Page 75 - Antennas for Base Stations in Wireless Communications

48 Chapter Two

Figure 2.7 Crossed-dipole element pressed and
bent from sheet metal with an air-spaced microstrip
feed (Photo courtesy of Andrew Corporation)

A Roberts balun provides additional optimization parameters com-
pared with a Pawsey stub because varying the Z 0 of the microstrip line
at any point is easy, and the open-circuit l /4 line on the unfed side of
the balun provides additional impedance compensation. It is possible
to arrange for the point at which the feed crosses from one balun leg to
the other to be at different heights above the groundplane for the two
members of the crossed pair and some very simple and elegant designs
have been created in this way. 11
A square dipole array, as shown in Figure 2.6c, provides a further option
for a dual slant-polar array with 65° azimuth beamwidth. The radiating
currents in the dipoles on the opposite sides of the square are in phase, one
pair providing +45° and the other –45° polarization. The individual dipoles
can be designed using any of the techniques described previously.
Log-periodic dipole antennas (LPDAs) are sometimes used as array
elements. These can be placed with slightly wider interelement spacing
than dipoles because the radiation pattern of individual LPDAs has
low sidelobes near the vertical axis of the array—the direction in which
grating lobes first appear as the element spacing is increased. In a long
array (say 8l or more), there is no significant gain advantage relative
to the use of other element forms, although some examples have been
produced that have very high F/b ratios. Although individual LPDAs
may have wide bandwidths, they are not easy to use effectively in long
arrays for applications requiring a bandwidth such as might be needed
to cover both low-band and high-band frequencies. Dual-polar LPDAs
are bulky, all their dimensions being a substantial fraction of a wave-
length at the lowest operating frequency, and the advent of polarization
diversity has reduced the use of this element form.