Radio direction finder
From Wikipedia, the free encyclopedia
A radio direction finder (RDF) is a device for finding the direction to a radio source. Due to radio's ability to travel very long distances "over the horizon", it makes a particularly good radio navigation system for ships and aircraft that might be flying at a distance from land.
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[edit] Introduction
RDFs work by pointing a directional antenna in "various directions" and then listening for the direction in which the signal from a known station comes through most strongly. This sort of system was widely used in the 1930s and 1940s. RDF antennas are particularly very easy to spot on German World War II aircraft, as loops under the rear section of the fuselage, whereas most US aircraft enclosed the antenna in a small teardrop-shaped fairing.
[edit] Method of operation
In more recent times the task of finding the signal has been automated in the automatic direction finder, or ADF. In one type of ADF system the antenna consists of a small cylinder of wire, a highly directional solenoid, which is spun by a motor. The electronics listen either for the repeated "peak" in the signal or, more commonly, for the "trough" when the signal drops to zero because the antenna is at right angles to the signal. A small lamp attached to a disk is timed to spin at the same speed as the antenna, so when the peak or trough is detected the lamp flashes briefly. To the human eye it appears to be a single spot of light on a compass rose.
[edit] Usage in navigation
Signals are provided in the form of radio beacons, the radio version of a lighthouse. The signal is typically a simple AM broadcast of a morse code series of letters, which the RDF can tune in to see if the beacon is "on the air". Most modern detectors can also tune in any commercial radio stations, which is particularly useful due to their high power and location near major cities.
RDF was once the primary form of aircraft navigation, and strings of beacons were used to form "airways" from airport to airport. In the 1950s these systems were generally being replaced by the VOR system, in which the angle to the beacon can be measured from the signal itself, with no moving parts. Since the signal being broadcast in the RDF system is non-directional, these older beacons were referred to as non-directional beacons, or NDB in the aviation world.
Today all such systems are being generally removed in favour of the much more accurate and user-friendly GPS system. However the low cost of ADF systems today has meant a comeback, whereas the expensive VOR systems will likely all be switched off in the near future.
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[edit] Automatic direction finder (ADF)
An automatic direction finder (ADF) operates in a low frequency band of 190 to 535 kHz slightly below the AM broadcast frequencies. ADFs consist of a receiver, a demodulator and an output indicator. The ADF equipment listens to all transmitted waves broadcast on the ground stations [non-directional beacon](NDB) isolated frequency, uses a demodulator to unscramble the 400 or 1020 Hz modulated signal being broadcast by the NDB, and then displays the output to the user by rotating a navigational direction needle towards the stations based on the strongest received carrier wave. The NDB transmits radio waves uniformly in an outbound 360 degree wave packet—similar to light waves being transmitted in all directions from a light bulb. As with a light bulb's light waves, radio waves lose their energy with respect to distance. The ADF receiver isolates the received radio waves and isolates the strongest received wave peak. If the amplitude of the received waves was graphed the graph would represent a bell curve, with the highest amplitude wave representing the best direction to the NDB transmitting ground station. In addition, the angles leading away from that heading would fall off on both sides as they represent the weaker received waves being broadcast out to the right and the left of the NDB station. The ADF's direction needle will always point directly to the broadcast transmission tower, regardless of the ADF's attitude or heading. An ADF can be used to determine your current position, track inbound and outbound, and intercept a desired bearing. These procedures are used to execute holding patterns and non-precision instrument approaches.
[edit] Typical NDB services ranges
| Class of NDB | Transmission Power | Effective Range |
|---|---|---|
| Compass Locator | below 25 watts | 15 NM |
| MH | below 50 watts | 25 NM |
| H | 50 to 1,999 watts | 50 NM |
| HH | 2,000+ watts | 75 NM |
[edit] Station passage
When you are near the station, slight deviations from the desired track result in large deflections of the needle. Therefore, it is important to establish the correct drift correction angle as soon as possible. Make small heading corrections (not over 5°) as soon as the needle shows a deviation from course, until it begins to rotate steadily toward a wingtip position or shows erratic left/right oscillations. You are abeam a station when the needle points to the 90° or 270° position. Hold your last corrected heading constant, and time station passage when the needle shows either wingtip position or settles at or near the 180° position. The time interval from the first indications of station proximity to positive station passage varies with altitude—a few seconds at low levels to 3 minutes at high altitude.
[edit] Homing
The ADF may be used to “home” in on a station. Homing is flying the aircraft on any heading required to keep the needle pointing directly to the 0° RB position. To home into a station, tune the station, identify the Morse code signal, then turn the aircraft to bring the ADF azimuth needle to the 0° RB position. Turns should be made using the heading indicator. When the turn is complete, check the ADF needle and make small corrections as necessary.
[edit] Tracking
The ADF may also be used to "track" a desired course using a known ADF station transmission and the current winds aloft which will influence the aircraft position along the desired route. Tracking is similar to "homing". Tracking is maintaining a geographically straight path or track by which the winds aloft have been factored into to the aircraft heading to evaluate a ground track that will yield the desired track over the ADF station. The main differences between homing and tracking is that in "tracking" the calculated track heading will provide the aircraft with a direct ground track course to the desired ADF and in "homing" the pilot will make several adjustments in the aircraft heading to maintain the orientation of the azimuthal needle to point direct in front of the aircraft. Tracking will yield the shortest distance and time to the ADF location. Homing will yield a curved and contoured ground track line to the ADF location with a gradual reduction in the curvature as the aircraft becomes closer and more on the correct path to the ADF station.
[edit] See also
- Amateur Radio Direction Finding
- Cardioid
- Battle of the Beams
- radio navigation
- VOR/DME
- Radio direction finding
- Traffic analysis
- Huff-Duff
- Wullenweber
- Real time locating
