Asingle-port uniplanar antenna with a built-in tunable filter is presented for operation inmultiple LTE bands for cognitive femtocell\napplications. The antenna is based on a monopole microstrip patch fed by coplanar waveguide.The frequency reconfigurability is\nachieved by using two PIN diodes to couple or decouple a ring slot resonator filter fromthe antenna feed line. By switching the PIN\ndiodes, the proposed design can operate in either wideband or narrowband modes.When the antenna operates in the narrowband\nstate, two varactor diodes are used to continuously tune the narrowband frequency from 2.55 to 3.2GHz, while the wideband\nstate is obtained over the 1.35ââ?¬â??6.2GHz band. The diodes and their biasing networks have nearly no severe effect on the antenna\ncharacteristics. Prototypes of the proposed structure using ideal and real switches, with and without varactors, are fabricated and\ntested. Measured and simulated results are in good agreement, thus verifying the good performance of the proposed design. The\nobtained results show that the proposed antenna is very suitable for cognitive radio applications, in which the wideband mode is\nused for spectrum sensing and the narrowband mode for transmission at different frequency bands.
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