Here is an article on ceramic filters by a popular manufacturer Murata.
Attached is an excerpt showing the importance of having a proper matching load for the device. This is for a 455 kHz. IF part with a recommended 3000 ohm load.
As Donald Kirk found in his last posted using a lower impedance load results in a downward shift of the center frequency. PortZ in the NanoVNA will not correct this. Also note that ripple will be affected.
Bottom line - use a proper test setup if you want accurate results.
Roger
Roger Need
