I built a couple of notch filters today. One which I intend to use just to test if the overtones when transmitting 80m is what is causing the problem or if its the actual 80m signal that is too strong for the receivers to handle. So if I insert this notch filter which provides about -61dB notch @3.65 MHz and the problem still occurs, then its the overtones which are the problem.
The notch filter is a 3-pole Chebyshev filter which I designed with Elsie. I should get a bit more notch by fine tuning it but I need to get hold of some decent trimmer capacitors. I noticed when I changed the fixed capacitors from regular ceramic ones (X7R or similar?) to NP0 I went from 18dB notch to 30dB (!) when measuring on a part of the filter, that is quite some improvement. So the conclusion of this is, NP0 rules!
I also built a hybrid lowpass-/notchfilter that I’ve built before for 40 and 20m. I really like these filters! They are easy to build, they have great overtone attenuation and they handle legal limit with a big margin. However, this filter I didn’t get as good as the earlier ones, probably because the stubs work less well the lower in frequency you go. I managed to get around -60dB of attenuation on 7.1 MHz but what I got disappointed about was that I didn’t get the attenuation through the filter down more than to 0.15dB.
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