Page 96 - Antennas for Base Stations in Wireless Communications

Base Station Antennas for Mobile Radio Systems 69

2.3.7 Passive Intermodulation
Products and Their Avoidance
The advent of multiband networks and antennas, the demand for addi-
tional capacity, and the adoption of tower and infrastructure sharing
have led to an increasing requirement to reduce PIMs to a level at
which they do not impair network performance. The standard test
level, −153 dBc relative to two carriers at a +43 dBm, is approximately
the ratio (10 15.3 ) of the distance from the Earth to the sun compared with
the thickness of a piece of thin paper. Its achievement is a factor that
strongly determines the choice of the materials, finishes, and mechani-
cal design of almost every antenna component. Not only must the PIM
limit be achieved consistently in large numbers of production antennas,
but antennas must maintain this level of performance for many years
while exposed to the corrosive effects of rain, salt, and industrial pollut-
ants as well as the mechanical stresses and vibration caused by wind.
Antenna design must be conceived from an early stage in terms of the
avoidance of PIMs—there is no easy way in which an inadequate initial
design can be modified later in the hope that PIMs can be avoided.
The achievement of low PIM levels depends on following clear rules,
both in design and at all stages of production. These include

■ Avoid design details requiring any interconductor joint that is not
essential to the electrical operation of the antenna. If two conduc-
tive mechanical components are in contact, consider how they could
be made from a single piece or insulate them so no current can flow
across the joint. If current must flow between two large flat surfaces,
consider separating them with an insulating membrane and rely-
ing on the large capacitance formed between the opposing surfaces.
Ensure that every essential conducting joint is tightly compressed
and that the force on the joint does not depend on any compressible
or creep-prone material. Where possible, reduce the area of contact so
the current path is clearly defined and the contact pressure is high.
■ Avoid dry solder joints. Well-soldered joints cause few problems, so
ensure good access and good lighting so joints can be soldered and
inspected easily and also that soldering equipment is right for the
job. If lead-free solder is used, the soldering temperature is usually
higher than with leaded solder, so these considerations become even
more important to success. For printed circuit boards, choose laminate
grades that are guaranteed to have low intrinsic PIM levels; these are
available from most manufacturers.
■ Ensure that printed circuit boards are cleanly etched and well washed
after processing. If boards are cut after washing, there is always a risk
of new contamination; routing or water-jet cutting are less likely to