So what is an antenna exactly? It is a device that takes an oscillating electric current and radiates that energy in the form of electromagnetic radiation and vice versa. More simply, it converts electric power into radio waves.
Understanding the important characteristics of the marine VHF antenna will allow you select the right antenna to best fit your vessel and your specific needs.
The Marine VHF Antenna
The most common type of Marine VHF antenna found aboard vessels is commonly referred to as a whip antenna. A couple of examples are shown below:
A fiberglass whip antenna of various lengths from 4’ to 23’. Again this type of antenna can be mounted on any type of vessel, however it is typically found on power boats.
A 3’ stainless steel whip antenna. While this type of antenna can be mounted on any type of vessel such as smaller power boats, it is most commonly found on the masthead of sailboats.
Marine VHF Antenna Characteristics
Let us start by saying that “Antenna Theory” is an exceedingly complex subject. The design and proper functioning of an antenna is comprised of determining the frequency of use, the radiation pattern desired, directivity, efficiency, gain, bandwidth, polarization, and antenna temperature just to name a few of the parameters involved. Needless to say the theory goes far beyond what the average mariner requires in attempting to select the proper antenna for their vessel. So we are going to attempt to keep this a bit simplistic and discuss in general terms what you need to know.
What is important to know is that all of this theory has been done for us by the antenna manufacturer. The goal being to produce a product that will provide some combination of performance and durability. Some antenna manufactures have excelled; others, not so much.
Below we will discuss 3 parameters of all common marine VHF antennas: Polarization, Radiation Pattern, and Antenna Gain. While the polarization and the basic radiation pattern is common to most antennas, it plays a part in understanding how antenna gain can affect performance depending on how the antenna is used.
Polarization and Radiation Patterns
Generally speaking, there are (2) types of polarization: Linear and Circular. In the case of mobile types of communications (think marine VHF,) the linear polarized antenna is the most common found.
Within linearly polarized antennas, you have horizontal polarized and vertical polarized. A linear polarized antenna radiates wholly in one plane; which is the direction of propagation.
At left is a view of the radiation patterns of a simple omnidirectional vertically and horizontally polarized antenna. The red antenna being located at the center of the "donut," or torus. Distance from the center represents the power radiated in that direction. The power radiated is maximum perpendicular to the antenna, dropping to zero directly in line with the antenna axis.
Looking at the diagrams above, unless you are interested in communicating with the moon or possible a submarine, a horizontally polarized antenna is pretty much useless for marine VHF communications.
Without getting into electrical-field theory and antenna design, suffice it to say that the common marine VHF antenna is a linear vertically polarized antenna. In its most basic configuration, the marine VHF whip antenna’s radiated signal is basically of equal strength through 360 degrees of the horizontal plane as shown below.
Another view of the radiation pattern from above looking down at the tip of a simple omnidirectional vertically polarized antenna. The antenna is at the center of the diagram
The radiation pattern of a simple omnidirectional vertically polarized antenna. The antenna is at the center of the "donut," or torus. Distance from the center represents the power radiated in that direction. The power radiated is maximum in horizontal directions, dropping to zero directly above and below the antenna.
The formal definition of antenna gain is the sum of antenna efficiency and directivity. For the non-expert in antenna theory, antenna gain could be stated as the "apparent" increase in power made available by the antenna alone.
In the simplest terms, it is a relative measure of an antenna's ability to direct or concentrate radio frequency energy in a particular pattern or direction. This measurement is typically measured in dBi (Decibels relative to an isotropic radiator) or in dBd (Decibels relative to a dipole radiator). However in most marine VHF antenna specifications it is simply stated as dB gain.
An isotropic radiator (think 0dB gain antenna) is nearly impossible to produce. However, the 0dB antenna is used as a starting point in theoretical design criteria and for a base line in measuring antenna gain.
Considering the difficulties in producing a 0dB antenna, for the sake of this article we will presume that all antennas have gain whether positive or negative.
While in fact higher gains in an antenna do not cause an actual increase in the power radiated, it does shape the radiation pattern to concentrate that power into a desired pattern enhancing the perception of increased power.
Below is a simple view of the radiation patterns for 3, 6, and 9 dB gain antennas:
The radiation pattern of a 6 dB gain omnidirectional vertically polarized antenna.
The radiation pattern of a 3 dB gain omnidirectional vertically polarized antenna.
The radiation pattern of a 9 dB gain omnidirectional vertically polarized antenna.
Here it is again with the 3, 6, and 9 dB gain antenna radiation patterns superimposed on one another:
So what does this all mean? Well basically as antenna gain increases, the radiation pattern narrows and elongates. More simply put, it means that the power being radiated by the antenna reaches out further the greater the increase in gain. All other things being equal, this provides much improved communications at longer distances.
So right now you might be thinking: I am going to get a 9 dB gain antenna since it will provide me with better and more reliable communications at longer distances.
My suggestion - Slow Down and Take a Deep Breath!
Higher gain may not always be the best choice.
The first thing that needs to be clarified is that the maximum communication range will not change with an increase in antenna gain. Basically, if the antenna on your vessel cannot see the antenna on the vessel you are trying to communicate with no communications can occur. No amount of antenna gain is going to change that. See Marine VHF Radio - Radio Range How Far Can I Transmit?
It does mean though, that the radiation pattern can be shaped to provide better communications within the maximum range of communication. So the question is; what is the best radiation pattern shape for you?
On power boats that are not subject to heeling or heavy rolling action, a 9 dB gain antenna will provide a significant improvement on your VHF radio’s performance. On the other hand if your power boat is often subject to seas where there is the possibility of rolling or pitching action you may want to consider nothing more than a 6 dB gain antenna, and finally, sailing vessels, especially those who occasionally like to bury the rail, should limit themselves to a 3 dB gain antenna.
So why is that? Anytime the antenna is not perfectly vertical, higher gain antennas will actually reduce the horizontal communications distance and increase the vertical as shown below.