Re: Antenna Advice
It[']s on top of a pilot house. I calculated it to be 16 ft from flat water to antenna tip with an 8 ft antenna.
Would I be better with a 9db rating to get further range?
In your case the antenna base is about 8-feet above the water. This is a reasonably good height for a small boat antenna. You can figure the effective antenna height by using the mid-point of the radiating portion of the antenna. For example, if you used a four-foot extension mast on your base, and mounted a 3-foot antenna on the extension, the center of the radiating part of the antenna would be about 8 + 4 + 1.5 = 13.5-feet above the water. Or, let's assume you used a common "8-foot" antenna which has about 2-feet of non-radiating segment at its base and a 6-foot antenna at the top. This antenna is too heavy for an extension, so we mount it on your base. Its height is going to be 8 + 2 + 3 = 13-feet. In either case, your antenna is going to be about 13.5 to 13 feet above the water.
The range to the radio horizon is found from the relationship
Range-in-miles = ( 2 x height-in-feet)^0.5
If you could double the height of the antenna, the range to the radio horizon would increase by a factor of 1.4. In other words, to get a 40-percent increase in range, you would have to increase the antenna height to 27 or 26 feet. That is typically not a practical installation on a small boat. We can say that when the antenna height is more than 10-feet above the water, a point of diminishing returns has been reached for improving range by increasing antenna height. It is just not practical to run around on a small boat with a 27-foot high antenna.
All antennas create an effective gain by concentration of the radiation into a main lobe and reducing the radiation in all other directions. In a vertical antenna the concentration is to narrow the range of vertical angle radiation coming from the antenna, and to concentrate all radiation, as much as possible, into one main lobe aimed at the horizon. The more gain increases in the main lobe, the less radiation there is everywhere else. As long as the antenna remains stationary and its main lobe is directed toward the horizon, the result is a beneficial outcome: more signal is produced. If the boat is in motion and the orientation of the antenna changes from vertical, the main lobe of radiation moves away from the horizon. If the main lobe of radiation is very narrow, it takes only a few degrees of change in orientation to moved away from the peak signal and into the region where there is much less signal. For small boats there is always some motion unless at the dock or operating in extremely calm water, and for this reason the use of high-gain antennas is often seen as a disadvantage. Lower-gain antennas have a much broader main lobe, and even when the boat rocks 10-degrees, the change in gain at the horizon is minimal.
The improvement in range that can be obtained by increasing antenna gain is hard to calculate. The basis for a range increase due to gain increase depends on the path loss for a certain distance. There are several ways to model path loss. In the most favorable model, path loss is calculated as increasing at a rate of 20 x log(distance); this is called the free-space model. This gives us a relationship between path loss and path distance. If we use this model and calculate the outcome for a loss of 3-dB, we find the distance would be in the ratio of 1.4:1. That means an increase in gain of 3-dB ought to compensate for 3-dB of additional path loss, which, with this most favorable model, implies the path length could increase by 1.4-times. That sounds rather good. It may be too good.
The actual path loss is not always so easily modeled. Some models for actual path loss suggest the path loss is related to distance by a relationship more like 40 x log(distance). That implies that a 3-dB increase in gain would make the path length increase to 1.19-times.
It is hard to say exactly which model best describes any particular boat-to-boat communication path. As a result, we are left with making something of a guess about how increase in antenna gain will improve range. We can say that doubling the antenna gain will likely improve the range by about 20-percent to perhaps 40-percent at best.
By the way, your original antenna, the Shakespeare model 5206-C is probably one of the least expensive, least durable antennas available. It may also not be a particularly good antenna. I don't recommend that antenna.
The claims made by antennas manufacturers about the gain of their antennas are all really quite fanciful, and there is little rigorous testing or verification. It is my opinion that at the very least, all of these antennas have their gain overstated. You should subtract 3-dB from any claimed gain.
Let me turn to your initial question: is it better to use a 9-dB gain antenna to get further range?
Any increase in antenna gain helps to increase range, as I demonstrated above. These "9-dB" antenna are typically not really 9-dB gain; they're probably more like 6-dB. They are physically longer or taller. The taller antenna comes with an increase in antenna height, but the effective height is really the mid-point of the radiator, not the height of the tip of the radiator. The increase in gain comes with the narrowing of the main lobe. If you have a very stable boat, you may find that the narrow lobe does not create a problem.
My general recommendation is to use a smaller antenna, typically a 3-foot metal whip or a 4-foot fiberglass enclosed antenna, and mount it on a 4-foot extension, in preference to using an 8-foot or 16-foot antenna. I also like the durability of the metal whip antennas. The metal whip will just bend and not break if you hit something.
For more on estimating path loss, see
http://continuouswave.com/whaler/reference/pathLoss.html
For more on how antenna gain affects main lobe width, see
http://continuouswave.com/whaler/reference/antennaVertical.html
For more on how to estimate the range of communication, see
http://continuouswave.com/whaler/reference/VHF.html
For a derivation of how the radio horizon is calculated, see
http://continuouswave.com/whaler/reference/radioHorizon.html
On my own boat I use a 3-foot metal whip antenna on a 4-foot extension mast. This has proven to be a very durable antenna. It has been in use for seven seasons of boating. I do a lot of trailering and it has survived many thousands of miles on the highway. The performance of the antenna has been extremely good. I can communication boat-to-boat at a range of more than 15-miles. I can typically receive NOAA weather radio broadcast transmissions at ranges of 75-miles or longer on a consistent basis. I describe the particulars of my antenna installation at
http://continuouswave.com/whaler/reference/VHFAntenna.html
and I have no reservation at all in recommending those components. They are not expensive, they are durable, and they work extremely well.