How to Buy Marine Electronics
Unlike a radio receiver, a VHF transceiver must have an external antenna
in order to transmit its signal to a distant listener. Receiving signals
is easy, getting the transmitters power into the atmosphere takes a bit
of doing. You choice of antenna will play a large role in how well your
radio's signal will be heard by other stations.
Success in transferring energy from the radio's radio frequency power amplifier
to the atmosphere depends on both the type of antenna you chose and the
specific coaxial cable used to connect the radio to the antenna. At the
frequencies used for marine VHF communication (centered on the two meter
band, 155 MHz) a considerable amount of the transmitter energy can be lost
during its passage through the antenna cable. The energy losses that occur
with the typical small diameter (3/16" o.d.) cable supplied with many antennas
can amount to 80% of the input power per 100 feet of cable. Power loss
in the antenna cable may not be of great concern on a small powerboat,
where the length of cable from the radio to the antenna is often only 10-20
feet. Conversely, a sailboat's antenna, mounted on the top of the main
mast may require a cable length in excess of 100 feet, making cable loss
a major concern. In general, the larger the diameter of the cable the lower
the loss per foot. Successful marine use requires the highest quality coaxial
cable. The insulation must be of a type that will not absorb water. The
woven shield layer must have a high shielding density and it and the inner
conductor wires must be tin plated to prevent corrosion. Although marine
grade coax cable may cost more than cable made for land-side use, it is
well worth the relatively small added expense.
The coaxial connectors used to connect to the cable must be marine grade if they
are to provide satisfactory service over time. Although they require some
skill in application, crimp type connectors installed with the proper crimping
tools are likely best for this application. Soldering the connector to
the cable is a good choice if you know how to solder. Citizens band and
closed circuit TV connectors are not suitable for this service. Buying
the best quality connectors is a wise investment.
The strength of the transmitted signal available to a distant receiver depends
largely on the performance of the antenna. Since the boat is free to maneuver
the antenna must radiate equally in all directions. VHF antennas are often
described as having a certain amount of "gain", an indication that they
can increase the strength of the transmitted signal. In fact, they cannot
increase the amount of energy received via the coax cable from the transmitter,
however they can redirect the available energy in a way that makes it most
effective in reaching a distant receiver.
A truly omnidirectional antenna would radiate energy equally in all directions.
The radiation pattern would resemble the light emitted from a spherical
light bulb. Clearly, sending energy directly upward or downward would be
wasteful, there are no marine receivers above or below the boat. Properly
designed, an antenna can redirect some of the energy that might have gone
up or down and thereby increase the amount directed toward the horizon.
A short vertical radiator, usually about 3 feet long will radiate little
energy upward or downward, thereby increasing the amount radiated horizontally,
toward the distant horizon. The increase in perceived energy is usually
referred to as antenna "gain". The short antenna will typically have a
gain of 3 db (3 db equals a doubling of signal power). This gain effect
can compensate for some of the energy lost in the passage of RF power from
the transmitter to the antenna. Making the antenna longer increases the
directional effect, the signal is further "squished downward", radiating
more energy in the direction of the horizon and still less in both upward
and downward directions. Depending on antenna length and design, this type
of antenna can have a power gain of 6 or even 9 db.
The law against free lunch applies to antennas. The greater the gain of the antenna
the more directional it becomes. Everything will be fine so long as a 6
or 9 db antenna is vertical and stable. However, as the vessel rolls and
pitches the antenna will no longer be vertical. The signal radiated from
the more directional 6 or 9 db antenna may then be directed upward into
the sky or downward into the surrounding sea, rather than in the direction
of the horizon. A good general rule for antenna selection is to use a 3
db antenna for sailboats and either a 3 or 6 db antenna for power boats.
In general, 9 db antennas should be reserved for use on land. The directional
characteristics of the antenna operate in the receive mode as well as when
the antenna is used for reception of incoming signals.
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make your purchase.