DJ0IP has collected a few interesting HF RX BPF designs. These are fairly narrowband. With todays EMI problems (that NRRL and the other radio amateur organizations seem to care little about), using filters such as these can have an improving effect. Not only for noise emanating inside the amateur radio bands but specifically noise outside the bands. Why?: broadband noise from LED switchers, SMPS, Solar arrays etc often cover a wide HF spectrum, also outside the amateur radio bands. If the input of a HF receiver lets in noise energy also from those frequency ranges outside this will often be altered in slight non linearity in the input stages of the receiver and turn into noise inside the amateur radio bands. I have seen this with comparing switching noise in the 40, 20, and 10m bands and just outside the bands. My FT2000 transceiver switches out the preselector filter when the RX is just outside the amateur radio bands. This results in quite significant noise increase from LED switchers, SMPS etc. while it is fairlig quiet inside the amateur radio band, indicating that the preselector filter indeed has a good effect. For radios that doesnt have this built in, these filters can be of interest.
Download the PDFs from the links below –>
