utmost importance when it comes to receiving AIS signals. My current AIS antenna is a 1 meter antenna, mounted on the mizzen mast, about 30 feet above the deck.
On the other hand, my AIS broadcasting station in Portsmouth, NH, does presently utilize a "rubber duck" 6 inch antenna, perhaps 25 feet above sea level and I do see targets about 6-8 miles out. And that is despite of 3 bridges and other obstructions.