Practical_Antenna_Handbook_0071639586
514 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s Figure 23.3 Addition of sense antenna overcomes directional ambiguity of the loopstick. Figure 23.4 Cardioid pattern of sense-plus-loopstick antennas.
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 515 Field Improvisation Suppose you are out in the woods trekking around the habitat of lions, tigers, and bears (plus a rattlesnake or two for good measure). Normally you find your way with a compass, a U.S. Geological Survey 7.5-min topological map, and a GPS receiver. Those little GPS marvels can usually give you excellent latitude and longitude indication when you can see sky above you, but often fail to deliver reliable position information in woodlands and dense forests. In such terrain, the answer to your direction-finding problem might be the little portable AM BCB radio (Fig. 23.5) that you brought along for company. Open the back of the radio and find the loopstick antenna, because you will need to know its orientation. In the radio shown in Fig. 23.5 the loopstick lies across the top of the radio, from left to right, but in other radios it is vertical, from top to bottom. Once you know its position, you can tune in a known AM station and orient the radio until you find a null. Next, point your compass in the same direction to obtain the numerical bearing (0–360°) of the signal. If you know the approximate location of the broadcast station, subtract the bearing you determined from 180 degrees (or vice versa, depending on which method gives you a positive value equal to 360 or less) and draw a line on the topo map at that angle, originating at the known AM broadcast station. Of course, it’s still a bidirectional indication, so all you know is the line along which you are lost. Now tune in a different station in a different city (or one at least far enough from the first station to make a difference) and take another reading. Again subtract (or add) 180 degrees to obtain a positive number between 0 and 360. Draw a line at this bearing originating from the second broadcast station location. These two lines should intersect at your approximate location. If you know the locations of additional broadcast stations, take a third, fourth, and fifth reading and you will generally be able to pinpoint your location pretty tightly. If you were smart enough to plan ahead, you will have se- Figure 23.5 AM portable radio. Dark bar represents position and direction of internal loopstick.
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514 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s<br />
Figure 23.3 Addition of sense antenna overcomes directional ambiguity of the loopstick.<br />
Figure 23.4 Cardioid pattern of sense-plus-loopstick antennas.
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