RADIO DIRECTION FINDING
The system of worldwide marine radio beacons is one of the oldest and most frequently used radionavigation systems in the world. There remain many advantages to the use of radio direction finders in navigation. Bearings can be taken off any transmitting station if the location of the antenna is indicated on a chart. A position can be fixed using two or more radio beacons. A radio bearing can be used to back up a visual bearing as well as any other type of navigational fix. There are over 900 radio beacons worldwide, a radio directional finder can be used for coastal navigation just about anywhere. Prime importance is the ability of one vessel to take a bearing on a second vessel that is transmitting a distress message.
All radio beacons are found in the 285 kHz to 315 kHz range. Modern radio direction finders are also equipped with the capability of receiving 500 kHz, the international calling and distress frequency for radiotelegraph, as well as 2182 kHz, the international calling and distress frequency for single-sideband. With the exception of calibration beacons, special low-power transmitters used for calibrating RDF receivers, most marine radio beacon transmitting stations operate 24 hours a day. In the United States and Canada the radio beacons are sequenced. Six stations broadcast on the same frequency, one at a time, in a order. This allows the you to get sufficient bearings for a fix without changing frequencies.
There are two types of receivers found aboard ship. They are the traditional radio direction finding receiver (RDF), also known as an aural null indicator, and the auto directional finder. The older style RDF receiver depends on the ability of the user to listen and, sometimes with visual assistance, locate the null of a transmitted signal to determine direction. The auto direction finder locks onto the signal's carrier wave to determine direction. It should be understood that in the case of radio direction finding it is the null of the signal, not its strongest point, that one uses to determine a station's direction. This is because you can more precisely locate the null rather than the strongest signal, usually to within plus or minus 2 degrees. At times the radio direction finder will have a visual indicator such as a meter or split beam tube to assist the user in more accurately determining the null. The modern auto direction finder can be used in a manual mode, as an RDF, or in an auto mode. When in auto mode, the unit searches for the direction of the carrier wave and then locks onto the broadcast signal.
There are three types of antennas used with radio beacon receivers.
One is the older and now less common movable loop antenna which is a circular antenna connected to a hand wheel above the receiver. This is turned by the user to find the signal. The second is the more common crossed loop antenna, known as a Bellini-Tosi antenna. This type of antenna has two fixed loops that are mounted rigidly together usually on a mast. The third type is the rotating loop antenna which is a circular antenna that rotates at a rapid speed within a protected case. There is a fourth type of antenna more commonly found on small portable radio direction finders. This is the bar antenna discussed earlier. RDF loop antennas work on a principle similar to water traveling over a waterwheel. RF energy is picked up by the antenna and induces a current which moves in a circular pattern around the loop. The loop corresponds to the wave of energy passing by it thus creating a current flow in a single direction. When the leading edge of the antenna is aligned with the direction of the transmitter, the signal is the strongest. When the loop is perpendicular to the direction ofthe transmitter, the RF energy will strike the antenna face and flow in two directions at the same time. When the current reaches the receiver it is in the form of different polarities. The current literally cancels itself out; it is at this point that the null is indicated. A direction indicator is aligned with this point on the loop.
While the earlier style directional loops employed this system, the newer crossed loop antenna employs the same principle but determines direction in a slightly different manner. Because there are two loops perpendicular to each other, the antenna as a whole will pick up the signal at varying strengths in different areas of the antenna. The signals then pass to the receiver where a device called a goniometer separates the incoming signals and by measuring their varying strengths determines direction. A direction indicator is connected to the goniometer.
An RDF loop antenna will pick up a signal when either edge of the loop is aligned with the transmitter; the user can have a potential 180-degree error in his radio bearing. This is called ambiguity, and is eliminated by the simple addition of a sense system to the unit. The sense system consists of a second whip antenna that is connected to the receiving circuit. A switch allows the current flow from the second antenna to mix with the flow from the loop antenna. Ifthe polarities are the same the signal direction will be indicated by the stronger mixed signal. If the polarities are different, the input from the sense antenna will cancel the input from the loop antenna and no signal will be heard, thus indicating the loop is facing the wrong direction. All loop antennas have a definite positive and negative side which allows the system to function. The user should carefully consult the instructions for his or her particular unit to become familiar with the specific manner in which the sense system works.
Auto direction finders operate in a similar manner. The unit has a goniometer with a motor attached. The goniometer focuses in on the null point ofthe received signals, and the motor moves the direction indicator to allow the user to determine a bearing to the station.
Rotating antenna systems operate slightly differently with the moving loop picking up a signal as it passes through the RF field. The rotating loop receiver utilizes a cathode-ray tube indicator to show the RF pattern as perceived by the rotating antenna. The RF pattern can be narrowed to determine quite accurately the direction of the transmitter.
