Radio Direction Finder (RDF)

A radio direction finder (RDF) is a device for finding thedirection, or bearing, to a radio source. The act of measuring thedirection is known as radio direction finding or sometimessimply direction finding (DF). Using two or more measurementsfrom different locations, the location of an unknown transmittercan be determined; alternately, using two or more measurements ofknown transmitters, the location of a vehicle can be determined.RDF is widely used as a radio navigation system, especially withboats and aircraft.

RDF systems can be used with any radio source, although thesize of the receiver antennas are a function of the wavelength ofthe signal; very long wavelengths (low frequencies) require verylarge antennas, and are generally used only on ground-basedsystems.

Radio direction finder

PICTURE DIAGRAM OF RDF

Types of Antennas

Yagi antenna

A Yagi-Uda antenna, commonly known simply as a Yagi antenna,is a directional antenna consisting of multiple parallel dipole elements in a line, usually made of metal rods.

Loop aerial

A loop antenna is a radio antenna consisting of a loop (or loops) of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line.

Dipole antenna

Is a straight electrical conductor measuring 1/2 wavelength from end to end and connected at the center to a radio-frequency (RF) feed line.

Parabolic antenna

A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves. The most common form is shaped like a dish and is popularly called a dish antenna or parabolic dish.

Phased arrays

Direction Finding Methodologies

Manual

Pseudo-Doppler DF technique

Watson-Watt

Correlative Interferometry.

Manual

Manual direction finding involves the use of a receiver and hand-held directional antenna : usually some type of yagi / log periodic.

The antenna is moved / rotated until the point of maximum signal strength is determined.

Rotation can also be performed using an antenna mounted on arotor or swivel. Accuracy decreases rapidly as distance to source increases.

Pseudo-doppler DF technique

The pseudo-doppler technique is a phase based DF method thatproduces a bearing estimate on the received signal by measuring the doppler shift induced on the signal by sampling around the elements of a circular array. The original method used a single antenna that physically moved in a circle but the modern approach uses a multi-antenna circular array with each antenna sampled in succession.

Watson-Watt

The Watson-Watt technique uses two Adcock antenna pairs to perform an amplitude comparison on the incoming signal. An Adcock antenna pair is a pair of monopole or dipole antennasthat takes the vector difference of the received signal at each antenna so that there is only one output from the pair of antennas.

Correlative Interferometry

The basic principle of the correlative interferometer consists in comparing the measured phase differences with the phase differences obtained for a DF antenna system of known configuration at a known wave angle (reference data set).

Automatic Direction FindersTheory of operation  A ground station is shown on the charts as Non-Directional Beacon (NDB). They are called non-directional because they don't contain any directional information. The NDBs transmit equally in all directions, like waves caused by a pebble that has been thrown into a pond. Radio waves from an NDB create an electromagnetic field. The electric field is called the E-field and the magnetic field is called the H-field. The E and H fields are perpendicular in space, and their amplitudes vary like a sine wave. If you're not confused yet, keep reading.

Non-Directional Beacon frequencies 

Aeronautical non-directional beacons broadcast on relatively low frequencies (200 to 415 kHz). The ADF will also receive the standard AM broadcast band at 550 to 1600 kHz. Land-based aeronautical navigation aids such as VORs and NDBs have a two- or three-letter identifier broadcast in Morse code. Some NDBs also broadcast audio, usually weather information

Two antennas are better than one

. All ADF systems have both loop and sense antennas. The loop antenna, is usually a flat plate antenna located on the bottom of the aircraft, while the sense antenna is usually a simple wire or foil type antenna imbedded in a fairing. The loop antenna consists of two perpendicular windings on a square ferrite core. The H-field induces a voltage into the two windings of the ADF loop antenna. Because the windings are on a closed loop, the phase angle of the voltages vary as the antenna is rotated. Rotating the loop antenna, you will find there are two points

where the voltages exactly cancel each other out. These points are called nulls. Only one of these nulls points to the NDB. The other null is 180 degrees away fromthe NDB. If we use only a loop antenna we could be heading in the opposite direction. This is not good. The sense antenna determines which null is correct.

