E-plane filters with selectively located transmission zeros

The possibility to produce transmission zeros at selectively located finite frequencies is reported for bandpass filters compatible with the conventional E-plane topology. The structure is based on etching resonant apertures on a thin all-metal E-plane insert, which are coupled in parallel. We demonstrate that different combinations of the horizontal (overlap) length and vertical distance between the resonators can produce the same coupling coefficient while introducing transmission zeros at different frequencies. This allows for selective location of the transmission zero for each pair of resonators, which is demonstrated by means of 2 nd order filter examples. Two prototypes have been designed and fabricated producing sharp roll-off at either the lower or higher cutoff frequency. Good agreement between numerical and experimental results validate our argument. Moreover compared to conventional E-plane filters, the parallel coupling topology is shown to reduce the overall dimensions by about 50% and shift the spurious resonance by about 1 GHz at x-band while maintaining the low-cost and mass-producible characteristics.