to accommodate the connector barrel
length plus about^3 / 4 inch on each end),
the tubing will shrink down onto the pre-
viously installed short lengths, effectively
sealing the entire assembly.
I don’t like this method because the
material only shrinks so much for its nomi-
nal size, and sometimes it’s not enough to
accommodate the difference in diameters
between the cable and the connector.
2 Tape wrap using 3M Scotch 2228 Rubber
Mastic Tape (my preference). This mate-
rial seals tenaciously to itself as it’s over-
lapped, and once in place, it is incredibly
difficult to remove. In fact, a razor knife is
always needed to cut it off the connection
when it’s time for service.Calculating Loss
To ensure that the run from the antenna or
transducer to your piece of equipment is
within the tolerance specified in Table 16-2,
you will need to conduct a simple loss calcula-
tion. This is an important step of any coaxial
cable design/installation procedure.
To perform this calculation, simply add
the attenuation through the connectors to the
attenuation from the coaxial cable. The
NMEA uses a constant of 0.5 dB to indicate
the loss per connector for this calculation. You
will also need to count the number of connec-
tions, and remember that inherent loss per
100 feet of cable run is based on the frequency.
Let’s try an example using RG-8X cable to
connect a VHF radio with its antenna. The
radio’s operating frequency is 162 MHz, which
we’ll round off to 160 MHz, and the cable run
is 50 feet. Based on the specifications for RG-
8X cable, the signal loss per 100 feet is 4.0 dB
@ 160 MHz. The math looks like this:
(L100)4 dB TCL
where L length, and TCL total cable loss174 marine electronics installation and troubleshooting
Plugging in the numbers above, we find
that the total cable loss is:
(50100)4 dB 2
Then we add the loss from the two connec-
tors (0.5 dB each), for a total loss of 3 dB. Since
the maximum loss allowed by the NMEA is
3 dB for a VHF system (per Table 16-2), we’re in!VHF Antennas
Like coaxial cable, antennas also greatly influ-
ence equipment performance. If the wrong
choices are made in antenna selection or
installation, serious problems will result.
For most boaters, their VHF radio is the
all-important link to the U.S. Coast Guard if
an emergency arises, so maximum radio range
is an important safety consideration. Since
the FCC limits power output from an
installed VHF transceiver to 25 watts, the
antenna installation is the key to maximizing
effectiveness.
That said, many radios rated at 25 watts
actually transmit at 20 or 21 watts. Lower out-
put translates into less range at the antenna, so it
is important to test for VHF power output to
ensure peak transmission range. Higher-quality,
name-brand radios tend to be very accurate. To
perform the test, use a power standing wave
ratio (SWR) meter, as outlined below:1 Connect the meter to the VHF radio’s
antenna jack.
2 Plug a dummy load into the meter’s
antenna jack.
3 Key the transmit button on the micro-
phone to take a direct reading of the
radio’s transmit power.
4 Read the wattage output on the meter’s
gauge. Don’t be surprised if a 25-watt radio
only reads 18 or 19 watts of actual output.We’ll return to SWR meters shortly.