That is only correct, for the most part, if you consider strength of signal and projection over flat plane ONLY (in other words - over short distance); however, if considering the Earth's curvature - a longer antenna will always give you MORE RANGE. For various reasons it is difficult to exceed 20-25 mile effective range radius but you will not get even close to that maximum possible distance/range with a short antenna mounted low.
Agree with 'the higher the antenna the better', especially on a non-rocking/rolling boat. Our previous boat had two VHF radios. One attached to an 8', 6 db antenna, mounted on the flybridge with the base of the antenna 11' above the water. (effective height, 19' tall). The other radio was hooked to a Shakespeare (4018) two-piece 19' tall 9db antenna where the base was also mounted 11' above the water (effective height, 30' tall/above the water). I could communicate between the Pungo Creek area of NC to Silver Lake on Ocracoke Island to a friends boat that had a similar 9 db set up on his boat. (We could NOT with the 8' 6db). The distance, (the way the crow flies) between Pungo Creek and Ocracoke is about 46 miles. Under certain conditions VHF can 'skip' and give a longer range than even the direct line of sight antenna-to- antenna graphics depict.; especially with the 9 db that concentrates the signal to the horizon. 9 db is a great way to go, but on rocking/rolling smaller boats, the 9 db signal is often just lost/directed into the water, and/or directed into outer space! On our Parker-Sized boats, the 8' 6db is a good compromise. We could use a 9 db, but it would work best, only on calm days.
A picture says a thousand words.
I'm with Warthog! I have many friends over the years that wasted time and $$ with a short, 3db whip antenna. They can sometimes RECEIVE signals from a great distance (say, from another boat or USCG base-station, and who are NOT using a 3db!), but your 'sending/broadcast' signal will fall short. I've been told many times, "Hey, I heard you calling me, why did you (meaning me!) not respond?!" The simple answer is, I didn't respond because it was impossible to receive their weak, short-range, 3db signal. I didn't hear them. A 3db will do a little better mounted 60 feet in the air on top of a sailboat mast, but that's just because of the height; the signal is still a weak 3db. (Sailboats use them because if they had higher db antennas, most of the signal would be shot into the water or into outer space when healed over).
Another vote for the bigger antenna. You should absolutely not be looking at anything less than a 4' aerial, preferably 8'. Look at the underside of your T-top; very likely you have mounting plates designed specifically for the mounting of antenna bases. If not, use a backing plate made from starboard or fiberglass and thru-bolt the mount. If you're really worried about it, you can fashion a mount that spans one of the T-top tubes, or install a product like this: Universal Mounting Bracket
