80m LP-filter built

I did build a filter today, which I hope will solve our harmonic issues caused when transmitting on 80m. The hybrid lowpass filter I built didn’t work well, it caused 2:1 VSWR and didn’t do the job it was supposed to do. I don’t know why, but when building it I could not get the insertion loss down as low as I managed to do on the 40 and 20m filters, so I guess that might be why. However, I scrapped that filter and decided to try to attenuate the harmonics on the 200w side, that way I can make sure that is any harmonics are created its 100% in the amplifiers, since to some extent harmonic distortion is impossible to avoid. I have also realized that these filters wouldn’t be too hard to build for HP use, so that might be the way to go, since its a lot easier to build compared to band-pass filters.

I will not take credit of the LP-filter design. I did mess around trying to make some decent CAUER filter in Elsie but I didn’t get the result I wanted when I stumbled across an already made design already done, which was en example design in the Elsie directory. It looked so good that I decided to build it straight off. I measured the inductances I wounded on T130-2 cores and got a Q of about 85. This would according to Elsie give us an insertion loss on 80m at around 0.3dB which I felt was reasonable and a notch around -75dB.

I did some simulations also in Orcad to see what kind of voltages to expect over the capacitors and it was nothing bad. Even with a bad load I got less than 300v when transmitting with 200w, so I believe I have a quite safe margin using the 500v silver mica capacitors.

So, I built the filters and on the first try it looked really good, except that the notch was a bit off. So I did two iterations of the capacitors causing the notch and voila it was “perfect”. I felt it was good enough that I didn’t even try to change any of the other values. I had already measured the inductance of the inductors and saw they were quite close to what I wanted.

Now I plan to build another filter for 20m which notches away 10m. These are the two bands that causes most problems at SJ2W. But if this works I will probably build filter for 160 and 40m as well and make 19″ rack box with them and automatically switched with the openASC system.

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