db refers to decibel. It's a commonly used measure of power. 3db = 2 X power, 6db = 4 X power, etc. Example: a 25 watt transmitter with a 6db gain antenna would have an effective radiated power of 25 X 4 or 100 watts, not counting losses in the transmission
frequencies, coax loss is important if the coax is very long. It can be greatly reduced by using proper coax. RG-213 or 214 is preferred and commonly available (better than RG8 and much better than the thinner RG58 or 8X). LM400 and other exotic varieties of coax have even lower losses. You can get an idea of what types of coax are available, and their properties, from the following: RF Connection Coaxial Cable
Generally, mounting the antenna higher is better. But, I doubt that 3' would make much difference at all. Put the antenna in the clear as best you can.
Assume the govt. of Grenada
is cool with this? It's a hassle getting a shore station license
in the U.S., but can be done if you have a clear commercial