Below is a comparison between calculated and measured performances of a narrow band (1% BW) 7-pole pure metal insert filter. The insert was etched from 0.1 mm thick Beryllium Copper foil placed in a split-block aluminum waveguide. 
In the filter calculations 25 modes were used. No tuning was applied afterwards.

Red and blue curves are calculated

The following applies:

• The manufactured filter is only 9 MHz too low in frequency, corresponding to a frequency error of 0.06%.

• Bandwidth is as predicted.

• Return loss of manufactured filter is better than 24 dB.

• Losses are not accounted for in the model. Lower losses in the physical filter can be obtained by silver plating of insert and waveguide.