Have we been "given the ax"
by the cable designers & namers of old? Well, no.
The "-ax" is short for "-axial",
of course, and refers to the lengthwise axis of the
cable. And when it's abbreviated as "-ax,"
the word "cable" is also implied.
Here's a rundown of the common "-axial" cables.
Coax (coaxial
cable) consists of two conductors which share the same
axis. To do this, and to separate them electrically,
at least one must be cylindrical and larger in diameter
than the other. Coaxial cables are inherently unbalanced,
which may be bad news concerning immunity to EMI, but
this is often overcome by their efficiency in connecting
to high frequency antennas. The choice of coax is ordinarily
dictated by the antenna design itself.
In referring, above, to “at least
one must be cylindrical,” this acknowledges use
of a hollow center conductor used in some large cables
to take advantage of the skin effect (at high frequencies)
in which nearly all the signal is confined to the outside
of the conductor. The core material can be eliminated
with little effect on performance, and can result in
reduced weight or cost.
Interestingly, there are specially
designed coaxial cables which, themselves, function
as antennas. They are constructed with a slit or perforated
shield, which ordinarily would be a poor design, but
leak enough signal to/from the center conductor that
they radiate over their entire length. These are used,
for example, in subway tunnels to provide RF access
to those systems operating underground.
There are many designs of coax, including
those with sophisticated, highly-effective multiple
shields, and many different cable diameters with corresponding
higher losses in smaller [center conductor] gauges and
low loss in the larger gauges. And there are many differences
in insulation materials which affect safety, flexibility,
and signal handling. The technology is constantly changing.
Multiple shields in coax are normally
"unitized," that is, electrically connected
to one another.
 Triax
resembles coax in that all the
conductors share the same axis, but there are three
of them. At least two of these must be cylindrical and
insulated from one another and the third conductor.
So it is a three-conductor "co-"axial cable.
Triax can be used in many coax
applications, but offers an additional, separate shield
— not just another layer of shielding. The outer
shield covers the "coax" inside and can add
an extra measure of EMI protection.
 Quadrax
is a four-conductor cable. The
two separate shields share the same axis, but the two
remaining conductors are a twisted pair. Like triax,
the shields are insulated from one another, which helps
improve noise immunity.
It is also well suited to confining
noisy signals, such as pulses, from interfering with
other low-level circuits. This explains its application
in radar display buses.
Its greatest usefulness is below 50
MHz.
Twinax also has two twisted
conductors, but they are surrounded by a single (or
double, but not isolated) shield. Twinax almost completely
violates the common-axis idea — unless you consider
the "pseudo"-axis of the twisted pair. (Historically,
the name was devised to imply a "next generation"
of coax, primarily in data communications applications,
though twinax is no longer a preferred medium for new
applications in this market.)
There are many
variations within all the "-axial" designs,
but a basic understanding of their family names may
help in making sensible application decisions, or at
least better appreciating the decisions made by avionics
and airframe engineers.
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