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In this letter, a low-cost continuously tunable substrate integrated waveguide (SIW) resonator and a low phase noise SIW voltage-controlled oscillator (VCO) are presented. A simple new structure for coupling varactor to the SIW cavity is proposed, which realizes a simple circuit layout for the tuning resonator. Equivalent circuit model is given for the varactor coupled resonator. Relation between the coupling coefficient and the relative tuning bandwidth is analyzed. Prototypes of tunable SIW resonator and a low phase noise SIW VCO have been designed and fabricated. Measured results show that the proposed resonator possesses unloaded Q factor of 286-299 for a tuning range of 492 MHz, and the phase noise of the SIW VCO is -93 --95.6 dBc/Hz at 100 kHz offset for the whole oscillating frequency range of 455 MHz at X-Band.