Where I live, just North of London, there are some VERY strong AM signals which broadcast on frequencies just under the 160m band. As a result, these can overpower Rx stages or ‘deafen’ a receiver to the rest of the HF spectrum. As I use a full-spectrum SDR receiver for the receiver side of my station, the goal is to filter these out with minimal effect on the HF bands.
Living in a typical UK property, i don’t have the space for an antenna to do the 160m band justice, i have accepted that to filter these signals out i may have to take the 160m band with them to reduce them and the harmonics they produce, but i would like to keep 80m clean so i can later enter some of the UK contests held on that band, I in intially considered a notch filter at approximately 1.2Mhz, but in reality at my station anything under the 80m band is not for me, so i figured a high pass filter sitting just below the 80m band would be the way to go.
My first attempt was a 5th-order High-Pass Butterworth filter just below the 80m band; while it did a good job attenuating the AM signals, testing showed it attenuated too much in the 80m band.
So the next attempt was to make a 7th-order Butterworth at 1.7 MHz; this worked fairly well, and it only had minimal effect on the 160m band.
Before adding the filter.
After adding the filter.
You can see just how much the dynamic range has increased by reducing the signals at 909KHz and 1.089 MHz. You can also see, after adding the filter, just how strong some of the other Short and Medium Wave stations are, absolutely overpowering the signals received on the HF bands.
Coming up in Part 2… Custom PCB’s to aid development and testing…