Selecting a VHF Antenna
By Tom Burden
VHF antennas are a crucial part of your VHF installation, and understanding how they work and how to select the correct antenna can greatly improve your ability to communicate with other boaters and shore stations. The main purpose of a VHF antenna is to provide a radiator for the power that the transmitter in the radio produces, and to radiate this energy in the correct direction. Antennas also have to be enclosed in a weather-protected enclosure (or be made of a corrosion resistant metal and plastic) and must be able to withstand the substantial forces generated by a boat pounding in heavy seas.
An antenna’s dB rating (or antenna gain) indicates the apparent increase in transmitting power due to its ability to focus energy. Antennas with a high dB rating concentrate energy perpendicular to the antenna shaft in a field that is shaped like a disk. This makes your radio signal appear stronger to receiving stations around you.
It also reduces the amount of energy transmitted above or below the antenna, which can be a problem if your boat is pitching or rolling in seas. A concentrated signal can actually fade in and fade out as the boat rolls and pitches. As the dB rating of the antenna increases, so does the height of the antenna, but the horizontal angle diminishes. The broad radiation pattern from a low dB antenna allows a sailboat to heel over and still send signals toward the horizon. Sailboats should use a 3dB antenna mounted at the top of the mast whenever possible.
Gains and losses
Customers have asked us if it’s necessary to match the antenna gain in dB to the loss (in dB) caused by the coaxial cable inside the antenna. While the concepts are related, you can’t trade one for another. Here’s how we explain it:
A gain increase of 3dB means a doubling of signal strength: 6dB is a fourfold increase, and 9dB is an eightfold increase. The actual watts transmitted doesn’t increase; it’s just that the power is concentrated, much like a reflector on a flashlight, in a more concentrated beam. Therefore, a radio with a 6dB antenna will sound like it has a larger transmitter than the same radio with a 3dB antenna.
Coaxial cable, while efficient, loses some energy with every foot. In the West Advisor on Coaxial Cable we’ve noted that RG-58U coax (the small stuff) loses 3dB for every 49' of run. That’s roughly the same as saying that if you have a 49' run of cable, your radio will sound like it’s transmitting with 12.5 watts instead of 25. If you run the same cable 98', you’ll sound like you’ve got a six-watt radio. Note that by putting your antenna at the top of an 80' mast, even using small coax, you’ll still get very good range, but your coax will have reduced your effective power. Therefore, it is prudent to use the largest coax you can, especially on long runs. The trade-off is more weight aloft, which increases heel and diminishes sailing performance, so you will have to compromise.
VHF antennas are made of stainless steel or fiberglass. For masthead mounting, short stainless “whip” antennas create less windage and provide the appropriate wide radiation pattern required by a heeling sailboat. Small powerboats may also use stainless whip antennas because they are rugged. Fiberglass tube antennas vary in quality. For high-end applications where maximum range and durability are important (like oceans and large lakes) we recommend Shakespeare Mariner Series and Galaxy antennas, which are coated with smooth polyurethane and will last five to ten years. They use stainless steel ferrules as well as brass and copper elements for maximum range and efficiency. Less expensive antennas use nylon ferrules, which is not as strong as the chromed-brass or stainless steel found on quality antennas.
Elements inside the antenna
There are three main styles of electrical elements: those using cut lengths of coax cable, those using a simple brass radiator, and those using a more complicated copper and brass radiator. While all provide acceptable performance, better antennas use brass or copper inside the fiberglass for maximum strength and durability.
So what kind of range can I expect?
To maximize the range of your VHF, there are a few rules that you should know:
Marine VHF radios for recreational boats are limited to 25 watts of output so, unlike Single Sideband or Ham radios, you cannot boost your range with a more powerful transmitter.
VHF radios operate on a line of sight principle between stations, meaning that the signals do not bend around obstructions or over the horizon (Okay, they do a little, but think of them as traveling about as straight as a beam of light.)
Antenna height, more than any other factor, determines how far you can transmit. An antenna mounted up high can “see” farther over the horizon.
Antenna gain is important. If several otherwise equal radios try to contact a single receiving radio, the higher gain antenna is the one the receiving radio will hear.
So how far can one VHF installation talk to another VHF installation? You can determine your likely range by using this simple bit of math:
To determine the range of an antenna, multiply the square root of its height (in feet) above water by 1.42. This gives the range in miles. Remember to perform the calculation for BOTH vessels, and then add the results to get the range between two vessels.
There are an infinite number of radio-to-radio distances and heights that we could examine, so we’ve simplified by using the examples shown below. The antenna heights are given for a handheld in a small boat, a typical powerboat antenna height of 10', and sailboat installations with 30ft. and 60ft. heights. Distances are in nautical miles.
|Transmit||Receiving Antenna Height|
Troubleshooting your antenna installation
If you suspect a problem with your VHF antenna installation, there are a limited number of possible suspects. Check this useful video from antenna supplier Shakespeare: