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 513
Figure 23.2B Pattern of loopstick antenna: nulling the source.
na’s axis. You will get exactly the same response from aiming either end of the stick in
the direction of the station. As a result, the unassisted loopstick can only show you a
line along which the radio station is located but can’t tell you which half of the line to
focus on. Sometimes this doesn’t matter; if you know the station is in a certain city
and that you are generally south of the city, and can distinguish the general direction
from other clues, then the half of the null line that is closer to the north will provide
the answer.
One solution to the ambiguity problem is to add a sense antenna to the loopstick
(Fig. 23.3). The sense antenna is an omnidirectional vertical whip, and its signal is combined
with that of the loopstick in an RC phasing circuit. When signals from the two
antennas have roughly equal amplitudes, the resultant pattern will resemble Fig. 23.4.
This pattern is called a cardioid because of its heart shape. Because it has only one null,
it resolves the ambiguity of the loopstick pattern.
Note, however, that a single RDF receiver can provide only bearing information.
Receivers at two or more sites, widely separated (relative to bearing angle of the target),
are necessary to also obtain distance information and pinpoint the location of the
target.