trademark held by Times Microwave Systems
(www.timesmicrowave.com). This cable is
generally considered superior to any of the R-
designated cables. The company has a lot of
experience developing special cable for mili-
tary applications; LMR cable is one by-product
of their research and development.
This discussion may seem cryptic, but
the electronic equipment manufacturers
generally make the decision for us in terms
of what cable to use. For example, Ancor
Marine (www.ancorproducts.com) makes
some very specific recommendations in its
catalog: RG-58C/U for interconnecting
electronic equipment; RG-59/U for televi-
sions and some satellite systems; RG-8X for
VHF radios in which the distance from the
antenna to the transceiver is less than 50
feet; and RG-8/U for VHF with longer cable
runs to the antenna.The NMEA on Coaxial Cable
When all is said and done, however, the best
resource for sorting through all of this is the
NMEA. It devotes ten pages of its electron-
ics installation standard to coaxial cable
requirements—in standards writing, that’s
significant. (But even the NMEA will tellyou to first consider the equipment manu-
facturer’s recommendations.)
The NMEA defines maximum acceptable
levels of signal loss for different types of equip-
ment, shown in Table 16-2. Use these values
unless the equipment manufacturer specifies a
lower value.Attenuation
Next let’s look at the specified attenuation for
the different cable types, as shown in Table 16-3.
Specifications for attenuation in cable with the
same basic designator (various brands of RG-58
or RG-8X, for example) may differ slightly from
one manufacturer to another. It is advisable to
double-check the values in the table against
those for the specific brand of cable you are
using, particularly if you’re installing long cable
runs. It could mean the difference between a
pass/fail situation for a given installation.Impedance
Of course the last spec to match is the inher-
ent impedance of the cable. Some applications
call for 50-ohm cable, others 75-ohm cable.
Impedance ratings of various cable types are
shown in Table 16-4.168 marine electronics installation and troubleshooting
Equipment Maximum Loss (dB) Operating Frequency
VHF radio 3 162 MHz
SSB radio 3 2—22 MHz
Cell phone 3 850 MHz or 1.9 GHz
Television 6 54—806 MHz
Satellite TV 6 1.6 GHz
GPS 3 1.5 GHz
DGPS 3 150—500 kHz
Source: NMEAMaximum Allowable Signal Loss by Equipment Type (per NMEA)TABLE
16-2