Choosing a VHF antenna is not as simple as 123, but you need to create a sense of balance between gain, cost, and size. The type of antenna that you need to acquire will depend upon the physical requirements, cost, and distance desired. When it comes to getting a good VHF coverage, the use of an antenna is necessary.
If you happen to have an extraordinarily good radio, it will have no value at all if you can’t connect it to a good antenna. A primary consideration here is the height of the antenna above the water.
The higher location you install your antenna in, the more coverage or range it will have. It is inherent for VHF radio to traverse the air space in a straight line, the direction of which is called line-of-sight.
The curvature of the earth’s surface is likely to create an impact on coverage, as can be evidenced by interruptions. Besides, any stumbling block in between the two radios can also affect the coverage.
If two stations attempt to establish communication between each other, it will be futile if either one or both of them fall below the horizon. If this happens to be the case, then there is no possible way of establishing a connection.
Due to the earth’s curvature, it is normal for VHF communication to be in a limited range, at 40-50 miles.
Since height is one of the prime considerations in range, sailboats are likely to enjoy better range if they will install their antenna to the top-most mast of their vessel, and of course, they need to make use of the proper kind of VHF antenna for this purpose.
Powerboats that are relatively smaller in size are just a few feet away from the water. Hence, making it impractical to mount such a high antenna.
If you want to calculate the range of an antenna, you can refer to the formula below:
Range in Miles = (height above water)2 x 1.42.
Keep in mind that range is additive, and would be highly dependent on the height of the antenna of the other station. It is typical for a 3-foot antenna to yield around a 2.5-mile range. But, if the other party also has a similar 3-foot antenna, then there is going to be a good chance that it will see 2.5 miles. For the communication range, anticipate it can be around 5 miles.
Ant Height Range
- 20 foot 13 miles
- 10 foot 9 miles
- 5 foot 6.3 miles
- 3 foot 5 miles
Height = Antenna height + height above water
One can program an antenna in such a way that it will provide gain to a radio. The gain can work in receiving and transmitting. By virtue of existing law, marine radio is confined to only have 25 watts of transmit power. Therefore, how can range get further enhanced by gain?
The VHF antennas do not possess any power that will help in increasing the power fed into it. What the antenna does is manipulate this power, and make it centered, focused. It will cause the marine antenna to take that power from the air and have it to the horizon. Much like how a balloon works, by smashing it — even with such a great force — its natural tendency is that its sides will only expand.
Connectors and the accompanying coaxial cables are other things to consider. They both have a shared amount of loss. The amount of loss can be referenced from the label specifications of the coax and is in decibels.
Coax, often seen as the total opposite of antenna gain, can be measured in dB at every 100 feet. Meaning to say, if you have a 100-foot piece of coax that comes with a 3db loss over the cable length — expect it to lose the power that you put into the line.
Subjecting your 100-foot piece of coax with 25 watts of power with 3db loss at 100 feet, you can anticipate it to have 12 output watts going into the antenna.