DISHAL's filter tuning method

I have some ceramic resonator bandpass filters I have designed and fabricated for a project. Some are at around 2.5 GHz, and Dishal's tuning method (both on the computer and in the lab) works wonderfully for tuning them up! I can short out 3 of the 4 resonators, tune the 1st resonator, unshort the 2nd resonator and tune it, unshort the 3rd resonator and tune it, etc, just like you read about.

But I has some other bandpass filters around 1.25 Ghz, exact same topology, same proportional bandwidth, same # of poles (just longer ceramic resonators), and Dishal's tuning method does not work at all! When I short out the resonators, either on the computer or in the lab, I do not see a resonance hump to tune the resonator on frequency. I just see a flat 0 dB return loss across the band.

I do not understand. If it works at 2.5 GHz, how come it does not work at 1.25 GHz? I looked them over pretty carefully and do not see any obvious operator error.

Anyone ever see anything like this? Or am I just going nutz?

It is something wrong with the circuit or the way you are coupling. I have used Dishal's method to do tuning and measure coupling coefficients between resonators from 100 KHz to well above 2 GHz.

Yeah, I too am not aware of any bandpass filter types or conditions where Dishal's method does not work, yet here I am with 2 filter designs that will not tune up that way (either on the computer OR in the lab). It is possible that I am making some sort of double mistake...just have not found it yet.

I assume you mean these are ceramic acoustical resonators. They may not short out acoustically like an electrical tank resonator.