Notch filter built and isolators made

I’ve done some work in the lab again. We’ve been seeing some QRM on 10m from 20m, so I decided to built another notch filter. I am very happy with the result, about -86dB notch and around 0.01dB insertion loss on 20m. I also spent some time in the lathe making some isolators for the 40m verticals for the 4SQ.

I really recommend these notch filters. They work great and are very easy to build. I start with making the stubs a bit too long. Then I cut them until I have the notch at the correct place. After that its a matter of decreasing the insertion loss, so I adjust the L until I get rid of the C from the stubs, giving a perfect 50 ohm match.


  1. Nice measuring equipment, I always enjoy your photos.
    73, Bas