Narrow BW (.1%) CombLine Filter

Theoretically yes

I think it is possable

Practically, no. A filter usually has a need for a low insertion loss, flat passband, and defined in-band riple. When you take an ideal prototype and try to make it into an actual real life filter, the Q of the filter elements tends to distort the passband so much that it looks nothing like the prototype prediction.

Now, if allyou want to do is pass a single frequency and reject out of band stuff by as much as possible, and you do not care too much about insertion loss, then yeah, you can make a narrow filter at 12 GHz. A single dielectric resonator puck losely coupled will do the job nicely.

I suppose you could use a saphire resonator or superconductor resonator and end up with superhigh Q elements that could do the filter job, but that is more science fair than engineering.

Temperature stability may be an issue at this narrow bandwidth. Usually a frequency conversion to a lower frequency is required prior to narrow band filtering, giving a larger relative bandwidth. For example, at 240 MHz, 12 MHz BW is 5 %.

Thanks VSWR.

I did design the filter, and it worked.
My concern is its performance over temp.
I used cold rolled steel for my resonator, and a silver plated aluminum housing.

Unit was plugged into a sub assy after I built it so I didn't have any chance to test over temp.

I'll build another unit to test over temp to see its performance.

Regards
Krytar