|How important is the Coax Cable?
The coaxial cable running from your radio to the antenna is
unbelievably important. Everything you transmit and receive must
travel along its length. All too often the coax is ignored and
performance suffers because of the lack of attention it receives.
Having a hi-performance antenna and a defective piece of coax is
the equivalent of having a hi-pressure nozzel at the end of a garden
hose that is full of holes. Complaining about the antenna or the
nozzel performance is unjustified. The fact remains, when it comes
to buying coaxial cable, bulk or in assemblies, quality is everything!
Although there are many types of communications grade coaxial
cables on the market, this discussion will primarily focus on those
used in mobile communications applications. Before starting, a
few terms and/or phrases need to be clarified.
Coaxial: two conductors sharing the same
Center Conductor: the wire at the very
center of the cable
Insulator: the material surrounding the
Shield: the outer conductor surrounding
Jacket: the outer covering of the cable
Propagation velocity: speed of signal
traveling in coax
For mobile installations, there are three (3) primary types of
coaxial cable used to build the assemblies.
RG-58 type: This
type of coax is used for single antenna installations and for
jumper wires that go between the radio and a test meter (SWR
meter). The RF resistance of this type of coax is 50-ohms. Within
this group you will find coax labeled with RG-58 or RG-58/U.
These cables have a solid center conductor. The second type is
RG-58A/U and they have a center conductor made up of many thin
wire strands (normally about 17) that are twisted together to
form the center conductor. The common outer diameter for this
type of cable is about 0.20" .
type of coax is also used for single antenna installations or
jumpers between pieces of equipment. Sometimes it is referred
to as " Mini-8" . Like the RG-58 type coax,
RG-8X also provides 50-ohms of resistance. In short, this cable
could be called hi-performance 50-ohm cable. It always has a stranded
center conductor and a high shielding percentage. It will also
handle higher power (wattage) and has a higher propagation velocity.
For the general user, it is more than what is required. However,
if you are using amplifiers or just like to get the absolute most
from your set-up, it will deliver. The outer diameter of RG-8X
is typically about 0.24" .
times a year we get tech calls from installers who place
dual antennas on their vehicle and run RG-58 or RG-8X from
each antenna to a T-connector at the back of their radio
only to find that the system "doesn't
get out" . You should not use 50 ohm coax on a ground plane
dependent dual antenna installation ... it MUST be 75 ohm
RG-59 type coax. These flawed installs can be misleading
because SWR tests can show exceptionally low SWR, making
the installer think that all is well. However, the impedance
of the antenna system does not match the requirements of
the radio and therefore, the output power of the radio is
greatly reduced. In several tests, we found that a 4 watt
radio would only generate 1.75 watts of output power which
is the equivalent of having an SWR reading that exceeds 6.0:1.
type of coax is used for dual antenna installations only. The
RF resistance of this type of coax is 75-ohms. Within this group
you will find coax labeled with RG-59 or RG-59/U. These cables
have a solid center conductor. The second type is RG-59A/U and
they have a center conductor made up of many thin wire strands
(normally about 19) that are twisted together to form the center
conductor. The common outer diameter for this type of cable is
about 0.22" .
About the center conductor: We strongly believe that mobile
installation should always use stranded center conductors. The
reason behind this is due to potential breakage of solid conductors
due to vibration and/or repetitive flexing. If you have ever picked
up a piece of wire and bent it repeatedly until it broke, you would
fully understand our reason for recommending stranded center conductors
on mobile installations.
insulation: There are two common types of insulating
material used in coax. First there are the foam (polyfoam) types.
Although most specifications sheets show that polyfoam insulated
coax has a faster propagation velocity, we do not recommend it
for mobile installations running close to a heat source such as
the auto's exhaust line or a coax run that will pinch the coax.
We prefer the plastic types (polyvinyl, polypropelene, etc.)
because they are just plain tougher. The small loss in velocity,
for all intents and purposes, is insignificant insofar as low
power, low frequency communications are concerned. The properties
of coax cable changes if the center conductor is not in the
physical center. Polyfoam insulation deteriorates faster than
the plastic types and also tends to collapse easier than the
plastics when pinched or sharply bent
About shielding: The shield surrounds the center conductor
and prevents internal leakage and external interference. The typical
shield used on two-way radio communication cables is a woven braid.
For the most part, it is formed with either bare or tinned copper
wire and is a very important consideration when trying to determine
the quality of the coax. Low percent coax has a loose braid and
exposes more of the center conductor to leakage. Unfortunately,
the cable industry did not invent a coding system that designates
the type and percentage of shielding used to build the coax. Shielding
percentage is the most abused part of the coaxial cable manufacturing
process. There was a time when 70% coverage was considered the
absolute minimum, but we have seen cheap cable with as little as
58% shielding being sold in recent years. Because it is readily
available, we do not recommend coax cables with shielding less
than 90%. Using anything less is the equivalent of watering your
lawn with a hose that is full of holes and has a restricted opening.
You must exercise caution when it comes to shielding.
About the length: This
is a testy subject with many engineering types. They have argued
with us on many occasions regarding this matter. They say that
if your system is set-up properly that the length of the coax
is irrelevant. We agree! However, mobile installations have so
many variables that a perfect set-up is the exception, not the
rule. One guy has a pick-up and another has a fiberglass motor home.
One wants the antenna on the bumper, another on the hood and
a third on the roof. Few people want to drill holes in their
vehicle so quality grounds are always a consideration. Because
of the imperfect world, we almost always recommend 18' (5.5m) when
our products are used. We do so with good reason too! At 18' the
voltage curve has dropped back to the zero voltage point where
the cable meets the antenna which reduces the reactance within
the cable itself (a null cable if you would). It has been our experience
that if the antenna location makes it somewhat out of sync with
its surroundings, cable lengths that are not multiples of our 18'
suggestion adds to the problem.
On that note,
when you have 18' of coax going to a radio that is only 8' away,
what should you do with the other 10'? We recommend that you
serpentine it like a skein of yarn so that it is 10-14" long
and tie it in the center with a wire tie then tuck it away. Do
not roll it up in a tight circle as this can cause it to act like
an RF choke, which often times will cause system problems.
Other ways to wreck your coax: Wear holes through it,
slam it in the door a few dozen times, attempt to splice it as
you would a wire going to your taillight, tie it in knots or make
real sharp bends in it.
For your information,
all of our Fire-Flex coaxial cables have stranded center conductors,
polyvinyl insulation and bare copper shielding in the 95% range.
Even though we offer some cables that are not 18', we do so for
the knowledgeable installer, who knows that if a problem shows
up, he must exercise the 18' recommendation.