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 517
Be aware that the small loop antenna has a figure eight pattern like the loopstick,
but the maxima and minima are oriented 90 degrees from those of the loopstick. In the
regular loop the nulls are perpendicular to the plane of the loop, while the maxima are
off the sides. Thus, in Fig. 23.6 the nulls are in and out of the page, while the main lobes
are left and right (or top and bottom).
Sense Antenna Circuit
Figure 23.8 shows a method for summing together the signals from an RDF antenna
(such as a loop) and a sense antenna. The two terminals of the loop are connected to
the primary winding of an RF transformer. This primary (L 1A ) is centertapped and the
centertap is grounded. The secondary of the transformer (L 1B ) is resonated by a variable
capacitor C 1 . The dots on the transformer coils indicate the 90-degree phase
points.
The top of L 1B is connected to both the sense circuit and the receiver antenna input.
Potentiometer R 1 is a phasing control. The value of this pot is usually 10 to 100 kΩ, with
25 kΩ common. Switch S 1 is used to disconnect the sense antenna from the circuit. Because
the nulls of the loop or loopstick are typically a lot deeper than the null on the
cardioid pattern, the null is first located with the switch open. If the receiver S-meter
reading drops when the switch is closed, the correct null has been selected. If not, reverse
the direction of the antenna and repeat the process.
Figure 23.8 Sense circuit for a loop/sense antenna RDF.