It was Warren WA8TOD brought the issues with harmonics on CW to our attention. Undaunted by the issues, he set out to design a bandpass filter that could replace Filter 4 for use on 12 and 10 meters. He wanted to see if it was possible to design a simple filter that had steep enough skirts to attenuate both the harmonics and the “45 MHz – Carrier” spurs. He also wanted to see just how simple a filter could be and still meet this requirement.
Here is a very simple third order filter that appears to fill the bill. Shown are the original design, the actual implementation, and the measured results. It does the job (just barely, in the case of 12 meters but good enough) and is only a third order filter as opposed to the fifth order designs for the original low pass Filter 4.
Warren used the online 66Pacific toroid calculator which called for 5, 20, and 5 turns respectfully on T-6-25 cores. He ended up measuring and trimming these to 4,17, and 4 turns in the final design. Similarly, the design called for 480, 32, 480 pf for the caps… He ended up using 440 (2 x 220), 27, 440 pf to get the profile shown in the measured spectrum.
In the end, he achieved with a single filter a practical tested design that meets both harmonic and spur requirements at 2 watts. It can be done!