It also has used an antenna that consists of a thick wire that protrudes straight down out of the empennage and then makes a turn toward the tail. For some installations, Cessna has used flush antennas that appear to be flat plates under the empennage. There are a few different types of marker antennas the more common types look like little canoes about 10 inches long. Marker beacon signals are highly directional, which means you have to be almost directly over the transmitting ground station to receive them therefore, marker beacon antennas need to be on the bottom of the aircraft. Sometimes a com antenna must be relocated to the bottom of the aircraft. Therefore, it is important that the com and GPS antennas be mounted as far apart as possible. This makes receiving the signal susceptible to airframe shadowing, thus mandating that a GPS antenna be mounted at the very top of the fuselage.Ĭommunications radios can cause a lot of interference with GPS, because of the proximity of the panel units or their antennas. Additionally, the GPS frequency is so high (in the gigahertz band) that the signals travel in a line-of-sight manner. Because of this, the GPS antenna has a built-in amplifier to boost the signal for the receiver. GPS satellites transmit less than five watts of power, so by the time the signal reaches you, it is very, very weak. Therefore, a failure in the nav antenna system would cause multiple systems to malfunction. The blade and towel bar antennas have equal receiving sensitivity from all directions.Ī single nav antenna almost always feeds mulitple nav receivers and sometimes the glideslope as well. The towel bar resembles the common bathroom fixture, one on each side of the tail. The dual blade is just that, two blades, one on each side of the tail. But the cat whisker antenna is poor at receiving signals from the side. The cat whisker consists of a couple of rods jutting out from each side of the vertical stabilizer at a 45-degree angle. The VHF nav antenna is almost always mounted on the vertical tail, and there are three types: the cat whisker, the dual blade, and the towel bar. Blade antennas are susceptible to delamination, which tends to detune the frequency response and distort the transmitted signal-that’s why the biennial transponder check is so important. This goes for all antennas a dirty antenna does not perform up to its potential. Often, just cleaning a spike antenna doubles your transponder range and gets rid of those intermittent Mode C problems. The spikes are prone to caking up with oil, reducing the transmitting range.
A spike would not work very well for a DME the blade antennas are preferred because the radiation pattern is better.
The blade antenna is also called a broadband antenna because it is tuned for a range of DME frequencies. The spike should only be used for transponders because the antenna length is tuned to one frequency-the transponder frequency. Two types are commonly used: spike and blade antennas. They are about four inches long, and the same antenna can be used for both systems because the transponder frequency is in the middle of the DME frequency band. UHF antennas are commonly used for transponders and distance measuring equipment (DME), and they are always found on the bottom of the aircraft. Know where your antennas are and how shadowing may affect their range and coverage. Shadowing is caused by structure, such as the vertical stabilizer or landing gear doors, in the transmitting path of the antenna. They can be mounted on either the top or bottom of the aircraft, but each installation is susceptible to shadowing from the fuselage. Each com transmitter has its own antenna, mostly for redundancy. Communication antennasĬommunication antennas are basic in operation. The frequencies at which they operate and their directional qualities usually determine their shape and placement. But taken one by one, those antennas are easier to understand. Often, a well-equipped airplane will have an antenna farm on the belly, and it can be confusing to try to figure out what each antenna does. Modern antennas come in many different shapes and sizes. Except for a few boxes (such as autopilots), avionics rely on antennas to talk with the outside world. Antennas are probably the most overlooked part of an avionics system, yet they’re among the most important.
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