Homebrew Pass-Band Filter
To comply with Part 97 of the FCC rules for 43dB harmonic suppression, I built a 40M Pass-Band Filter to use with my homebrew QRP transmitters. Searching through the ARRL archives, I selected the September 1988 QST article written by Lew Gordon K4VX called "Band-Pass Filters for HF Transceivers". I wanted to build a five-pole Butterworth filter but decided to build the three-pole version as described in the article.
In the above picture, I cut the single sided circuit board material with metal snips, then used a spot weld bit to drill the circuit board pads. Keep in mind I use this Pass-Band Filter with QRP transmitters. If you plan to make one for high power, see my Upgrade Update below or refer to the article because the gap between the pad and the rest of the circuit board needs to be significantly wider. I wound the toroid coils as described in the article using T-80 cores. I also had two aluminum boxes on hand so I used one for the project.
The article also stated to adjust each toroid coil using a dip meter or antenna analyzer. I could not get my antenna analyzer to recognize a dip, so tuned the three LC circuits using my WA1FFL Direct Digital VFO and Techtronic TAS485 attached in parallel to each circuit. I tuned each of the three LC circuits around 6.8Mhz by observing the sine wave peak. I had to remove the number of turns as depicted in the article to get them to resonate at 6.8Mhz. It also took time to locate some package peanuts to make a batch of Q-Dope. My lovely XYL finally got in a HSN shipment with pink peanuts, so I dissolved them in Methyl Ethyl Ketone to make a batch.
The above picture depicts the six components soldered to the circuit board.
The above picture depicts the BNC connectors mounted to the Aluminum Box after drilling out the holes.
And finally the 3 pole Butterworth filter. Using my WA1FFL Direct Digital VFO and Techtronic TAS485, the filter passes 6.9Mhz to 7.3Mhz. It looks like a winner for QRP CW work!
7-4-20 Upgrade Update
I decided to upgrade the 3 pole Butterworth filter to accomodate up to 100 watts. The upgrade consists of a one sided circuit board I etched to provide a wider gap between the three pads and the circuit board ground.
Here is the 3 pole Butterworth filter upgrade using the existing components from version one above. With my WA1FFL Direct Digital VFO and Techtronic TAS485, the filter now passes 6.82Mhz to 7.22Mhz. Given the goal is to use it when operating my homebrew 40M CW transmitters, I am good to go.