Cell towers broadcast in a circle around the tower. Under the old scenario, a tower that happens to be close to the water would project out over the water. (See photo.) But by putting in more towers that broadcast in smaller circles, the coverage on land remains good or improves, while the coverage on the water may be reduced.
Above, shows a small number of towers placed along the coast, each tuned for more range. This gives some signal access offshore. Below, there are more towers but with less range. The result is decreased access on the water. 
And the mobile carriers aren't the only ones sabotaging your signal strength. The Federal government is involved, too. In the same way your VHF has a much bigger antenna than your mobile phone, it also has more power, which allows it to send out a stronger signal. The Federal Communications Commission limits the power on mobile phones.
This actually makes sense for the typical user. Just think of how many mobile phones are being used at once in your average shopping mall. If these phones were all blasting away at high power, the interference would prevent them from all working at the same time.
Your dashboard VHF radio typically has 25 watts of power and your handheld VHF usually has 5 watts of power. But the FCC limits the power on a mobile phone to about 0.3 watts. That's a substantial difference. So you have a device sending out a less powerful signal using a tiny antenna. Add to this the interference from the other electronic devices on your boat and the boat itself and it is amazing we are able to get a signal at all!
THE SOLUTION
Luckily there is reasonably-priced equipment available that can greatly boost your signal, allowing you to use your mobile phone in all but the most remote locations. These devices will enhance both your access to voice connections, letting you stay in touch with loved ones and giving you another form of communication in emergencies, and to data, offering increased safety and convenience through access to real time weather and other marine databases.
To access this increased level of connectivity you will need an external antenna and an amplifier installed on your boat, and they will have to be installed correctly. Let's look at some of the information you need to purchase the correct equipment.
You will remember from earlier in our series that in the U.S. there are two broad types of mobile networks, GSM and CDMA. Like all radios, your phone will communicate across certain frequencies. The specific frequencies used will depend on which type of mobile network you have. In the U.S. there are four ranges, typically referred to as 850, 900, 1,800 and 1,900 MHz. For this discussion it is not important which one you use, but you will most likely see these terms so you should be aware of what they mean. Note that all of this becomes far more complicated when you move outside the U.S., so we will focus here on U.S. installations only.
To get a signal you must be able to reach the correct frequency. The good news is that you do not need to worry about this for your phone if you are purchasing it from your carrier, because AT&T and other companies will ensure that the mobile phone they provide you will match their frequencies. However, you will need to consider these frequency issues when selecting a compatible amplifier and antenna.
Other terms you will see are Multi-Band, Dual-Band, Tri-Band and Quad-Band. This refers to the number of frequency bands a device supports. They are called "bands" because a frequency actually covers a range. For example, the "1,900 band" spans frequencies from 1,850 to 1,995 MHz. These terms do not define which bands are supported and they often will support frequencies outside of the U.S. This is typically true when you purchase a quad-band amplifier.
BUYING AN AMPLIFIER
How do you know which amplifier will work with your phone and carrier? Most suppliers – companies like Digital Antenna and Wilson Electronics – make this fairly easy. They allow you to select the amplifier by carrier or by the specific make and model of your phone. Remember, each carrier has specific frequencies which must be supported by your phone, so if you tell them the make and model they can determine the frequency range needed. In general, you should select a multi-band amplifier to give you the most options and flexibility in the future. There are amplifiers that will work with both AT&T and Verizon, for example.
