Page 92 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 65

it has become general practice that they do. They also usually have the
same ±45° slant linear polarization—antennas where the two bands
require different polarizations are usually constructed as side-by-side
arrays.
A dual-polar dual-band antenna may contain up to six independent
and well-isolated antennas in one housing. Although creating a concep-
tual design for such an antenna is relatively easy, optimizing it is not.
Both band groups have wide bandwidths, so stable performance is not
easy to achieve, and many of the parameters available to the design
engineer simultaneously influence the performance of both the high-
band and low-band arrays.
The requirement for isolation between the cross-polarized ports of
a dual-polar antenna is that it exceeds 30 dB. The isolation between
the ports for the high- and low-band arrays should also typically
have an isolation of 30 dB to avoid unwanted interactions between
the transmitters. The close proximity of the elements for each band
makes it difficult to provide this isolation directly, so it is common
to incorporate a filter circuit in the feedlines where this is needed.
The high- and low-band arrays are sometimes combined into only
two ports (dual band ±45° ports) by the use of a diplexer—a simple
microstrip filter can usually provide the required isolation and power-
handling capability.

Wideband Arrays The reader may wonder why a satisfactory base sta-
tion antenna design cannot be created by using true broadband radiat-
ing elements, such as LPDAs. This is not as easy as it may sound.
A dual-polar LPDA with sufficient bandwidth is a quite large device
of very considerable mechanical complexity. To avoid grating lobes and
consequent loss of gain the required interelement spacing will be around
one wavelength at the high band, whereas the physical dimensions of
each element are of the order of one half-wavelength at the low band.
Even if the element groups can be compressed by using miniaturization
techniques (which usually reduce the available bandwidth and result in
lower gain), the very high mutual impedances between the elements of
such a close-spaced array create major problems in optimizing elevation
pattern performance and input impedance.
Independent Antennas Mounted End-to-End Two antennas can be mounted
end-to-end, one above the other, but for high-gain antennas this leads
to very long structures. Antennas mounted end-to-end have occasion-
ally been used to obtain space diversity on a single frequency band,
but their small vertical separation results in high signal correlation.
(Lateral separation of antennas is more effective than vertical separa-
tion because scatterers are predominantly dispersed in the horizontal
plane around the MS, not in the vertical plane.)