Time difference of arrival (TDOA) array 453
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23-15A Time difference of arrival (TDOA) RDF array
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23-15B Block diagram of typical TDOA circuit
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454 Antennas for radio direction finding (RDF) signals 180 x < 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 intended 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 position, we can determine the bearing of the arriving signal
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Switched-pattern RDF antennas
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Suppose we have a unidirectional pattern such as the cardioid shown in Fig 23-16 If we can rapidly switch the pattern back and forth between two directions that are 180 apart, then we can not only discern direction, but also we can tell whether an off-axis signal is left or right This feature is especially useful for mobile and portable direction finding In Fig 23-16 we have three different positions for a signal source When the signal source is at point A, it will affect the pattern to the left more than the pattern to the right, so the meter will read left If the signal source is at point B, on the other hand, the signal affects both pattern positions equally, so the meter reads zero Finally, if the signal arrives from point C, it affects the right-hand pattern more than the left-hand pattern, so the meter reads right Figure 23-17 shows how such a system can be constructed This system has been used by amateur radio operators with rubber ducky VHF antennas and a sin-
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23-16 Switched-pattern RDF system
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Switched-pattern RDF antennas 455
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23-17 Double-ducky RDF system based on the switched-pattern system
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gle receiver, where it is commonly called the double-ducky direction finder (DDDF) The antennas are spaced from 025 to 1 apart over a good groundplane (such as the roof of a car or truck) If no groundplane exists, then a sheet metal groundplane should be provided In Fig 23-17 we see the antennas are fed from a common transmission to the receiver In order to keep them electrically the same distance apart, a pair of identical half-wavelength sections of transmission line are used to couple to the antennas Switching is accomplished by using a bipolar square wave and PIN diodes The bipolar square wave (see inset to Fig 23-17) has a positive peak voltage and a negative peak voltage on opposite halves of the cycle The PIN diodes (D1 and D2) are connected in opposite polarity to each other Diode D1 will conduct on negative excursions of the square wave, while D2 conducts on positive excursions The antenna connected to the conducting diode is the one that is connected to the receiver, while the other one is parasitic The active antenna therefore switches back and forth between ANT1 and ANT2
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456 Antennas for radio direction finding (RDF) This antenna is coupled to the receiver through a small-value capacitor (C1) so that the square wave does not enter the receiver This allows us to use the transmission line for both the RF signal and the switching signal An RF choke (RFC3) is used to keep RF from the antenna from entering the square-wave generator The DDDF antenna produces a phase modulation of the incoming signal that has the same frequency as the square wave This signal can be heard in the receiver output When the signal s direction of arrival is perpendicular to the line between the two antennas, the phase difference is zero, so the audio tone disappears The pattern of the DDDF antenna is bidirectional, so there is the same ambiguity problem as exists with loop antennas The ambiguity can be resolved by either of two methods First, a reflector can be placed /4 behind the antennas This is attractive, but it tends to distort the antenna pattern a little bit The other method is to rotate the antenna through 90 (or walk an L-shaped path)