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Nick Noll.
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Choosing a Shakespeare Antenna Choosing a marine antenna for
your boat VHF Marine TV
The most frequently asked question
regarding marine antennas is the most basic one: Which antenna should I choose?
However, that's a very difficult question to answer simply, because coming to
such a decision is often a very involved process.
This document
attempts to address some of the many issues of choosing an antenna for your
boat. Over time, the material will be expanded and hyperlinked to provide as
thorough an exploration of the process as possible.
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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.
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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.
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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.
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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.
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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.
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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. |
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." | Choosing a Shakespeare Antenna
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