# Antenna tutorial



## Fishers of Men (Jul 29, 2005)

The best marine radio will not perform well when connected to the wrong antenna.
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: 
&#8226;	Rail Mounting Available in a variety of sizes and include ratchets to allow easy lowering. 
&#8226;	Mast Mounting: For sailboats 
&#8226;	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: 
&#8226;	Mount as high as possible to take advantage of line-of-sight. 
&#8226;	Mount away from large metal objects 
&#8226;	Mount away from other antennas 
&#8226;	Mount at least 3 feet away for marine radio 
*Other Considerations*
&#8226; Use quality coaxial cable and connectors: You can loose considerable signal strength with poor cable and connections. The longer the cable, the more signal loss there will be. Cable lengths of 10 to 20 feet are not of much concern, but a sailboat using small diameter cable running 100 foot long can loose 80% of its signal strength. 
&#8226; Combination antennas: Whenever possible, you will get the best results with using an individual antenna for each purpose. You can get a VHF antenna that is also your AM/FM antenna, but it won't work quite as well as the single purpose antenna.

to be continued...


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## KaGee (Sep 8, 2006)

The Achilles heel is the RF connector, commonly known as a "UHF" plug or "PL-259". The connector has to be removed in the majority of installations in order to route the coax. The problem enters when the connector is not properly soldered back on. 

WEST MARINE and others sell a "solderless" connector. These should be shunned. They rely on a mechanical connection which can easily become corroded. Use a soldered connector, with a good solder job you will not have that problem.

Another thing to avoid is shortening the coax. Coil up any excess and stow it... never cut it. The length of the coax is figured into the antenna's tuning.


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## Fishers of Men (Jul 29, 2005)

Good call Keith,
Reminded me also, keep the coil away from other electrical items, such as a transducer cable. lol and when you coil it, it creates another magnetic field.
You need to keep all these things and metal away from your compass. 
I guess we need a separate thread for compasses tho.


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## I Fish (Sep 24, 2008)

So, does the antenna send the signal from the very tip, or from the whole thing? I would like to add one, but with a 18' center console and a bimini top, I'm very limited to locations, especially with consideration to other electronics. Could I mount a shorter one on some kind of telescoping tube to get more height, just when using the radio?


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