A wideband rejection filter may be used to prevent medium wave overload while still keeping the wideband nature of your receiving system intact. Learn more.
My wideband receiving system, using active loops and wideband SDR, suffers from some medium wave overload challenges. These problems can be serious if you live near high power broadcast transmitters. Three solutions are generally available, as shown above.
First, consider your receiver itself. Does your radio have internal band reject filtering or enough ADC dynamic range to handle the overload? Perhaps.
My Flex 6300 does not have any band reject filtering. However, it does have attenuation and a fairly high dynamic range 16 bit ADC. So, while my loop provides some MW birdies on 160 meter reception, IMD is not a major problem. My Flex is good enough to use with a wideband active loop.
On the other hand, the SDRplay RSPduo has a wonderful MW Notch Filter. When activated, it drops medium wave signals by 40 dB or more without effecting shortwave reception. Actually, I don’t think of this as a notch filter, more a band reject filter for the full MW band, as described in the RSPduo data sheet. Works like a charm.
Perseus contains bandpass filtering for all the major short wave ranges above 1.7 MHz, so it does a decent job of knocking down MW overload. But I still get lots of birdies and a bit of IMD on the lower ham bands. Finally, my Afedri SDR-Net dual channel does not have inboard filtering so is difficult to use with a wideband active loop out of the box, unless control software does a good job of setting AGC.
Wideband Rejection Filtering – Other Options
Second, you can place an outboard filter between the wideband amplifier and the receiver input. My homemade 1.7 MHz high pass filter will reduce medium wave signals by 30 dB while passing through higher frequencies. This is what I use with the Afedri to combat overload. You can buy various filters online that do a fine job of wideband rejection filtering for your SDR. You can even figure out a way to switch such a filter in or out, to maintain wideband operation,
Finally, you might consider placing wideband rejection filtering between the physical loop and the input of the loop amplifier outside. LZ1AQ suggests a couple of approaches including a MW high pass filter or L/C notch arrangements to knock down the strongest signals. Such a filter would have to be designed for very low impedance, in the range of 7Ω to match the amplifier input. I plan to experiment with this soon.