As you read on my last post about this novel, we were getting a lot of static discharge noise on our SDR receiver which mades it practically unusable. Yeah really, at the point that some stations were lost below all this noise. Take a look:
Working on 40-meter band the noise in central frequency is really incredible, same behavior on lowest side (6700-6900Khz). When using the Yaesu FF-501 inline filter, we got
Something surprising. The Yaesu FF-501 filter is doing pretty well here. Just a bit of the noise shadow on central frequency and now we got a lot of stations there, those were eclipsed by the strong noise signal
The fifth challenge consists on the design and fabrication of a low pass filter to get rid of the annoying noise that needs to work on both 40 and 80 meter band.
My father have several schematics about HAM radio filters so let’s get to work on this.
- L1 and L5 are air coils with a 9mm internal diameter and 3 turns.
- L2 and L4 are air coils with a 13mm internal diameter and 7.5 turns.
- L3 is an air coil with 13mm internal diameter and 8.5 turns.
This is the schematic we used in order to get the filter done, and looks like this:
But we noticed it’s HUGE in aspect of size for our WebSDR box.
Anyways let’s give it a try
Hey! It’s super nice, better than we expected. But, as long as it’s too big we built another one, in a smaller size. Pretty much smaller than the original version, using telephone copper wire instead of 2mm wire. We also removed one turn of the L2, L3 and L4 coils.
Noticed a bit of overall attenuation using this small filter, it’s probably due to the new coil design (one less turn than the bigger version). To solve this, I set the receiver gain from -25dB to -20dB to compensate the different waterfall color.
We are pretty much satisfied with the results of both big and small filters, now we took the small one and got it installed inside the WebSDR server box. It’s running at this moment so we’ll check the filter performance now on 40 meter and then in 80 meter band overnight.
We left the small filter working since yesterday afternoon so it been working on 40 meter band until 10 PM, then switched automatically to 80 meter band until 9 AM today. Filter went excellent, we are pretty much happy with the results we got.
Next thing in the agenda is the power supply. The power supply will feed the +5v Raspberry PI input, +5v upconverter input and the 12V antenna switching relay. See you in the next post!