This paper reports the results the application of sapphire rod trimming to improve HTS filter properties. First, we examined the optimum position of the sapphire rod above the hairpin filter to shift the resonance frequency. The resonance frequency could be changed about 20 MHz, a value which is sufficient for filter trimming. Next, we examined sapphire rod trimming for a 5-resonator hairpin filter. We found that sapphire rod trimming is a very useful technique for improving filter properties; for example, reducing the ripple and increasing the steepness of the filter properties. We also discuss improving sapphire rod trimming using an equivalent circuit model. The improvement and sapphire rod position could be explained by the equivalent circuit model.
