COAXIAL CABLE VADEMECUM
Many consumers, when they think of "coaxial cable," tend to think it refers only to the cabling used by cable and satellite companies for cable TV. However, this is only one example of coaxial cable. In this article we will breakdown what makes up a typical coaxial cable as well as go over a few common types.
Coax cable is defined as any cable with the following properties:
Each of these components plays a specific role. Let's take a look at each in more detail:
Center
Conductor:
At the heart of a coaxial cable is a center conductor. Typically
constructed of either pure copper (in higher-end cables) or copper-coated
steel or aluminum (in less-expensive cables), the center conductor
is responsible for transmitting the cable's signal. As such, it
must meet certain electrical properties (such as wire resistance).
The rest of the cable construction is primarily designed to help
the center conductor maintain its electrical integrity.
Dielectric
Insulator:
The dielectric insulator's purpose is two-fold; first, it acts
as an insulator between the center conductor and the outer braided
/ foil shielding. Second, it helps physically hold the center
conductor in the center of the cable. This is important, as signal
loss can occur if the center conductor strays too close to the
outer area of the cable. Various materials are commonly used for
the dielectric. A few of the more common materials, in order of
quality (from best to worst), are below:
1. Foamed Polyethylene (FPE)
2. Teflon
3. Polyethylene (PE)
4. Polypropylene (PP)
5. Polyvinylchloride (PVC)
When reading specifications on coaxial cables, you may see references to the dialetric constant. The closer this number is to 1.0, the better. Foamed Polyethylene (FPE), for example, generally has a dielectric constant somewhere around 1.5, while PVC's dialetric constant is around 3.0 to 4.0. (Foamed PE basically uses gas, often nitrogen, to create gas bubbles in the material to lower the dielectric constant. Marketing literature that refers to "gas-injected dielectric" usually indicates the use of FPE. It is one of the best dielectric materials in common use.)
Braided
Shield:
Long copper cables have a tendency to act like antennas, picking
up stray signals from the environment. These unwanted signals,
known as "interference", disrupt the signal that the
cable is supposed to be carrying. Interference tends to come in
two different flavors: electromagnetic interference (known as
EMI) and radio frequency interference (RFI). EMI interference
is often caused by heavy power lines, cell phone signals, etc.
A braided shield protects the signal from EMI interference.
When looking at cable specs, the braided shield will often be expressed in a percent coverage, which often ranges anywhere from 30% to 95% coverage. The higher the coverage, the better the protection.
Foil
Shield:
Although not always present on coaxial cables, the foil shield
serves to protect from RFI interference. Foil shields are almost
always made out of aluminum foil, and simply wrap around the inner
parts of the cable. Unlike braided shields, which have a percent
coverage, foil shields always cover 100%.
Outer Jacket:
The outer jacket is generally made out of flexible PVC (polyvinyl
chloride) and serves primarily to hold the cable together and
protect it from the elements.
Connectors
There are many different connectors that can terminate a coaxial cable. We will now go over a few of the more common connectors.
RCA
The RCA connector was developed in the early '40s by the Radio
Corporation of America to connect record players to amplifiers.
The same basic connector is still in wide use today, and it represents
a large portion of the connectors used for home theater cables.
The fact that they are so easy to connect and disconnect makes
RCA connectors a popular choice for home theater applications.
RCA cables
can be used for audio, video and digital audio.
The biggest drawback with RCA devices is that each signal is sent
on a different cable. For example, a single RCA-terminated coax
cable only carries the left audio channel, or only the right,
etc. Three RCA cables are needed for high-def video, along with
two more for the audio. This makes for a mess of cables behind
your equipment. This is one reason that has made HDMI cables
so popular.
Attaching RCA connectors can be a bit more time-consuming as,
with some types of RCA connectors, the coaxial cable's wires need
to be soldered to the connector after stripping the cable with
a stripping
tool.
RCA connectors
come in solder-on, weatherproof and compression styles. A special
tool is required for compression connectors, and a soldering iron
is needed for solder-on connectors.
BNC
The BNC connector has two bumps on the female side that slide
into corresponding grooves on the male side. The connector is
then rotated a quarter turn to lock into place. BNC connectors
are widely used in commercial applications such as closed circuit
television systems, where its ability to lock in place (unlike
the slip-on RCA) makes BNC cables
a perfect fit. BNC connectors come in a wide variety as well,
including twist-on and weatherproof connectors.
F-Pin
The F-pin connector
is probably the most recognized of the coaxial connectors as it's
been in use with televisions and VCRs for decades. The familiar
threaded connector makes for a secure connection that will not
easily slip out of a device. This connector is also one of the
easiest to attach to a coax cable as it does not require any soldering.
Many different types are available including twist-on, crimp-on
and compression. For outdoor use, weatherproof connectors are
also available to create an F-Pin cable
with a secure connection and loss-less signal transfer.
Many manufacturers are doing away with the F-pin connector on their TVs. This is because that little F-pin connector is actually part of a device called a tuner. The tuner is what sifts through the signal coming through the cable and separates all of the TV channels. Since so many people change channels using a cable box or satellite receiver, these tuners are becoming obsolete. This can pose a problem for folks who have standard cable TV that still requires the TV to have an F-pin connector. Unfortunately, a small adapter or special cable won't do the trick. A tuner is required. There are a couple ways to do this. One way is to use an old VCR. VCRs have built in tuners. The other way is to use a device called an RF demodulator. A demodulator is essentially a TV tuner that will let you change the channels.
Crimp-on,
solder or twist-on?
There are 3 main styles of each connector available on the market.
With each of these styles the coaxial cable must be stripped in
such a way that a portion of each component of the cable is exposed:
Types of Coaxial Cable
RG58
Largely used in the commercial security camera industry, RG58 cable
is a low profile, inexpensive choice for large projects where
a high-bandwidth cable is not needed. Most often terminated with
BNC connectors, this type of cable can also be found attached
to testing equipment and 2-way radio systems.
RG59
Once the standard for cable TV, RG59 cables
are still found packaged with VCRs and televisions. RG59 was a
good low-cost option for cable TV for years until the cable industry
recently began its move into digital cable television, which needs
a thicker cable. Modern satellite television also requires a higher-bandwidth-capable
cable and so RG6 coaxial cable is becoming much more popular,
making RG59 no longer the industry standard.
RG6
RG6 cable
is differentiated from RG59 cable by having a thicker copper center
conductor. RG6 is primarily being used today for satellite and
digital cable TV, where higher frequencies are required that RG59
cable cannot support. RG6 cable is most often sold with F-pin
connectors for cable or satellite applications.
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