the narrow gate the strong cannot prevail; double tuned circuit
This page describes a simple bandpass filter that can be used to eliminate very strong signals from either below 475 kHz (NDBs) or above (MW broadcasting stations). This filter can be built in a few minutes from components purchased from the local Jaycar store or from junk box components (old AM radios). The design is the well known "double tuned circuit", and it is built with 455 kHz can IF transformers, normally used in AM radios. The transformers come in a great variety of input/output impedances and capacitor values. Some do not even have a capacitor. The ones I used are the white type, which have a nominal impedance of 50k to 500R. They also come with a capacitor in the high impedance side, which normally is tuned to 455 kHz. The good news is that the range of those transformers can easily cover 475 kHz. In the double tuned circuit, two of these transformers are placed back to back, with their high impedance, tuned outputs loosely coupled via a very small capacitor. In my case, 15 pF seem to give a reasonably wide peak to cover the whole band. The low impedance windings are used as the 50 ohm in and 50 ohm out. The following schematic shows the exact configuration:
The effectiveness of this bandpass filter can be seen in the pictures below. In the first photo, a PA0RDT mini whip active antenna is connected directly to the input of the spectrum analyser.
The frequency coverage is 0 MHz to 2 MHz. A number of strong MW stations can be seen, especially at 666 kHz and 846 kHz. After the insertion of the filter, the spectrum looks like this:
The NDBs have completely dissapeared and the MW stations are severely attenuated. This will help in the situation where the receiver is overloaded with the strong MW signals, however it will not help if the overloading occurs at the active antenna.