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you shoud'nt use antenna for tag because of the tag is be expensive.
you should design agood antenna for reader for example you can use fractal antennas.
for tag you can use a simple antenna for example a simple coil.
The usual themes for "improving" antennas include:
reduce the cost
reduce the size
increase the sensitivity (sometimes gain, sometimes efficiency)
make it less sensitive to the environment
make it stronger
make it usable in more applications