Pim said:I once managed to assemble a very high performance notch-filter for about 230 MHz by using a helix resonator with top-tuning (by adjusting the distance of a grounded plate closer or further from the top of the helix). I used it for a simple cable-tv descrambler as they just put on an interference signal on the RF for the channel.
I recall that I found some useful info regarding helix-based designs in the ARRL handbook (don't remember the year of the book, but it was at least 10 years old). The most work was not in calculating things but done by trial-and-error. I would strongly recommend a good book on the subject before trying anything like I did, i.e. trial-and-error as it was a very time-consuming task to get it all perfect.
Also, my design was sensitive for temperature (the dimensions of the helix, which was made of about 1,5 mm dia copper wire), changed sligthly with the temperature. So one had to fine-tune it now and then. But correctly tuned it had a superior performance indeed.