Choosing an antenna for your
boat
A boat can have many types of
antennas for communications, and others for navigation and entertainment.
In each category so there are many types of antennas to choose from. To
complicate matters even more, there are combination antennas that do multiple
jobs in one package. However, the only way to get to the end is to
start... somewhere... so let's take a look at the most common antenna
need, VHF, first.
A VHF antenna normally broadcasts and receives within the
VHF band. VHF is a method of encoding transmitted signals, but by world
accord (isn't that amazing!) certain frequencies and "bands" of
frequencies are reserved for maritime VHF communication. Connected to a
transceiver, a VHF antenna puts you in touch with your fellow boaters, with
drawbridge operators, and if needed with the Coast Guard (or other maritime
authority in your country). VHF systems let you call for help, receive
weather information, and exchange necessary communication with fellow
boaters. The range for VHF is not very far - line of sight from the
antenna, actually. For longer range communication other choices abound.
In a VHF system, the antenna is the most important
link. A poor antenna will give poor system performance from even the
finest transceiver, and a great antenna will help you get the absolute best out
of a marginal transceiver.
- What boat?
In choosing an antenna for your boat, the first consideration is the boat
itself, as well as what type of boating you do with it. If yours is
a simple fishing boat that would be dwarfed by a 23-foot antenna, then of
course the longer, better, higher-gain antennas are out of the
question. If yours is one of the luxurious boats with the multiple
decks and the hot and cold running champagne, then that 23-footer might
not even do - you might need the longer range communications of SSB and
Shakespeare's 35-foot Styles 222
and 229-F antennas. Or
check into Shakespeare's other large VHF Commercial
Antennas, instead. Remember, though, that the important
consideration is not just the size of the boat, but what you do with it.
If you never take your 90-footer out of Lake
Fishbitten,
then you don't need a huge antenna, after all. And it's not likely
that you'll take your bassboat Pacific island
hopping.
If that small fishing boat at the
fishing boat store got you hooked, look into Shakespeare's three-foot antennas
like the Style 5240-R. It's a half-wave, Low Profile, end-fed antenna with
a 36" stainless steel whip and chrome plated brass ferrule. The Style 5241-R is the
same design as the 5240-R, but with a heavy-duty stainless steel whip
for stability at high speeds. The 5247 is basically the same thing, but
its heavy duty whip Lifts and Lays down out of the way
while you cast around the boat in search of the elusive basscreature.
VHF antennas can be mounted on a flat
surface, or attached to a rail. For sailboats, you can mount low-profile antennas
like these or the 5215 series on a mast, and the extra height of the mast will
give them superior performance. The Style 5215-C-X even comes
with a 60-foot coax for just such mounting. We'll come to a more
thorough treatment of antennas for sailing vessels later on.
These low-profile antennas are
three feet long. One size larger would be the five-foot Style 396-1.
Remember, with VHF, longer and higher is better. The smaller antennas
might all you need for close-in, however, or your boat might not accommodate a
longer antenna. Can you see one of those 23-footers on a bassboat? The bass would laugh themselves silly.
- Consider the available
space for mounting an antenna
(and a radio), and how the antenna will have to be mounted.
- Rail mounting. Antennas attach
easily to a variety of sizes of rails with handy rail mounts. The ratchets on them
make lowering the antenna quite easy - for fishing or trailering,
whatever. The rails can be horizontal or
vertical, slanted or not. The simplest rail mount is Shakespeare's
Style 4720 Economy Rail Mount. It's a simple clamp and a simple
1"-14-thread bracket that holds the antenna to the clamp.
- Surface (deck or
bulkhead) mounting, flat or vertical. Mounting an an antenna to a flat
surface requires either a flange mount,
or a ratchet mount.
Larger antennas have their own special needs - discussed shortly.
On most boats, it's a safe bet that most of the available flat surfaces
aren't exactly vertical or horizontal. They slope. The
easiest way is with a ratchet mount that handily adjusts for the sloping
surface and still permits quickly raising or lowering the antenna in the
direction you want.
- Mounting Kits. Even if your boat
is large enough so you don't have to take down your antennas to fish,
many longer antennas probably have to be lowered to clear bridges, boat
houses or other overhead obstructions. The two-part antenna
mounting kit provides that option. The upper bracket snaps open, so
you can lay down the antenna when you need to, and raise it again quickly
when clear of the obstacle.
- Mast mounting
(for sailboats, or attached to some mast-like structure on your
boat). Antennas can be strapped to a mast quite easily. Some
antennas are intended for this mounting method, having an elongated
sleeve at the bottom for the purpose.
- Where will it be on
the boat?
Your antenna should definitely be mounted as high as possible.
That's super important, so here it is again: Your antenna should
definitely be mounted as high as possible. VHF signals are
line-of-sight only. so, the higher you can mount
your antenna, the farther it can "see" to the horizon, and the
more boats or other radios it can reach.
In this respect, sailboats have it
made. Installation of a small antenna on top of a sailboat's high mast
can give performance equal to or better than a long antenna on a small boat.
Choosing a location for your
antenna is an extremely important part of choosing the antenna itself.
Remember to keep the antenna away from large metal objects, and especially away
from other radiating devices (like other antennas). The antenna should not
be mounted closer than three feet from the radio, but it shouldn't be so far
away that the radio's signal is depleted before it gets to the antenna.
Don't forget to consider where you
have to put the boat when it's not in use. If the antenna will have to be
lowered when you come in to port, you'll have to be able to get to it to do so.
- What kind of boating
do you do?
The real question is: How much range do you really need from
your radio? The farther you venture from shore the longer reach
you'll need from your radio - and thus from your antenna - to communicate
with base stations like the Coast Guard. The farther out you go, the
farther you have to reach to communicate with other boats, too. It
can get pretty sparsely populated out there. If yours is a fishing
vessel, and you need to communicate with others in your fleet who are some distance away, you'll need a large
antenna, high up. For VHF, output power is limited to 25
watts. So, the antenna has to "reach" with its antenna
gain, a function of antenna height, length, and quality.
- Safety first
The main reason people have VHF radios on board is to call for help, if
help is ever needed. So, make sure the system you choose will have
enough reach and power to be of service in an emergency. Simply
rapping with the other boaters around your dock is fine, but if you need
help, you want to be able to call for help and be heard. Don't
shortchange yourself into a small antenna that won't get your signal
across the water when you need it most. When it comes to safety, err
on the side of caution.
Here is a formula for
calculating the range of an antenna:
Calculation for Range of an Antenna:
Square Root of Height (in feet) above water x 1.42 =
Range in miles
Remember to perform the calculation for BOTH vessels, then add the results to get the range between two vessels.
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- Antenna Gain
Shakespeare's VHF antennas come in gain categories, like 3dB, 9dB,
etc. This is a measure of how efficient the antenna is with the
signal you feed it, but it depends a great deal on the length of the
antenna. The gain of an antenna is stated in deciBels
(dB) of effective radiated power. Gain is an increase (or even a
decrease) in ERP. You do not get antenna gain from any so-called
amplifiers built into the antenna, and it does not mean you can put more
than the maximum legal 25 watts into a VHF antenna. You only
get dB gain from longer, more efficient antennas. High gain factors
are 6dB, 9dB, and thereabouts. Unity means no antenna gain - a
multiplier of "one."