Functional Analysis of a Typical Direction Finder

Examine the block diagram of a typical direction finder used for intelligence on page 153. It has, of course, the three basic units antenna, receiver, and indicator. In addition, it has a modulating voltage generator and an azimuth indicator. These two units are not used at the same time. One is a substitute for the other. When the modulating voltage generator unit is used, instantaneous electrical indication is provided by the cathode ray tube indicator. When the azimuth indicator unit is used, indication is by means of an aural null associated with a manually operated indicator.

Antenna array. The usual antenna array for a typical direction finder consists of four monopole vertical antennas arranged in the form of a square with the antennas oriented to north, south, east, and west. The total output of the four antennas is developed across a combining impedance and represents the vectoral sum of the outputs of the individual antennas. The phase of the output voltages of the individual antennas depends on the direction of the RF wave. The total output developed across the combining impedance contains directional information.

In a direction finder used for intelligence work there is usually an arrangement by which the length of the monopole antennas can be varied to conform to the various wavelengths at which a transmitter may be operating. Such an array of monopole antennas cannot be used in place of a loop antenna in most navigational work. The array is too big and cumbersome to be rotatable. However, the effect of rotation can be achieved by use of a device called a goniometer.

A goniometer is a special kind of transformer. The primary consists of two coils arranged at right angles to each other. Each coil develops the output of two monopole antennas of opposite phase. The secondary consists of a single rotatable coil. When the secondary coil is rotated within the fields of the two primary coils, the response is the same as if the antenna array were rotated.

Modulating Voltage Generator. The modulating voltage generator unit contains an oscillator which supplies a 147-cps signal. This audio signal is used to synchronize the response of the cathode ray tube indicator with the response of the antenna circuits. The signal also modulates locally any RF wave picked up by the antennas. Thus, the direction finder receiver can detect a response even though the RF wave may be unmodulated. It can also detect a response if the RF wave represents FM, SSB, or pulse transmission.

The 147-cps signal passes through a phase splitter circuit and two phase inverter circuits before it is applied to the balanced modulators of the antennas. The 90° phase splitter produces two versions of the original 147-cps signal, one 90° out of phase with the other. Each of these two signals is passed through a phase inverter. Each inverter produces two signals 180° out of phase. This means that the two inverters put out a total of four signals. As the waveforms on the block diagram show, these signals are 0°, 90°, 180°, and 270° out of phase with the original 147-cps signal.

The four phased signals are fed to the four balanced modulators. There is one balanced modulator at the base of each of the four monopole antennas. In each balanced modulator, the phased 147-cps signal is mixed with the output of the antenna. Thus, the output of each balanced modulator consists

Direction Finder Azimuth Indicator

o m n m of an RF signal modulated by a phased 147-cps signal. The phase of the RF signal is determined by the direction of its arrival at the antenna. The modulation is the phased 147-cps audio signal. The output of the combining impedance consists of the vectorial sum of the four RF signals modulated by the vectorial sum of the 147-cps signals. The vectorial sum of the RF signals depends on the bearing and direction of the RF while the vectorial sum of the 147-cps signals shows time. This means that the directional response of the antennas occurs at a definite point in the 360° of one cycle of the 147 cps.

Bearing indicator unit. Note that the original 147-cps signal is also fed into the bearing indicator unit. Here it passes through a phase splitter and two phase inverters. This produces four 147-cps signals, out of phase 0°, 90°, 180° and 270° with the original signal. These four phased signals are passed through balanced modulators and applied to the vertical and horizontal plates of the cathode ray tube. There, they cause the trace of the tube to rotate 360° for each cycle of the 147-cps signal. The face of the tube is calibrated in degrees. Since the circular movement of the trace is controlled by the 147-cps signal, the directional response of the antenna is applied to the scope at the same point in the 360° of the 147-cps signal as the point at which it occurs in the indicator unit.

Note that the bearing indicator unit contains a 200-kc oscillator. This 200-kc signal is eventually applied to the cathode ray tube. There it provides the voltages that cause the trace to move from one side of the tube to the other, with one complete movement back and forth for each cycle. The 147-cps phased signal is also applied to the cathode ray tube to cause the trace to move in a circular direction through 360° around the face of the tube. The trace moves through 360° of rotation for each full cycle of the 147-cps signal. Thus, the circular movement of the trace is coordinated with the antenna response, since both are governed by the 147-cps signal.

For the indicator response pattern, the voltages of the antenna response add to or subtract from the voltages of the 200-kc signal. This means that in some directions the trace moves clear to the edge of the scope, but in other directions, the trace does not move from the center of the scope. Thus, the bearing pattern which results is a propeller-shaped pattern. The tips of the propeller-shaped pattern indicate the null points in the antenna's response pattern and thus the line of direction.

Sense operation. Since there are two propeller-shaped tips to the pattern, the indication is ambiguous as to direction of arrival. Sense operation to resolve this ambiguity is necessary. A single switch puts sense circuits into operation.

