[Elecraft] Filter Selection Approach

[Guy, K2AV]

Casual operation has rarely strained my radios, and my MP and my K2 are quite adequate for that.  When we get to contesting, all facilities are strained.  I prefer the K2 audio to the K3, and sometime soon I will get to the bottom of that.  The quickest, sharpest, deepest skirts on selectivity still belong to my MP with matched 8 pole INRAD filters in 8 Mhz and 455 kHz IF's.  (I know that's heresy, but so be it...)

On receiver front-end, overload immunity and SDR-driven DSP magic (like blanking key clicks), with more to come, the K3 leaves everything else in the dust. But I pick roofing filters to get as close as I can to the MP+INRAD+INRAD skirt performance. 

BOTH K3 selectivities (roofing and DSP) need to be matched to produce optimal rejection of signals just outside of the desired listening window.   I have the "400" and "250" 8 poles in my K3 and list them as 450 and 350 in the filter setup. The bulk of my operation in a contest is RUNNING, not search and pounce, for DX tests mostly using a very large 40m antenna fixed on Europe, and with huge differentials (80+ db) between desired weak signals and undesired strong signals just above and below.   I also specifically use the [sometimes complained about] "mild" difference between the two to advantage, as I use BOTH for running, and use 350/300 when I need it just a bit tighter. 

While I could narrow down with a 5 pole 200 on a weak station, 1/3 or more of the stations calling are off my frequency, often because the current packet spot is up or down.   And I'm not allowed to spot myself to fix it.   I have to keep a window to hear them or someone else gets the top score. 

The next CW station can be up/down only 350 Hz, meaning that outside the listening window, the skirt needs to dive for ultimate rejection as fast as possible. Being flat at +/- 175 is not the issue when setting at 350 width, it's how far down the combination of skirts has gone at +/- 225.  The most important thing is where are the skirts halfway down and how much more do they drop with another 10 Hz, NOT the 3 db or 6 db points.  The 8 pole roofers are part of making those skirts dive more in 10 Hz halfway down. 

I set up my filters at the +/- 50 dB points, and eagerly await the day when CW widths and shifts are in 10 Hz increments. 

I have had some extremely vociferous contradictions from some insisting that the 8 pole roofers are simply a waste, and a wide and narrow 5 pole are all needed, but my contest experience continues to suggest otherwise.

All of the above are in stark contrast to casual operation.  Buy filters for how you use the radio. If you are into hard contesting, do it with 8 poles.

73, Guy
K2AV


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