have certain shapes and be placed in certain locations to work properly.
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When most lay people think of antennas on airplanes, they assume they are there for efficient communication, which is true. Com antennas are usually mounted on both the top and bottom of the aircraft, and their only concern is being affected by the shadow of the fuselage. Each transmitter has its own antenna, and the antennas are placed strategically, mainly because their range and coverage can be adversely affected if they are placed incorrectly.
The way they work is quite simple and their placement is crucial for them to be effective in their purpose. For example, a radio that feeds the top antenna usually works best for communications when the plane is still on the ground, while one that feeds the antenna on the bottom of the plane usually works best when the plane is in the air. It's not hard to see why.
Emitting less than five watts of power, GPS antennas result in signals that are typically very weak. Because of this, most GPS antennas consist of built-in amplifiers designed to amplify the signal for the receiver. Additionally, the GPS frequency is very high, usually in the gigahertz range, requiring a GPS antenna to be attached to the top of the trunk.
Communication antennas can interfere with GPS antennas, which means that the two antennas should be placed as far apart as possible. Suction cup antennas are often used with handheld GPS, but they can spell disaster if placed in certain areas, such as windows. This is just one reason why IFR certification with handheld GPSs is unlikely to happen anytime soon.
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Remote navigation or Loran antennas look a lot like communications antennas until you look inside. Loran antennas usually contain either an amplifier built into their base so that the signal is better than a smaller amplifier under the skin. They are designed to be mounted on the top or bottom of the aircraft, but you must adjust the receiver to match the correct antenna position for them to work properly.
Loran systems are very sensitive to P-static, which is caused by the build-up of electrical charge when the aircraft flies through heavy dust or rain. However, if you connect the aircraft structures and antennas properly, this often prevents this from happening. Static build-up also occurs when the vinyl stickers found on the vertical fin decide to attract static build-up and other types of interference.
Aircraft antennas also include loop antennas, which are shaped like, you guessed it, rings. They are also called directional antennas because they can actually determine which direction the signal is coming from. They consist of two or three separate coils, which make them look like a flattened cookie, and each signal is received at a different strength between the coils.
Loop antennas are usually short and wide, hence their cookie shape, and can be found on both the top and bottom of an aircraft, although they are usually located on the bottom. These are the types of antennas commonly used by lightning detection systems. They tend to retain oil and water, so a good seal is always recommended to prevent water build-up and extend the antennae.
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Marker beacon antennas must be on the underside of the aircraft, as the antennas must be almost directly above the transmitting ground station to receive any signal. There are many different types of beacon antennas on the market, with the most common looking like small canoes that are about 10 inches long. They are both simple and reliable.
Cessna has used modified versions of the marker beacon antenna with great success. These include flush antennas under the fuselage that look like flat plates, and antennas that have thick wires that stick straight down and then turn towards the tail of the aircraft. Both of these types of marker beacon antennas are very successful.
Almost always found in the vertical tail, nav antennas come in three basic types: The cat's whisker has several rods that protrude from each side of the stabilizer at a 45-degree angle. It's a good antenna if you're flying low because it can't pick up signals from the side. The second type, the double blade, has antennae on both sides of the tail.
A third type of ship antenna, the towel bar, is a balanced loop antenna that can easily receive signals from all directions. Wiper antennas are located on either side of the tail of the aircraft and are often required for area navigation (RNAV) systems.
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These antennas, which look like six inch square plates, are mounted on the floor of the plane. They are usually either one or two antenna systems and the radar signal is transmitted straight down and literally bounces off the ground. Radio altimeters involve high frequencies and therefore require a secure electrical connection to the aircraft skin.
You can determine the distance above the ground by measuring the time between the transmission of the signal and the reception of the signal. Again, secure antenna connection is a must. otherwise the system talks to itself and produces false readings.
Primarily used for range-measuring equipment (DME) and transponders, UHF aircraft antennas are only about four inches long and are always located on the floor of the aircraft. They can be used for both DMEs and transponders, and the two main types are blade and spike antennas. Spike antennas should only be used for transponders, while blade antennas work best with DMEs.
When an aircraft's landing gear is down, it can shadow UHF antennas due to their small size, and protruding antennas are even susceptible to problems such as misaligned cleaning brushes. Biannual transmitter inspections are also highly recommended, in part because blade antennas can collect oil and water and thus interfere with the transmitted signal.
Electronic Navigation Research Institute
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