Strong local broadcasters can create medium wave overload which causes problems for shortwave reception on higher frequencies.
Recently, I have been enjoying medium wave DX using my wideband active loop. My one meter loop does a wonderful job pulling in distant AM broadcast band stations, day and night. Since I have orthogonal crossed loops, I can switch between them to optimize directional reception.
But, while the good news is that active loops work great on medium wave, that is also bad news for reception on higher frequencies. I find that my wideband receiving system – wideband antenna and wideband radios – suffers from a lot of medium wave overload. The result is a ton of harmonics and inter-modulation products stretching up into the middle of HF.
While my wideband loop amplifier seems to have enough dynamic range to handle strong nearby signals, the wideband SDR receivers do not.
Last year, I wrote about how well SDR receivers can handle large signals without much or any front end filtering. Turns out that’s only true if your ADC has enough bits for excellent dynamic range and you are sampling fast enough to use a lot of decimation. While I will talk about this in more detail shortly, my experience is as follows.
My higher end 16 bit Flex 6300 handles medium wave overload pretty well without front end filtering.However, lower performance radios may require some attenuation or rejection filtering to cope with medium wave overloading if you live near some strong local transmitters.
Medium Wave Overload – Take a Look Around
I live on the south side of Calgary, as shown by the green VE6EY marker on the map above. Four local broadcasters, labelled in yellow, are generally within 20 km from me and have patterns aimed my way. You will find these stations generate signals at my receiver input of between -10 to 0 dBm. These are strong AM broadcast signals producing between 200 to 700 millivolts at my receiver input terminal.
The other four marked stations produce -20 to -10 dBm, or around 100 millivolts at the input. For comparison, a strong shortwave signal arriving from China will produce around -70 dBm, or 0.2 millivolts, while a weak Indian station arrives at -90 dBm or 20 μV.
Most SDR are designed for maximum input voltages of between 1 to 4 volts, which doesn’t leave much headroom. Mixing wideband antennas and receivers might need some filtering help.