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VHF Antennas
VHF radios are one of the most
cost-effective pieces of safety gear that you can put on your boat. Using relatively
inexpensive transceivers, and one of many styles of antennas, a VHF radio enables you to
contact the Coast Guard or other vessels when you’re in trouble. Of course, most VHF
use is for more enjoyable pursuits, like chatting about the day’s fishing, or letting
a bridge tender know that you would like to pass.
To maximize the range of your VHF,
there are a few rules that you should be aware of:
 | Marine VHF radios
for recreational boats are limited to 25 watts of output so, unlike Single
Sideband or Ham radios, you cannot boost your range with a more powerful transmitter.
A quality installation always helps, though. That includes a good 12V DC power connection,
and quality coax connections. |
| Even if you could increase your
power, it wouldn’t help much. VHF radios operate on a “line of sight”
principle between stations, meaning that the signals do not bend around obstructions or
over the horizon (Okay, they do a little, but think of them as traveling about as straight
as a light beam.) |
| This means that antenna height, more
than any other factor, is responsible for determining how far you can transmit. An antenna
mounted up high can “see” farther over the horizon. |
| Under the right conditions, a very
small transmitter can communicate over vast distances. For example, the signal from an EPIRB (emergency beacon) is less than one tenth of a watt, yet they can
be heard by satellites orbiting 528 miles above the earth. |
| So, the wattage of the radio is much
less important than antenna height in determining range. Antenna gain is important,
however. This is a measure of how much the antenna focuses the radio’s energy in the
horizontal direction. If several otherwise equal radios try to contact a single receiving
radio, the higher gain antenna is the one the receiving radio will hear. |
| It is also extremely important to
have good, solid, low resistance connections between the cable and
antenna and between the radio and the cable. Splices in the cable are not
recommended! We are dealing with RF power here, and any discontinuity will cause
power to be reflected back to the transmitter and lost. |
So how far can one VHF installation
talk to another VHF installation? The following table, taken from a special section on VHF
radios in Motorboating & Sailing, gives typical reception distances for selected
antenna heights. The antenna heights are given for a handheld in a small boat, a
typical powerboat antenna height of 10 ft., and sailboat installations with 30 ft. and 60
ft. heights. Distances are in nautical miles.
Transmit Antenna
Height
|
Receiving Antenna Height |
|
5' |
10' |
25' |
100' |
250' |
| 5' |
5 mi. |
7 mi. |
9 mi. |
15 mi. |
23 mi. |
| 10' |
9 mi. |
10 mi. |
11 m. |
18 mi. |
25 mi. |
| 30' |
10 mi. |
12 mi. |
13 mi. |
20 mi. |
28 mi. |
| 60' |
12 mi. |
14 mi. |
15 mi. |
21 mi. |
30 mi. |
|
Coast Guard shore stations are
commonly located on the highest mountains along the coast or mounted on tall towers, which
gives them tremendous range. They also maintain a network of repeaters so that even if a
station or vessel is not located nearby, they can still communicate with you.
dB Rating
An antenna’s dB rating indicates the apparent increase in transmitting power due
to its ability to “focus” energy. Antennas with a high dB rating concentrate
energy perpendicular to the antenna shaft in a relatively flat disk. This makes your radio
signal appear stronger in receiving stations around you. It also reduces the amount of
energy transmitted above or below the antenna, which can be a problem if your boat is
pitching or rolling in seas. A concentrated signal can actually fade in and fade out as
the boat rolls. Antennas with high dB ratings are taller than those with low dB ratings.
For example, a six dB antenna is 8' tall, while a nine dB antenna is 18'-23' tall.
Shorter, 3dB antennas transmit available energy in a less concentrated, more
“spherical” pattern. The “broad” radiation pattern from a low dB
antenna allows a sailboat to heel over and still send signals towards the horizon.
Sailboats should use a 3dB antenna mounted at the top of the mast whenever possible.
Construction
VHF antennas are made of stainless steel or fiberglass. For masthead
mounting, short stainless “whip” antennas create less
windage and provide the appropriate wide radiation pattern required by a heeling sailboat.
Small powerboats may also use stainless whip antennas because they are rugged. Fiberglass
tube antennas vary in quality. The best are Galaxy, which are
coated with smooth white polyurethane and will last 5-10 years. Polyurethane-coated
antennas don’t have Galaxy's attractive mirror-finish, but they are durable.
Standard fiberglass antennas last a few years, especially when used on covered boats. When
the fiberglass begins to discolor and sliver, it should be replaced. One major difference
in antenna construction is in the ferrule used to attach an antenna to its mount. Less
expensive antennas use nylon, which is not as strong as the chromed-brass or stainless
steel found on quality antennas. It is a good idea, especially when using metallic
mounts, to match the ferrule material to the mount material to reduce electrolytic
corrosion problems.
Elements inside the Antenna
There are three main styles of electrical elements: those using cut lengths of coax
cable, those using a simple brass radiator, and those using a more complicated copper and
brass radiator. While all provide acceptable performance, better antennas use brass or
copper inside the fiberglass for maximum strength and durability.
Do antenna extensions increase
gain?
A few years ago, we were surprised to see a marine publication suggest that adding an
extension to your VHF antenna to get it higher off the water resulted in an increase in
antenna “gain.” The gain of the antenna is primarily determined by how many
stacked elements, or radiators, it has. This results in longer antennas for higher gain
levels. The addition of an extension does not affect the construction or electrical
properties of the antenna, and therefore has no impact on gain. However, an extension does
get your antenna higher off the water, which results in greater range. Make sure that you
brace your antenna/extension (and brace antennas over 8' in two places) to prevent damage
at high speeds or in rough seas. |
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Metz VHF Whip Antenna
VHF Galaxy 8 FT Antenna
Antenna Mounts
Whip Only for Metz Antenna
Antenna with 20'
for RG58 cable and PL259 Connector
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