Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

Other 75-ohm cable sizes sometimes used
in marine applications are RG-6 and RG-11,
but these are rather difficult to find in most
chandleries.

Installing Coaxial Cable

When installing cable, be careful about mis-
matches; that is, connecting two cables
together that have different resistance values or
connecting cable with the wrong impedance
value to a piece of equipment. The longer the
cable run, the more this mismatch will affect
the performance of the equipment.

coaxial cable and antennas 169

Cable Frequency (MHz)

Size 30 50 100 1,000 2,400

RG-58A/U 2.5 4.1 5.3 24 38.9

RG-59 —— 2.4 3.5 12 ——

RG-8X 2.0 2.1 3.0 13.5 21.6

RG-8/U —— 1.3 2.2 9.0 ——

RG-213 1.2 1.6 1.9 8.0 13.7

LMR-240 1.3 1.7 2.5 8.0 12.7

LMR-400 0.7 0.9 1.3 4.1 6.6

Attenuation in dB per 100 Feet of Coaxial Cable

TABLE

16-3

Cable Lengths

The NMEA suggests that cable lengths be

modified for the specific installation. The coils

or bundles of coaxial cable that we so often

find hidden behind panels are not desirable or

wise. Cut the cable to the length you need and

install a matched cable end. Allow a minimum

12-inch service loop at the equipment end.

(This, of course, would only apply if the

equipment manufacturer does not specifically

prohibit the alteration of the coaxial length,

which, as discussed in Chapter 14, is often the

case.) The goal is to make the most direct con-

nection possible in order to reduce the attenu-

ation between the antenna and the device to a

minimum.

In routing coaxial cable, pay attention to

its minimum bend radius. Sharp bends in

coaxial cable increase attenuation and con-

tribute to signal loss in the run. Minimum

radius bends for different cable types are spec-

ified by the NMEA, but you should always

confirm the cable manufacturer’s minimum

bend specification for a type of cable, as the

numbers vary from one manufacturer to

another. Table 16-5 shows minimum bend

radii per NMEA requirements.

Cable Size Impedance (ohms)

RG-59 75

RG-58 50

RG-8X 50

RG-8/U 52

RG-213 50

LMR-240 50

LMR-400 50

Inherent Impedance by

Coaxial Type

TABLE

16-4