Will additional coax stubs improve Bandpass Filter performance?

4O3A 20m Filter with coax Stub

I was curious if the performance of our 4O3A high power Bandpass filters could be improved with additional coax stubs in parallel. We were operating at ED1R for some time now with the added stubs, and we are pleased with the results. Check out the details!

In an earlier post I explained how to build coaxial stubs for 20m and 40m. Why 20m and 40m? A decade of hardcore contesting from stations all around the world has shown me that these two bands are the most critical ones, when it comes to In-Station interferences.

High Power Filters with additional coax stubs

Stubs connected after High Power Bandpass Filters

The picture above shows the bank of 4O3A high power bandpass filters we use are ED1R. One filter per band, from 10m to 80m. On 20m and 40m a coaxial stub is connected in parallel to the filter’s entrance, using a PL259 T-Connector.

Results 40m

4O3A 40m Filter with_without Coax Stub and Stub only

Click the diagram to enlarge

To be honest, I was very positively surprised about the result. In the diagram above you can see the S21 attenuation of the 4O3A 40m bandpass filter (red), the coax stub only (green) and the coax stub in parallel to the Bandpass filter (blue). The coax stub adds through its sharp notch another 30db(!) on the second harmonic. The attenuation from 7MHz to 14 MHz increases from 52dB to 82dB!

Results 20m

4O3A 20m Filter with_without Coax Stub and Stub only

Click the diagram to enlarge

This diagram shows the S21 attenuation of the 4O3A 20m bandpass filter (red), the 20m coax stub only (green) and the coax stub in parallel to the bandpass filter (blue). Again, the stub’s notches on 7MHz and 21MHz clearly improve the overall attenuation on these bands. Attenuation of 40m is kicked up from 63dB to 97dB. As a side effect, 15m gets also additional 25dB, resulting in a total attenuation from 20m to 15m by 95dB.


The 4O3A bandpass filters are doing a tremendous job at ED1R. But unfortunately due to the direct harmonic relation of the two bands, the 20m station was still suffering severely when 40m was operating at the same time. The coax stubs give us additional attenuation between the two bands at a very inexpensive price point.
With the stubs in parallel to the bandpass filters, the interferences are still not completely gone, but at least have been reduced by another 30dB.

I can unconditionally recommend the addition of coaxial stubs in parallel to your filters if you also suffer from interferences between 20m and 40m.

About Tobias (DH1TW)

Self-confessed Starbucks addict. Loves to travel around the globe. Enjoys the technical preparations of Amateur-Radio contests as much as the contests themselves. Engineer by nature. Entrepreneur. For more, follow him @DH1TW


  1. My KISS approach is with eliptical filters with deep notches on narrow ham bands. Coax stubs surely help as you already have filters high reactance outside of working band. Several coax stubs can be combined but it becomes messy on HF. Great work Amigo appreciated by using Firefox :-)

  2. Hi Tobi!

    There is an arrangement you can make using stub-L-stub that will yeld alone more than -70db on 2nd harmonic. I am on road now, but I can give you details later. It is described in inrad book.

    73 de py1nb

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