I was trying this weekend to build a Cauer LP filter that I designed for 80m with notch at 7.1 MHz. I did not get the performance of the filter as I wanted and they are quite hard to tune with those big pipes etc to bend. So instead I decided to go for a design very similar to the one ON4UN describe in his book with notches instead. Those are easy to adjust since you see very well what you are doing and also they do not have a big impact on the other notches. Another advantage is that you can get very low insertion loss and for this filter I was aiming for better than 0.1dB. I decided to skip the notch for 160m but I might add a notch small enough so that It does not impact the insertion loss and give maybe 20dB of depth. I also need to add the rest of the box and fine tune the notches so that they are behaving as they should.
The idea is that I will build these filters for the radio 1 and radio 2 position in the switching system. On 160/80m where we just have one antenna it will sit directly at the antenna and be used on all four radios. The notches both remove the harmonics generated by the amplifier but will also remove other transmitting radios main tone. I will build another filter for 40 and probably also rebuild the 20m LP-filter I built earlier as well to a notch filter.
I got VERY happy with the result, maybe I would have wished the 40m notch would be slightly deeper and could probably be possible by fine tuning the series inductor but I would need to make another one, so I think I will just stick with the current design.
Attenuation on 3.755 MHz is 0.05dB which is a lot better than the 0.1dB that I had as goal.
40m -> -56dB
20m -> -53dB
15m -> -66dB
10m -> -72dB
