CAVITY FILTER NOTCH

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The article below appeared in Amateur Radio Magazine of  August 1994 and has been modified to show the effect of using a cavity filter in parallel notch mode by being "T" into the Repeater's receiver's input. The repeater frequency shown is 147.875, VK6RAV's receive frequency. The cavity filter is a standard cavity filter with no modifications. When simply placed in parallel with the receiver's input, a notch depth of better than 20dB can be achieved 250 KHz higher than the receiver's frequency. If the pager is closer than 250 KHz, useful pager attenuation can still be achieved as close as 100 KHz, as little as 6dB pager attenuation can eliminate pager overload to a repeater's receiver. The tests were done on a standard 4" cavity filter and it would be expected that a 6" cavity filter would do even better.