I bought an MKU 13 LPF PCB from Alan G3NYK, and built it into one of his excellent tinplate boxes. Loss at 23cm is about 0.45dB. Best rejection is 46dB at 1.977 GHz and above 3GHz where it is over -50dB. I did not measure the response at the third harmonic.
That mirrors the spec on the DB6NT site fairly closely.
Equipment used: HP E4421B signal generator, HP 436A power meter, 3dB pad before and after the filter.
Next, I tested the output voltage using my 23cm transverter as source and a 30dB 20W 4GHz attenuator to measure the power out of the filter versus voltage on the detector. It is pretty much log-law up to about +33dBm.
I used an 0805 51R, face-down, as the terminating load, a 470 ohm 1/16th watt in series with the feedthrough and the recommended 1nF 0805 for the diode bypass. I couldn’t find a BAT62-03W, so I have used a SMS7630-079LF from Farnell. Measurements were made using a 10M input resistance DVM. and the 30dB 20W attenuator on the output, followed by the HP436A using 8481A and 8481D sensors.