A Cathode-Ray Direction Finder

Author: J.B. Hoag

In one system of direction finding which has been applied to the reception of atmospheric pulses (static) and other equally important work of similar nature, two loops or directive antennas are mounted on the same shaft, at right angles to each other. Assume that one of these has its plane in the north-south line, the other in the east-west line, and that they are connected through suitable amplifiers to the deflecting plates of a cathode-ray oscillograph, as in Fig. 34 S.

Fig. 34 S. A cathode-ray direction finder

Voltages in the N-S loop move the spot vertically while those induced in the E-W loop give a horizontal deflection. If the signal arrives from the east, a horizontal line appears on the screen, of a length proportionate to the strength of the signal. If the signal is from the north, a vertical line is formed. If the signal arrives from the northeast, the spot is deflected simultaneously in both directions and therefore travels along a line at 45° to the E-W or N-S line. If the signal arrives from some other direction, say more nearly north, the voltages in the N-S loop will be greater than those in the E-W loop, the spot will be deflected by a larger amount in the vertical than in the horizontal direction, and will mark out a line on the screen which literally points out the line of transmission and has a length proportionate to the strength of signal.

In order to determine the direction or sense of the transmission along this line, a non-directive single-wire antenna is mounted along the vertical axis of the loops. Voltages from this are amplified and applied to the control grid of the cathode-ray tube so as to change the brightness of the light on the screen. The signal from the vertical antenna is properly phased with the signals from the loops. Suppose, then, that the signal comes in from the east, not from the west. The E-W loop moves the spot into a horizontal line while the single-wire antenna brightens it from the origin to the right and back again, but dims or cuts it off entirely to the left of the origin.

Certain necessary adjustments must be made for proper operation of this system. It is assumed that the X-plates give horizontal lines and the Y-plates give vertical lines. To test this point, a and b of Fig. 34 S are joined together and to a single alternating source. Then the line on the screen will be formed at its true " 45° " position (even if the tube be rotated about its axis). A scale marked off in degrees is then set with its 45° mark coinciding with the line. There are electrical methods of setting the line at any desired angle: by keeping the phases on a and b the same, but changing their relative strengths. Furthermore, the gain of the N-S and E-W amplifiers must be the same. To be sure of this, a common test oscillator is used to inject equal voltages into the two loops, at which time the line on the screen must appear at 45°, or the gain of one of the amplifiers must be altered until it is at 45°. If the pattern is an ellipse instead of a circle, the N-S and E-W units do not have the same time delay or phase relationship. (In fact, the ellipses have been used¹ as a means of studying phase shifts.) Phase correction can be made by changing the capacitance of a condenser connected across one pair of the deflecting plates. The test oscillator should be applied to the input of the superhets, while the loops are shorted; then, with the loops in operation, in order to check for unsym-metrical conditions in them. It will be found that " closing the ellipse " is a sharper and better final check on the complete identity of the two systems than the angle setting. One-half a degree phase difference will noticeably open a straight line into an ellipse. Complete screening of the test oscillator is essential.

To adjust the phase and magnitude of the control-grid voltage from the vertical or " sense" antenna, the test oscillator is applied to it, and to first one of the loops, then the other. The sense system is altered in gain and phase until the line (E-W or N-S) is undistorted and extends only from the origin (center of the screen) in one direction. For the final checkup, all three systems are put into operation and a test oscillator (not wired to any portion of the outfit) is moved around the loops, while directions are observed on the screen. In this, and in practical operation, the loops will need to be set by hand, with their planes roughly at 45° to the oncoming signal. This is to prevent the strong signal from the loop whose plane is more nearly that of the direction of transmission, from becoming so great as to overload the amplifier. Such distortion will give false direction readings.