Design of the varactor-tuned bandpass hexagonal resonator filter with source-load coupling

A varactor-tuned bandpass filter using hexagonal open loop resonators was designed and fabricated. The source-load coupling was utilized which allows the filter to obtain additional transmission zeros for high stopband selectivity specifications. The filter has a tuning range of 670-770MHz with a 3-dB fractional bandwidth of 2.9%-3.5% and an insertion loss of 3.0-6.0dB. The experimental results validate the theoretical analysis and simulations with good agreement except for a frequency shifting of about 30MHz which is mostly caused by the tolerance in manufacturing.