The type of antenna you should get depends on the distance you expect to transmit, available space on your boat, whether you need to lower your antenna for bridges or transporting your boat, and amount of "gain" your antenna should have.
The two major decisions you'll have to make are regarding the length of the antenna and its gain.
Length
When it comes to antennas, size matters. The higher your antenna is above the water, the greater the distance you'll achieve. The VHF radio wave travels in a straight line. This is called line-of-sight. Your antenna has to be able to "see" the other antenna. Because of the curvature of the earth, as the distance between 2 antennas increases, they eventually fall below the horizon and can no longer communicate with each other. In most cases, communicating by way of VHF is limited to about 35-50 miles. Sailboats have a distinct advantage here. You can mount an antenna at the top of the sailboat mast and reach great distances with a short antenna. Your type of boat obviously determines the length of your antenna also. A 32 foot antenna on a bass boat is just not a good match.You will have to determine how much range you really need. A 3-foot antenna can usually get you about 5 miles of range. In case of an emergency, you will want to be heard. Don't short change yourself with an antenna that is too short.
Here is the formula for calculating the range of an antenna:
Square Root of Height Above Water (in feet) times 1.42 equals Range in Miles
Example: Highest point of your boat is 6 feet above water. You attach your 3-foot antenna at that point. The antenna is now 9 feet above water. The square root of 9 (which is 3) times 1.42 equals 4.26 miles
So how can you receive that marine radio weather broadcast that is 75 miles away? You have to perform this calculation for the "other" antenna also and add the 2 distances together to get the maximum range between the antennas. The weather transmission may be from an antenna several hundred feet high and on top of a mountain giving it enough range to connect to your antenna's 5 mile range. You would be able to communicate with a boat that was 10 miles away from you if it had an identical set up as you. Each of your antennas could "reach out" 5 miles.
Gain
By law marine radios are limited to 25 watts of transmitting power. An antenna cannot increase the amount of energy it receives from the radio, but it can manipulate it and focus it in certain directions to provide a perceived energy increase. This increase energy is what is called the "gain". Gain is measured in units called dB. A short antenna usually has a gain of 3 dB. This equals a doubling of the signal power. A 6 dB gain antenna can increase signal power by 4 times, and a 9 dB antenna by 8 times.So it would look like higher gain is always better. This is not always true.
If an antenna has zero gain, it transmits the signal equally in all directions. Imagine looking at the antenna at night and seeing thousand of thin laser beams going out of the tip of the antenna in every direction. Some of these beams would be shooting straight up into the sky and some shooting straight down into the water. Well obviously there would be no boats above you or below you so sending a radio signal in these directions is a waste of energy. An antenna increases its gain (and power) by redirecting these useless beams into a more horizontal pattern. The more it squishes down these beams, the more gain you get. When you have increased the gain to 9 dB, the beam is a very thin and horizontal. Now instead of a "ball" of beams coming out of your antenna in all directions, imagine a laser level perched on top of your antenna with a horizontal beam shooting out of each end parallel to the water's surface. As the boat rolls and pitches, this horizontal beam will begin to shoot upwards into the sky and down into the water. Your radio signal will go in and out the same way as the movement of the boat effects the direction of the signal.
So what gain should you choose? Sailboats should always use a 3 dB gain for their antenna mast. The mast movement will least affect it. The 9 dB gain antennas should be primarily used for land use or on boats with less pitch and roll. Larger boats frequenting calmer waters could get buy with a 9 dB antenna. Most powerboats should stick to 3 dB or 6 dB antennas.
Mounting
How you attach your antenna to your boat depends on the features of your boat. The 3 most common mounting methods are:- Rail Mounting Available in a variety of sizes and include ratchets to allow easy lowering.
- Mast Mounting: For sailboats
- Surface Mounting: You can use a flange mount if you have a truly vertical or horizontal surface to mount to. In most cases you will use a ratchet mount that can adjust for the slope of the mounting surface. You can easily lower the antenna with this mount also.
Mounting Location
You should follow these general guidelines for mounting:- Mount as high as possible to take advantage of line-of-sight.
- Mount away from large metal objects
- Mount away from other antennas
- Mount at least 3 feet away for marine radio
Other Considerations
Finding VHF Marine Antennas on eBay
A simple search of eBay auction titles using the search terms marine antenna will give you the best results. You'll have several manufacturers to choose from but the pioneer and innovator in marine antenna design, Shakespeare is a top choice. The U.S. Navy and Coast Guard rely on Shakespeare for antenna engineering technology and fiberglass reinforced composite antennas for all classes of ships - rescue vessels to aircraft carriers.Other Useful Marine Electronic Guides on eBay
- VHF DSC Marine Radio Buying Guide
- Fishfinder Buying Guide
- Pier, Dock, or Shore Fishing With a Wireless Fishfinder
- Marine Boat Compass Buying Guide
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