Note that sense operation in this system does not require a separate nondirectional antenna. The sense signals are taken from the same antennas that supply the balanced modulators. This is made possible by the balanced modulators. The output of a balanced modulator is an RF signal modulated by the 147-cps signal. However, because of the action of the modulators, the direction of the RF bearing signal reverses itself. Through the first. 180° of the 147-cps signal it has the opposite phase of what it has through the other 180° of the 147-cps signal. The RF sense signal, on the other hand, does not reverse itself.

Thus, for a half cycle of the 147-cps signal, the bearing RF and the sense RF are in phase. For the other half cycle, they are 180° out of phase. When the RF signals are in phase they add; when they are out of phase they cancel. The resultant response is the characteristic cardioid pattern. There is only one null and it shows direction. The resultant pattern of all four antennas developed across the combining impedance is a cardioid pattern showing the true direction of arrival of the RF wave.

The cardioid sense pattern has its null displaced 90° from the two nulls of the bearing pattern. To keep the indicator from being 90° in error, therefore, the 147-cps synchronizing signal applied to the indicator is passed through the sense pattern rotation circuit in the indicator unit.

The same unit has a sense blanking circuit to cut off the 200-kc signal for one half of each of its cycles. The purpose of this is to prevent ambiguity. Here's why: the 200-kc signal pro vides the sweep voltage for the indicator tube. It provides a complete sweep for each half cycle. This means that the propeller-shaped pattern, formed with no blanking, is really a double pattern, one pattern superimposed on the other. The two patterns are opposite in polarity. Yet, since each pattern is composed of two identically shaped lobes, this difference in polarity does not matter in bearing operation. However, for sense operation, which requires a pattern of only one lobe, this difference in polarity would produce ambiguous results. The two patterns produced by a double sweep would not be superimposed. They would be produced in opposite directions, and no sense indication could be obtained. This difficulty is overcome by using a single sweep for sense operation. Blanking one half of each 200-kc cycle provides the single sweep.

Azimuth indicator. The azimuth indicator provides an alternate means of getting bearing and sense indications. For azimuth indicator operation, the azimuth indicator unit is switched in to replace both the modulating voltage generator unit and the bearing indicator unit.

To replace the modulating voltage generator, the azimuth indicator unit uses a sinusoidal potentiometer. The sinusoidal potentiometer is a variable resistance across which a DC voltage is applied. The potentiometer has four output arms coupled to a common, rotatable shaft. An azimuth scale is affixed to this shaft and serves as indicator.

As the shaft is rotated through 360°, the DC potential at the end of one of the potentiometer arms rises from zero to a maximum positive voltage. As rotation continues, the potential falls back to zero. With further rotation, it goes to a maximum negative; then, it rises again to zero. Thus, during a complete cycle, voltages at the end of one arm correspond to the voltages produced at an AC source.

If the shaft were rotated at a rate of 147 cps, the output of one arm would be identical with the oscillator output of the modulating voltage generator. The output of each of the other potentiometer arms would be similar. However, the four arms point in four different directions. Therefore, the four voltages at the ends of the arms at any one instant are 90° apart in phase. If one represents 0°, the others represent 90°, 180°, and 270°.

This means that if the shaft were rotated at a rate of 147 cps, the signal presented to the balanced modulators would be identical with that presented to the balanced modulators by the 147-cps oscillator and the phasing circuits. If the shaft were rotated at 147 cps, the sinusoidal potentiometer would do exactly the same job as the modulating voltage generator. However, for azimuth operation, it is not necessary to rotate the shaft that fast. The phased voltages presented by the potentiometer arms are DC voltages. They are present even when rotation is stopped. These phased DC voltages are supplied to the modulators and react with the signals of the four antennas to give a response pattern which shows bearing. When the sense circuits of the antenna system are connected, a sense response results.

Thus, operating the azimuth unit for bearing indication produces the characteristic response pattern with two nulls. Operating the azimuth unit for sense indication produces the characteristic response pattern with a single null. The nulls are detected aurally in the receiver output by rotating the shaft of the sinusoidal potentiometer. An azimuth scale and a pointer are mounted on the shaft and are calibrated for bearing and sense operation. Consequently, the calibrated position of the shaft when aural null is reached can be used to indicate either bearing or sense.

The azimuth dial consists of two scales (red and white). These are calibrated in degrees (0 to 360) and displaced 180° from each other. In operation, the azimuth dial is rotated to either of the two null positions. Then the sense switch is thrown to either the red or white position. In the vicinity of the null, the aural signal will be louder in one position than in the other. The position that gives the louder response indicates the direct or true azimuth. The exact indication is therefore on the azimuth scale that has the same color as the sense switch position which produces the louder signal.

The general system just described is basically the system of all homing and direction-

finding equipment. Airborne homing equipment usually has a rotatable directional antenna (the loop) and an indicator coordinated either electrically or mechanically with the position of the antenna. For homing opera tion, the RF signals (from the radio beacons) are usually coded to identify each station. These coded signals can appear visually on a cathode ray tube indicator, or can be heard aurally in a headset.

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