I am looking to buy a wifi antenna for a boat to connect to shore based wireless hotspots that may be "far" away. Due to the waves, I am assuming that the antenna mast sways +-15° from vertical. The antenna will be mounted approximately 18m above the water.
Clearly if I choose an omni-directional antenna with too high gain, the swaying mast will lead to periodic drops of the the connection.
If I choose the gain too low, I will not reach shore based access points.
Can somebody advise me, what gain of an omni-directional antenna corresponds to an opening angle of 30°?
In principle it should be possible to define a relationship between gain and opening angle but all my internet searches for a graph showing this relationship have been in vain.