ELECTRONICS
MAKING THE ANTENNA CONNECTION
There are two terms that are often encountered when
examining antenna performance. Both are based on the
decibel (dB), a ratio or comparison between two values,
and often include a base value specified by a suffix
attached to the abbreviation. The gain of an antenna is
usually given in dBi (or just dB), which is a comparison of
the antenna’s performance to a theoretically perfect or
isotropic antenna that radiates equally in all directions with
a gain of zero. It’s a mistake to think of an antenna as
amplifying the signal. An antenna delivers a gain in
performance when compared to the theoretical isotropic
antenna because it is more efficient. Often, this efficiency is
created by making the antenna directional so that it receives
and radiates signals in a specific direction or plane.
Several antenna designs are used with wi-fi
communications. Examples include the Yagi, which looks like
a small version of a TV antenna; the vertical; and the dish or
grid antenna. The dish antenna may have a solid or grid-like
dish, but both it and the Yagi are quite directional in nature.
Highly directional antennas usually offer the most gain or
improvement in performance but aren’t practical unless they
can be easily aimed at the marina’s access point antenna.
Vertical antennas, like marine VHF antennas, provide some
gain over the isotropic antenna because they are to some
extent directional in the horizontal plane. Gain realized
from improvements in antenna performance is beneficial in
two ways, since both the received and the transmitted
signal are enhanced by a more efficient antenna.
The second term, dBm, refers to a base value of 1
milliwatt; an increase in power to 100 milliwatts would be
a gain of 20dBm. For larger comparisons, dBw refers to a
base of 1 watt. Because dBm varies at a logarithmic rate,
1,000 milliwatts ( 1 watt) and 500 milliwatts are equal to
30dBm and 27 dBm, respectively. The use of a
bidirectional amplifier to increase the strength of the signal
can improve wi-fi performance, but the comparison of
amplifier performance to antenna gain shows why
improving the antenna system first really pays off. Installing
an antenna with 3dBi of gain is equivalent to doubling the
transmitter’s power output, while an antenna with 5dBi of
gain would require that the transmitter power be slightly
more than tripled to provide equal performance. If the
amplifier isn’t bidirectional and only increases the
transmitter’s output power, no improvement in the strength
of the received signal will occur, and wi-fi performance
might still be less than optimal.
must carefully filter the received signal before increasing
its strength in order to prevent amplifying the noise.
The comparison between the strength of the wi-fi
signal and the background noise is referred to as the
signal-to-noise ratio (or SNR).
PUTTING IT ALL TOGETHER: ONBOARD WI-FI
INSTALLATIONS FOR MAXIMUM PERFORMANCE
The built-in wi-fi adapter is usually not the best
choice for wi-fi on your boat. Although newer laptops
have fairly sizable built-in antennas, the power output
is usually low, and there is rarely any provision to
connect an external antenna. As mentioned earlier,
the laptop, with its internal wi-fi antenna, is frequently
located in the chart table area, where the antenna is
in a prime position to pick up radio frequency noise
from sources such as inverters, digital instruments,
and possibly Bluetooth networks, which also operate
on the 2.4GHz ISM band.
PCMCIA and ExpressCard wi-fi adapters tend to be
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