Antenna's are a black art! First of all decide wether you need an omni or directional antenna (omni radiates equally in all directions). Then decide what level of boost you need. You have also to remember the signal degrades very quickly as the antenna lead increases in length (typically you loose 2db/metre so a 3m run reduces a 6db gain to zero gain). In many ways a short extension antenna has the advantage of allowing you some say in the positioning of the antenna.
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