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This paper presents tunable lumped-element bandstop filters for the UHF-band cognitive radio systems. The two-pole filters are implemented using lumped elements with both single- and back-to-back silicon varactor diodes. The single diode filter tunes from 470 to 730 MHz with a 16-dB rejection bandwidth of 5 MHz and a filter quality factor of 52-65. The back-to-back diode filter tunes from 511 to 745 MHz, also with a 16-dB rejection bandwidth of 5 MHz and a quality factor of 68-75. Both filters show a low insertion loss of 0.3-0.4 dB. Nonlinear measurements at the filter null with Δf = 2 MHz show that the back-to-back diode filter results in 12-dBm higher third-order intermodulation intercept point than the single diode filter. A scaling series capacitor is used in the resonator arm of the back-to-back diode filter and allows a power handling of 25 dBm at the 16-dB rejection null. The cascaded response of two tunable filters is also presented for multiband rejection applications, or for a deeper rejection null (>;36 dB with 0.6-dB loss at 600 MHz). The topology can be easily extended to higher order filters and design equations are presented.