The VHF radio spectrum consists of 55 channels in the 156-163 MHz band. VHF radio-range is line of sight so the higher the two antennas are mounted, the greater the distance. There are theoretical ways to work out the range but for simplicity I will leave them out. Factors such as atmospheric conditions and the installation itself also affect the actual range. Typical range with a coast station can be up to 35-40 nm.
I have encountered people who say they switch off their VHF once they leave on a trip to save battery power. VHF radio power consumption is typically for a 12 volt system, 5-6 amps consumption on transmit when set on the 25 watt setting. When set on the 1 watt setting this is typically around 1.5 amp. Reception only power consumption is typically around 850 mA (nearly an amp). For the electrically power challenged this can add up t0 20 amp-hours in 24 hour period. The VHF radio is however one piece of equipment that should be left on regardless of power consumption. The merchant ship that sights you and tries to communicate with you will do so well before you may be aware of it, and it may be hull down on the horizon. Ships are not limited to what are called shipping lanes, they voyage everywhere. The same applies when you have a DSC VHF radio with AIS incorporated.
When referring to VHF radio aerial performance the term gain in decibels (dB) is used when describing the rating of boat VHF aerials. This also describes the increase or gain in transmission power with respect to concentration or focusing of the emitted radio energy. A boat VHF radio antenna with a high dB rating will concentrate radio energy perpendicular to the shaft of the antenna in a field with a disk like shape. This concentrates the radio energy so that the radio signal is not wasted either above or below the antenna, so increasing range.
A high gain antenna has a greater range, but tends to cause fading when vessel is rolling and pitching such as that experienced in rough weather. As the gain of an antenna increases, the height of the antenna also increases, however the horizontal angle decreases. The lower gain antenna of 3 dB is more reliable with a broader radiation pattern has greater range under heavy pitch and roll conditions. To illustrate these changes consider that a 3dB antenna gain at 5 feet has a horizontal angle of 80 degrees. A 6dB antenna gain at 8 feet has a horizontal angle of 35 degrees. A 9dB antenna gain at 20 feet has a horizontal angle of 20 degrees.
VHF radio signals penetrate the ionosphere rather than reflect off it. There are circumstances in which VHF signals can reflect back from the ionosphere to give what are sometimes referred to as “freak” long distance communications such as during very strong solar cycles. This occurred during cycle 19 in 1957/58, cycle 21 in 1980 and cycle 22 in 1990. During these peaks, the monthly sunspot average rose to extremely high values and the ionosphere reflected higher frequencies than normal. VHF waves can also be reflected from clouds of increased ionization in the E layer of the ionosphere, and during auroras, which are the amazing green alight curtains caused by charged particles from the sun.
Many boats, including myself, also carry a handheld VHF radio. They provide very easy communications back to the boat when in the dingy, ashore or working on board. They are also ideal for smaller trailerboats. Most handheld VHF radios are not part of the GMDSS system however that has changed with units available but subject to different rules. The handheld VHF has a power output limited to 6 watts unlike the larger cousins with a 25 watt rating.
Typical range for a handheld VHF radio is around 3-8 miles. Battery power is also limited with rechargeable battery packs in the range 500 mAh up to 1400 mAh with power consumption on transmit being around 2 amps. This means that the battery pack soon runs down with talking, so either a spare pack is required or limited conversation.
When using a handheld VHF the same procedures apply, being handheld does not mean its to be used as a personal communication source. Handheld battery lives have been enhanced a lot with new technologies, such as the introduction of Nickel Metal Hydride (NiMH) and Lithium Ion (Li-Ion) types, and these can give up to 20 hours. Unlike Ni-Cad battery packs they do not suffer from memory effects.
Some radios claim to be splashproof and some waterproof, and tests by some resellers has shown that they do not always perform well in keeping out water. Investing in a waterproof VHF bag is a good idea if your handheld is an important part of your safety. More about fishing and boats and all about boat radios.