Today I added a couple of 7/16″ chassis connectors to the 20m LP filter and did the fine tuning of the filter and I got very happy about the result. I managed to get less than 0.1dB attenuation in the passband which was one optimistic goal and I got -75dB attenuation in the notch which is at 28.2 MHz. It’s about -57dB on 28.0 MHz and -55dB on 28.5 MHz.

So now I will try this for a while and if it works fine I will build one for 160,80 and 40m as well. But I need some 7/16″ female chassis connectors, so if you have any lying around please contact me!

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I’ve been a bit lazy lately but now I started to do something useful again. I started building the housing for the input filter for one of the amplifier projects going on. I also found a big fan which is capable of moving 21 m3 of air each minute that they were going to throw away at work. So I took it and when I tested it would not run, so I took a guess and replaced the motor capacitor and now it runs just perfectly.

I also finished up the box for the LP-filter for 20m. I have almost run out of 7/16″ connectors (chassie females) so I will need to pick a couple from the 80m filter at SJ2W which did not work properly and install it on this box instead. I did some quick measurements and managed to get about -65dB harmonic attenuation on 10m and about 0.15dB insertion loss on 20m. I think I can tweak these values slightly more, maybe I can get down to 0.1dB insertion loss and I will be pleased with it.

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The LP filter I built earlier for 200w was today built for high power instead. It was a real mess, its hard to bend these copper pipes to fine tune the things and after some problems with the network analyzer I managed to get something that looked pretty good, but it took me hours. It takes a long time to solder one of those coils to the copper, so that is what is the biggest problem and of course after you have un-soldered it the thing is really warm.

Anyway, I managed to get 0.1dB insertion loss and about -65dB notch BUT when I was fine tuning the notch I managed to break one of the 220pF capacitors, and of course I did not have anyone left. I had lots of 150pF and lots of 330pF but nothing between and I did not want to redo the layout and use some other values in parallel. So I ended up ordering 18 pcs of 220pF capacitors from an Ukrainian.

Here are some pictures, now I need to make one for 80m too. On 40m the current hybrid low pass filter works really well, but the one I did for 80m did not work well at all so I am gonna make a similar HP filter as the one previously made.

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Today I spent an hour building the filter for 20m which I also as the previous filter found in Elsie. This time since the inductances were supposed to be 500nH and 340nH I decided to use air wound inductors. Probably gives a higher Q since the insertion loss got so low. I have however not measured the Q of these ones as I did with the ones wound on T130 cores.

I got very happy about the performance, so it will be very interesting to test them.

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