Bend Radius & Cable Installation
Workmanship
Like modern avionic instruments which
are more sophisticated and sensitive (and capable) than
in the past, modern coaxes with softer, lower-loss dielectrics
are more easily damaged.
Bending, stretching, and kinks can
force the center conductor to one side. This disturbs
the common axis of the conductor and shield —
they are no longer concentric — and shows up as
a change in impedance at the point of "injury."
This can actually be seen on a Time Domain Reflectometer
(TDR), an instrument which examines cables for signal
reflection, appearing as an electrical "bump"
(or dip).
The penalty for mishandling comes in
increased losses, reflection quirks and impedance mismatches
translating to higher VSWR (Voltage Standing Wave Ratio),
and instability of other characteristics. Any of these
can have a serious effect on overall system performance.
 Bend
Radius is relatively easy to visualize (see Figure
1). It is harder to estimate, though, because
it is a radius, which by definition is a measurement
with an imaginary reference point. Bend Radius can be
critical to the installation of coaxial cables —
especially where the routing of the cable passes by
or through structural elements where an edge may touch
the cable. Under any amount of tension, this can make
a "corner" in the cable which violates the
bend radius limits.
Too often, the damage is invisible,
an internal flaw caused by undue forces. Even though
the Teflon® jacket on most coaxial cables used on
aircraft today is tough and flexible, it can also hide
such damage; all the more important to use caution during
installation.
 To
maintain published performance, each cable type has
a recommended minimum bend radius. This is usually 5x
the outer diameter of the cable, but may be specified
as some other figure by the manufacturer. (See Table
1.)
A technician needs to remember that
physical force, while not always avoidable when making
an installation, must be moderate. There are no "cable
stretchers" in the tool catalogs.
The most easily damaged part of a cable
is at the connector. Here the stiffness of the ferrule
or clamp which holds the cable to the connector body
encounters the flexibility of the cable itself. This
is known as a "stress riser," an edge which
concentrates the damaging force on the cable. In some
cases, pulling the cable at a right angle to the axis
of the connector ferrule can even damage the termination
or the connector itself. Where the planned cable routing
involves tight bends, it may be better to use a right-angle
connector or one of PIC’s new 75° connectors.
A full-size template is incorporated
in our RF Cable Guide, along with a lot of other useful
data for RF coaxial cable applications. It is available
on request. Click Here to order one.
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