On one of VE7BPO’s websites, which you can view HERE, he describes a low-pass filter he designed to reduce spurious, second-order intermodulation products caused by a nearby 10 kW AM station at 1150 kHz that were “all but” wiping out his reception of some of the weaker NDB (Non-Directional Beacon) stations below 500 kHz. He packaged his filter in a Hammond die-cast case.

I have a similar problem with a 50 kW station at 1170 kHz less than five miles from my QTH. In addition to the second-order intermodulation products, I have various other mystery signals below 500 kHz, including whistles, beeps, pops, buzzes and growls to contend with.

In hopes of eliminating or minimizing those sources of interference I duplicated VE7BPO’s filter electrically. It has turned out to make all the difference and i highly recommend it to anyone interested in NDB reception.

Another reason for this web page is to illustrate a simple packaging means that anyone can duplicate at home at very low expense.

This is VE7BPO’s circuit which I copied (with his permission) from his web site.

These photos are self-explanatory. The main point of distinction, if there is one, lies in soldering the connectors directly to the copper-clad board, thus making a very simple enclosure possible.



Note also there are four 4-40 hex nuts soldered on the edges of the board to match holes in the cover.



The cover is cut from a 5-inch by 7-inch sheet of 0.013-inch aluminum available at many hardware stores for the astonishingly low price of 19 cents.



That material can be cut with scissors or, better, a paper cutter, which prevents the tendency of a part to curl when cut with scissors.



Bends are easily made in a small vise using a hard wood block to sharpen the bend radius.