The sense antenna simply receives the electric portion of the electromagnetic field and produces a voltage that is always in phase with the transmitter. By measuring the combined voltage of the two windings in the loop antenna and comparing that to the voltage received by the sense antenna, the ADF is able to determine the direction to the beacon.

The receiver will "electronically" rotate the loop antenna (and pointer on the RMI) to achieve a minimum voltage output or null. Prior to the null, the ADF receiver compares the loop antenna voltage to the sense antenna voltage. If both loop and sense antenna signals are in phase prior to the null, they will add to each other. If both signals are out of phase prior to the null, they will subtractfrom each other. By adding and subtracting the two signals, the ADF can tell the difference between the two nulls.

The manufacturer will set one null as the "to" null. For example, if both the loopand sense antenna voltages are in phase and add to each other prior to the null position, the loop antenna is facing towards the transmitter. This will be set as the "to" null. The ADF needs both antennas to correctly point to the station.

Operation and controls 

ADF receivers have several operating modes that the pilot can select. In the ADF mode, both the loop and the sense antennas are used, the pointer is activated, and the ADF tries to point to the station. (This is the normal mode of operation.) Rotating the loop antenna control switch in either the left or right direction will cause the loop antenna and pointer to move left or right. Releasingthe loop antenna control switch causes the loop antenna and pointer to rotate and point to the station again. The ADF indicator (RMI) consists of a compass card and

pointer. In dual ADF systems, the RMI will have two pointers (for systems one and two). The pilot manually sets the compass card or, if the airplane has a flux valve, the compass card is slaved automatically to the aircraft's magnetic heading.

In the ANT mode, the loop antenna is disabled and all receiving is done through the sense antenna. The pointer should not move even if you turn the loop antenna control switch left or right. (This is the best mode for listening to yourfavorite ball game.) It is also good for identifying the station.

If the loop mode is selected, the sense antenna is disabled and all the receiving is done through the loop antenna. Rotating the loop antenna control switch in either direction will cause the loop antenna and pointer to rotate in the selected direction for as long as the switch is held. Releasing the control switch causes the loop antenna and pointer to stay where it is. You may also notice that the reception gets better when the pointer is 90 degrees away from thestation and worse when the pointer is either pointed directly at or 180 degrees away from the station.

Some ADF control panels have a test position. Rotating the mode control switch to "test" will perform a self-test of the ADF system. In the self-test mode, the pointer will move to 45 degrees left of the lubber line to a heading of 315. The lubber line is a little arrow or line at the 12 o'clock position on the RMI.

ADF (Automatic Direction Finder) is the radio signals in the low to medium frequency band of 190 Khz. to 1750 Khz. It was widely used today. It has the major advantage over VOR navigation in the reception is not limited to line of sight distance.

The ADF signals follow the curvature of the earth. The maximum of distance is depend on the power of the beacon. The ADF can receives on both AM radio station and NDB (Non-Directional Beacon). CommercialAM radio stations broadcast on 540 to 1620 Khz. Non-Directional Beacon operate in the frequency band of 190 to 535 Khz. ADF COMPONENTS ADF Receiver: pilot can tune the station desired and to select the mode of operation. The signal is received, amplified, and converted to audible voice or morse code transmission and powers the bearing indicator.

Antenna : The aircraft consist of two antennas. The two antennas are called LOOP antenna and SENSE antenna. The ADF receives signals on both loop and sense antennas. The loop antenna in common use todayis a small flat antenna without moving parts. Withinthe antenna are several coils spaced at various

angles. The loop antenna sense the direction of the station by the strength of the signal on each coil but cannot determine whether the bearing is TO or FROM the station. The sense antenna provides this latter information.

