Doug, W4DML was having trouble with WSM AM 650 interfering with the 17m band of his uBITX v6 radio.
No stranger to radio hardware and filter design, Doug designed a notch filter to deal with the problem.
Doug designed the filter using Elsie software from Tonne Software ( http://tonnesoftware.com/elsie.html ), creating a notch filter to block WSM on 650 kHz. The program gives values for capacitors that do not exist so he substituted available capacitor values close to the calculated values. The inductors are wound to the revised calculated value. Doug settled on 0.01 uF caps and 6 uH toroidal. The toroids are T68-7 with 34 turns of copper wire (magnet wire). Doug notes that the wire gauge does not control the inductance and used 22 gauge.
Doug shared some photos of the design and build and a short video demonstrating the results:
Here is a screenshot of the filter schematic in the Elsie program (Doug says the student version is sufficient for the kinds of ham radio filters he designs.) (click on the images to see larger versions)
along with a plot of the design using the Elsie software:
After wiring up the design:
… the spectrum analyzer with tracking generator confirms prototype:
Doug’s comment on the analyzer plot above:
The [plot above] shows the marker at 640 KHZ at the top of the photo and the marker pointer is on 640 instead of at the bottom of the dip in the notch. This is because of the scaling of the marker. It skips several KHZ between each movement of the adjustment dial. The next click up is 680 and the next click down is 620. I could have set the stop frequency to a lower frequency and this would give more resolution in the marker. I think it was more important to show the stop frequency of 30 MHZ in order show the minimal loss on other frequencies in most HF radios.
Here is the plot displaying about 2MHz:
The filter can be evaluated with a relatively inexpensive Nanovna as well:
Doug finaly transfered the prototype to a box with connectors:
This video ( https://youtu.be/0ycqMaBs2CA ) demonstrates just how well the filter works:
No more WSM interference!
Doug has designed and built a number of other filters as well:
Thanks Doug for taking the time to share the details of this build.