Antenna Gain
Antenna gain is a measure of an antenna’s
effectiveness. It has to do with the radiated
power of the antenna, but it isn’t about the
antennageneratingpower; it’s about how the
antenna concentrates the electromagnetic
energy—the radio frequency—received from
the radio. Gain is measured in decibels just like
the loss factor we discussed earlier. The gain
of the antenna, as well as its length, determines
the radiated pattern emitted from the antenna.
As the dB rating increases, the transmitting
power is compressed into a progressively nar-
rower beam, making the signal go farther, but
also making its direction more critical for suc-
cessful reception at the other end.
You can increase the radiated power from the
VHF radio by installing an antenna with a
higher gain rating, although it doesn’t always
improve performance in real-world conditions.
A 9 dB rated antenna transmits farther, but its
beam is narrower, so if it is mounted at the mast-
head of a sailboat and the sailboat is heeled, the
signal would be angling up into the sky on one
side of the boat and down into the water on the
other. For sailboats, therefore, a 3 dB antenna is a
more desirable choice. Even though the radiated
coaxial cable and antennas 175Test rig for checking VHF power output. The connector on the left plugs into the antenna jack on the radio set, while the
dummy load on the right takes the place of the antenna. (Note: Never bend coaxial cable as far as shown here. The cable
was bent far beyond its specified minimum radius and held in place with a cable tie only for the purposes of the photo.)
no gain (handheld
6–10" antenna)VHF-FM
antenna9 db 3 db 6 dbbeam doesn’t extend as far as the 9 db beam,
it’s more likely to reach a receiving station.
Powerboats, on the other hand, are inclined to
run in a more level position, and they can use
6 dB or 9 dB antennas more effectively, although
as a boat rolls in beam seas, transmissions may
still fade in and out.Typical radiation patterns of VHF-FM radio antennas.
A 9 dB gain antenna, which might be 24 feet long, offers
greater potential range but also a vertically compressed
signal. When the boat rolls, you might be transmitting
down into the sea on one side of the boat and up into
space on the other side. Many powerboaters choose
a 6 dB, 8-foot antenna as a good compromise.
(Reprinted with permission from Boating Skills and
Seamanshipby the U.S. Coast Guard Auxiliary)