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 525
The Doppler RDF antenna of Fig. 23.13 rotates at a constant angular velocity. The
signal wavefront approaches from a single direction (the lower left corner of the illustration
in this example), so there will be a predictable Doppler shift at any point on the
circular path of the antenna. The magnitude of the frequency shift is maximum when
the antenna is moving directly toward the source or away from it—i.e., when it is on
either side of the figure. When the antenna is closest to the source or farthest from it,
there is very little Doppler shift. The maximum shift at the two sides can be found to be
RwFc S = (23.1)
c
where S = Doppler shift, in hertz
R = radius of rotation, in meters
w = angular velocity of antenna, in radians per second
F c = carrier frequency of incoming signal, in hertz
c = velocity of light (3 × 10 8 m/s)
In theory this antenna works nicely, but in practice there are problems, one of which
is getting a Doppler shift large enough to easily measure. Unfortunately, the mechanical
rotational speed required of the antenna is very high—too high for practical use. However,
the effect can be simulated by sequentially (i.e., electronically) scanning a number
of antennas arranged in a circle. The result is a piecewise approximation of the effect
seen when the antenna is rotated at high speed. Byonics (www.byonics.com) offers a kit
by Daniel F. Welch, W6DFW, that incorporates the N0GSG Doppler system with digital
signal processing (DSP) originally described in the November 2002 issue of QST. In this
system, four verticals are electronically switched at high speed to create the Doppler
shift that allows pinpointing of the source to a 22.5 percent segment of the compass
rose. The user is responsible for constructing the four verticals. Googling the words
“Doppler RDF antenna” will provide numerous hits on the topic.
Wullenweber Array
One of the problems associated with small RDF antennas is that relatively large distortions
of their pattern result from even small anomalies because they have such a small
aperture. If you build a wide-aperture direction finder (WADF), however, you can average
the signals from a large number of antenna elements distributed over a large-Â
circumference circle. The Wullenweber array (Fig. 23.14) is such an antenna. It consists
of a circle of many vertical elements. For HF the circle can be 500 to 2000 ft in diameter!
A goniometer rotor spins inside the ring to produce an output that will indicate the
direction of arrival of the signal as a function of the position of the goniometer. The
theoretical resolution of the Wullenweber array is on the order of 0.1 degree, although
practical resolutions of less than 3 degrees are commonly seen.
Time Difference of Arrival (TDOA) Array
If two antennas are erected at a distance d apart, arriving signals can be detected by
examining the time-of-arrival difference. Figure 23.15A shows a wavefront advancing