Practical_Antenna_Handbook_0071639586

522 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 23.10 Adcock pattern.
nant (which means the antenna
can be used over a
wide band). The elements are
spaced from 0.1l to 0.75l, although
the example shown
here is spaced 0.125l. Spacing
is ultimately limited by
the need to avoid distorting
the circular (omnidirectional)
pattern of each element in
the pair, so a given dipole
configuration is suitable over
only a 3- or 4-to-1 frequency
range. Pickup of horizontally
polarized signals on the feedline
to each element is minimized
through the use of
balancing or shielding techniques.
Other Adcock arrays are
designed around two pairs of vertical dipoles or monopoles. All Adcock antennas are
vertically polarized and respond only to vertically polarized waves, so they have earned
an excellent reputation as being superior to loops for precise high-frequency shortwave
RDFing.
A typical pattern for an Adcock antenna is shown in Fig. 23.10. This pattern was
generated by modeling the array using the NecWin Basic for Windows program (Chap.
25). The example antenna is a 10-MHz (30-m band) Adcock that uses 1.455-m elements
(total 2.91 m on each side), spaced 4 m apart. The pattern is a traditional figure eight
with deep nulls at 0 degrees and 180 degrees. The antenna can be rotated to find a null
in the same manner as a loop.
Watson-Watt Adcock Array
Figure 23.11 shows the Watson-Watt Adcock RDF array, consisting of two Adcock pairs
arranged orthogonally to each other. It is common practice to align one Adcock with an
east-west line and the other with north-south. These are fed to identical receivers that
are controlled by a common local oscillator (LO). The outputs of the receivers are balanced
and are used to drive the vertical and horizontal plates of a cathode-ray oscilloscope
(CRO). Figure 23.12 shows the patterns that result when signals of various phases
arrive at the Watson-Watt array. The patterns of Figs. 23.12A and 23.12B are from signals
180 degrees out of phase, while the pattern of Fig. 23.12C is the result of a 90-degree
phase difference.