Page 97 - Antennas for Base Stations in Wireless Communications

70 Chapter Two

lead to contamination than laser cutting. Good material badly pro-
cessed will always fail to deliver the expected result.
■ Observe good engineering practice in the selection of metals and
finishes at any intermetallic joint. Corrosion, and consequent PIM
generation, is driven by the galvanic contact potential between the
contacting metals. Protect essential joints from corrosion and wher-
ever possible protect them from contact with water.
■ Avoid the use of metals with nonlinear conductivity or contact poten-
tials—nickel is a known PIM source and cannot be tolerated in electro-
plating or metal alloys. Stainless steel fasteners are frequently used in
antenna construction, but they should not form the current path at an
interconnection.
■ Avoid the use of ferromagnetic materials; the skin depth on these
materials is reduced and their RF resistance is correspondingly
raised.
■ Aluminum is widely used for antenna construction, but it is a very
electropositive metal and its surface is always covered by a thin layer
of aluminum oxide (alumina). Joints between aluminum components
must be provided with corrosion protection and high contact forces.
■ Maintain a high level of cleanliness during the handling and storage
of components as well as in assembly, testing, and packaging areas.

2.3.7.1 Some Universal Principles Many important design techniques
relate to the configuration of joints between conductors, whether these
are part of a radiating structure, an internal feed system, or the reflec-
tor surface and its supporting hardware. RF currents flow only in
a very thin layer on and below a conductor surface; the skin depth
at 2 GHz in aluminum is only 1.8 µm. Only the extreme surface layer
of the conductors takes part in current conduction, and at every point
where conductors touch, the current will pass across the shortest avail-
able path. The impedance encountered by the current will change if
the surface path geometry changes. At every interconnection, the sur-
face current path must be well defined, maintained independently of
production tolerances, and bearing sufficient mechanical pressure to
ensure that the path remains stable whatever forces the joint may expe-
rience on account of temperature change, vibration, wind load, or other
causes. It is essential to remember that in the real world no surfaces are
flat—they are always rough and they slope in one direction or another.
No surfaces are exactly square, and no conductors are completely rigid.
Table 2.3 shows examples that illustrate simple principles that must be
followed in every antenna design. These examples are not exhaustive,
and they apply whenever the radiating structure and feed system are