Practical_Antenna_Handbook_0071639586

274 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
phases. To develop maximum gain along a diagonal of the square, if the rear vertical on
the diagonal is defined to have zero phase shift, the front vertical (i.e., the one in the
direction of maximum gain) is –180 degrees and the two side verticals are both –90 degrees.
Obviously, all aspects of the array’s performance can be identical every 90 degrees
around the compass, simply by switching the phase of the drive current going to
each specific vertical.
An amateur DXer in the northeastern United States would likely orient his or her
4-square so as to put the center of the four switchable pattern lobes at 60-, 150-, 240-, and
330-degree compass headings to favor Europe/North Africa, South America, Australia/New
Zealand, and the Far East, respectively.
Other variations of the 4-square utilize somewhat different combinations of phase
shifts and/or smaller spacings between elements. In most cases, 4-squares are constructed
with ground-mounted monopoles and extensive radial fields, but today many
employ elevated radial systems. If the user has the means to “force” drive currents of
arbitrary phase and amplitude relationships to the four elements, there’s virtually no
limit to the number of customized azimuth patterns that can be obtained.
Receive-Only Vertical Arrays
Much work has been done in the past few years on phased arrays for receiving at MF
and the lower HF bands. In stark contrast to the challenges of weak signal reception at
VHF and above, the abundance of atmospheric noise below, say, 10 MHz gives the system
designer additional leeway in selecting techniques for digging weak signals out of
the background noise. At these low frequencies, receive-only arrays can differ from
their transmitting antenna counterparts in two important ways:
• Because receive-only arrays are not used for transmitting, antenna efficiency is
of only secondary importance; as a result, array elements much shorter than
l/4 and having very low feedpoint impedances as well as inexpensive tubing
and support requirements can be employed, and preamplifiers (if needed)
added at the array.
• Because no transmitted RF energy is directly applied to the elements, there is no
need for high-voltage or high-current components that are normally required
for transmit applications.
Over the past decade or two, leading 160-m and 80-m DXers fortunate enough to
have the necessary acreage have designed and built six-circle and eight-circle arrays for
these bands. Typical of these arrays, which provide outstanding receive directivity and
signal-to-noise ratio (SNR) on these notoriously weak-signal frequencies, is the eightcircle
for 160 m designed by Tom Rauch, W8JI. In this design, the output signals from
four of eight short vertical elements equispaced around the circumference of a circle are
combined with appropriate relays and phasing lines to provide more than 7 dB gain in
the desired direction, relative to a single such element. Because the circle of elements is
eight-way symmetrical around the compass rose, the main lobe can be switched in 45-
degree increments.
At any given switch position (i.e., compass heading), only four of the eight elements
are active: Two adjacent elements on one side of the circle form an end-fire two-element
array, as do two adjacent elements directly opposite, on the far side of the circle. These