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 cps signal. However, because of the action of the modulators, the direction of the RF bearing signal reverses itself.
Through the first. The RF sense signal, on the other hand, does not reverse itself. Thus, for a half cycle of the cps signal, the bearing RF and the sense RF are in 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 same unit has a sense blanking circuit to cut off the kc signal for one half of each of its cycles. The purpose of this is to prevent ambiguity. Here's why: the 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 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 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 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.
This means that if the shaft were rotated at a rate of cps, the signal presented to the balanced modulators would be identical with that presented to the balanced modulators by the cps oscillator and the phasing circuits. If the shaft were rotated at 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. 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. 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.
Thus, the response of a single loop or U-antenna, shows the line of movement of a radio wave but does not distinguish between two possible directions of arrival. A null indicates only that a t ransmitter lies in either of two opposite directions.
To get a response which will show true direction, the response of a directional antenna must be combined with the response of a nondirec-tional sense antenna.
A monopole vertical antenna meets the requirements for a non-directional antenna. You can see the combined response pattern in the accompanying illustration on page It is the figure in solid outline. Its shape is cardioid heart shaped , and it represents the resultant of the directional and nondirectional responses shown in broken outline. The two lobes of the directional response the figure 8 are of opposite polarity. However, the nondirectional response has the same polarity in all directions.
This means that, in combination, the nondirectional response cancels one lobe of the figure 8 and increases the amplitude of the other. The resulting cardioid pattern has only one null. Thus, the null of the cardioid pattern can indicate only one direction. In direction finding, the null is used instead of the maximum, for it permits more accurate bearings than the maximum.
Operation to provide a cardioid pattern showing true direction is called sense operation. Operation to provide a figure-8 pattern showing the plane of movement of a radio wave is called bearing operation.
The receiver used in a direction finder amplifies and detects the varying response of the antenna system and makes it available for the indicator. The direction finder receiver is usually a highly selective superheterodyne.
Cathode Ray Tube Patterns indication. For visual indication a pointer is usually used against a scale calibrated in degrees. On the scale, north either magnetic or true is the reference point,.
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