Practical_Antenna_Handbook_0071639586

C h a p t e r 2 3 : R a d i o D i r e c t i o n - F i n d i n g ( R D F ) A n t e n n a s 527
at an arbitrary angle to the line between two antennas. If the wavefront is parallel to
the antenna axis, it will arrive at both antennas at the same time. But if the signal arrives
at an angle, it will arrive at one antenna some time before it reaches the second.
From the difference in the arrival times at the two antennas we can calculate the arrival
angle.
There is an ambiguity in the basic TDOA array in that the combined output will be
the same for conjugate angles, i.e. at the same angle on opposite sides of a line bisecting
the array. This problem can be resolved by adding the electronic system of Fig. 23.15B.
Signals from ANT1 and ANT2 (designated V 1 and V 2 , respectively) are detected by
separate receivers (RCVR1 and RCVR2) and then threshold detected in order to prevent
signal-to-noise problems from interfering with the operation. The outputs of the threshold
detectors are used to trigger a sawtooth generator that controls the horizontal sweep
on an oscilloscope. The two signals are then delayed, and one is inverted so that the
operator can distinguish between their traces on the CRT screen, thus eliminating the
ambiguity.
Example: If the antennas in Fig. 23.15A are arrayed east and west, the line perpendicular
to the line drawn between them runs north and south. If we designate north as
0 degrees, the signal shown arrives at an angle of 330 degrees, and it will arrive at ANT1
before it arrives at ANT2. A signal arriving from a bearing of 30 degrees will produce
the same output signal, even though it arrives at ANT2 before ANT1. All signals of
bearing 0 ≤ q < 180° will arrive at ANT2 first, while all signals 180 ≤ q < 360° will arrive
at ANT1 first. Yet both will produce the same blip on the oscilloscope screen. The solution
to discerning which of the two conjugate angles is the true arrival angle is to invert
the ANT1 signal. When this is done, the ANT1 signal falls below the baseline on the
CRT screen, while the ANT2 signal is above the baseline. By noting the time difference
between the pulses and their relative positions, we can determine the bearing of the arriving
signal.
Figure 23.15B Block diagram of typical TDOA circuit.