Bearing Indicator : displays the bearing to station relative to the nose of the aircraft.Relative Bearing is the angle formed by the line drawn through the center line of the aircraft and a line drawn from the aircraft to the radio station.Magnetic Bearing is the angle formed by a line drawnfrom aircraft to the radio station and a line drawn from the aircraft to magnetic north (Bearing to station). 

Magnetic Bearing = Magnetic Heading + RelativeBearing.

 TYPE OF ADF INDICATORFour types of ADF indicators are in use today. In every case, the needle points to the navigation beacon.Those four types are: 

Fixed Compass Card : It is fixed to the face of instrument and cannot rotate. 0 degree is always straight up as the nose of aircraft.

The relationship of the aircraft to the station is refered to as " bearing to the station " MB or aircraft to magnetic north. This type of indicator, pilot must calculate for the bearing by formular 

MB = RB + MH

Rotatable Compass Card : The dial face of the instrument can be rotated by a knob. By rotating thecard such that the Magnetic Heading (MH) of the aircraft is adjusted to be under the pointer at the top of the card.

The bearing to station (MB) can be read directly from the compass card without calculation and make

it easy for pilot. Today , they designed automatically rotate the compass card of the instrument to agree with the magnetic heading (MH) of the aircraft . Thus MB to station can be read at any time without manually rotating the compass card on the ADF face. 

Single-Needle Radio Magnetic Indicator : Radio Magnetic Indicator is an instrument that combines radio and magnetic information to provide continuousheading , bearing , and radial information.

The face of the single needle RMI is similar to thatof the rotatable card ADF.

Dual-Needle Radio Magnetic Indicator : The dual needle RMI is similar to single needle RMI except that it has a second needle. The first needle indicated just like single needle. inthe picture , the yellow needle is a single which indicate the Magnetic Bearing to the NDB station . The second needle is the green needle in the picture.

The second needle (green) is point to VOR station .The dual needle indicator is useful in locate the location of an aircraft.OPERATIONADF operate in the low and medium frequency bands. By tuning to NDB station or commercial AM radio stations. NDB frequency and identification information may be obtained from aeronautical chartsand Airport Facility Directory. The ADF has automatic direction seeking qualities which result in the bearing indicator always pointing to the station to which it is tuned. The easiest and perhaps the most common method of using ADF , is to " home " to the station . Since the ADF pointer always points to the station , the pilot can simply head the airplane so that the pointer is on the 0 (zero) degree or nose position when using a fixed card ADF . The station will be directly ahead of theairplane. Since there is almost always some wind at altitude and you will be allowing for drif, meaning that your heading will be different from your track.Off track , if the aircraft is left of track, the head of the needle will point right of the nose. If

the aircraft is right of track , the head of the needle will point left of the nose.

For fixed compass card , if you are not fly Homing and you want to fly heading at some degrees. You must use the formula MB = MH + RB to find out what degree the ADF pointer should be on. Today , the fixed card indicator is very unsatisfactory for every day use which can still be found on aircraft panels but not many planes that pilot actually uses it due to it has easier type of indicator.

For rotatable compass card, it was a big step over the fixed card indicator. The pilot can rotate the compass card with the heading knob to display the aircraft MH " straight up " . Then the ADF needle will directly indicate the magnetic bearing to the NDB station.

For Single needle Radio Magnetic Indicator , the compasscard is a directional gyro and it rotates automatically as the aircraft turns and provide continuous heading . It is accurately indicates the magnetic heading and the magnetic bearing to the beacon. This instrument is a " hands off " instrument.

For dual needle Radio Magnetic Indicator, it is give the pilot information the same as the single needle such as aircraft heading and magnetic bearingto the NDB . The second indicator will point to VOR station. This help pilot to check the location of the aircraft at that time.

Technological Institute of thePhilippines -Manila

1338 Arlegui St., Quiapo Manila

ECE122 (Electronics Navigational Aids)

EC42FA1

RDF & ADF Draft Report

Submitted to:

Engr. Philander U Lomboy

Submitted by:

Bolido,Kathryn Ann Z.

Cariño, Fritzie Faye S.

Date:

July 24,2015