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Gain is a measure of the efficiency of an antenna to convert energy into radio waves emitted in a specific direction. It is not a great way to measure tag readability. A tag with high gain will direct most of its energy in a specific direction. But if you are reading the tag from an angle to that direction, you might not be able to read the tag at all.
The ability to read a tag depends on multiple factors. If all the tags are in the same orientation—say, horizontal on the front of an item—then a linear-polarized antenna will give you the best results. If the tags are in random orientation, the tag antenna should be designed to be read in any orientation and you should use a circular-polarized antenna.
How densely the items are packed together is also an issue, as is the material of which the tagged item is made. If you have a lot of boxes containing dense objects or metal objects, all stacked close together on a shelf, it will be harder to read the tags, regardless of their gain or antenna design. If you are tagging metal items, even with metal tags, you will not be able to read the tags on items behind other items.
Reading tags consistently involves choosing the right tag, placing the tags at the best spot on the items to be tracked, and stacking the items in such a way that the tags can be read consistently. A good systems integrator can help you with these issues.
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