WFAX 1220 AM was starting to bother me. Each morning, I'd be drinking my coffee, listening to nice roundtables on 160 meters, when, right at 6 AM, WFAX would fire up its 5 kW AM transmitter, 1.5 miles from my location. And they would crush the "front end" of my R2 phasing receiver. It doesn't take much to do that, since the only thing between the SBL-1 mixers in the R2 and the antenna is a signal splitter. Obviously I needed some filtering.
I turned to the free program called Elsie (L-C, get it?) and quickly whipped up a design for a seven element, capacitive input high-pass filter that promised to take about 45 db out of WFAX's sails, without attenuating even the lowest end of Top Band.
Last night I scrounged through the junk boxes and found suitable capacitors. A visit to an on-line toroid calculator showed that around 35 turns on a T-50-2 (red) iron powder core would yield the needed 6 uH coils. I built the filter this morning -- picture below.
It works very well. You can see the results in the picture above. The yellow trace on the 'scope shows the signal at the antenna terminal. Yikes, it shows around 4 volts rms at 1.220 MHz (the scale is 5 volts/div). The blue trace below is on the same scale -- this is the signal coming out of the filter. Not enough to really measure on the 5 volts/division scale.
This was a very satisfying "quick and easy" build. I really like the sound of the R2, so much so that I'm not firing up the DX-100 as much as I had been. Instead I find myself just listening to the R2.