Preselector is a quite narrow input filter intended for receivers, especially needed with present cheap SDR devices where the input filters should be improved in general.
My goal was to build up such one mainly for two bands : 80m & 40m. My version is based in 100% on DL2NBU & DL6RAI project. It can be currently found on Bavarian Contest Club site.
Please find a schema:
Here you can find also a list of necessary toroids and welding methods:
I screened the case with copper tape before I installed all listed components. PCB was prepared using just hand drill due to quite not complicated connections(in case of just 2 bands to be covered). If you decide to build up the complex version, PCB is available within the PDF linked in the bottom of this page.
I added the single bistable switch for changing band from 80m to 40m (it’s only about shorting one coil).
nanoVNA shows a quite good characteristics:
I noted that it’s important to use exactly the same toroids like author did. It decreases the attenuation of a filter.
Ready-to-go documentation can be downloaded as PDF from here
Jakub/SP5TOF