A directional antenna has several advantages. The gain gives you more range because your sent and received signal levels are higher. You also pick up less interference from other devices because you don't "see" in as many directions. You also cause interference with fewer other users. Of course, you lose coverage area if you use a high gain antenna due to the narrower beam width. It's a good way to make a link between one pair of stations, or perhaps connect to a few stations that happen to be in the same general direction. One example would be a node on a high vantage point on the edge of town at some distance from a number of nodes concentrated in one direction such as the downtown area. You can use the two antennas to make directional links in two different directions. If you don't have a specific purpose for the second antenna, it's usually a good idea to just put a standard antenna on it. It's often useful to be able to make an connection to the mesh node from somewhere nearby. However, there is no single answer. You make many tradeoffs with antenna cost, coverage area, etc.
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