[Elecraft] K3 - A case for wider roofing filters?

[Lance Collister]

Joe Planisky wrote:
> ... or There's No Such Thing as a Free Lunch.
> 
> Until recently, I only had the 13 kHz and 2.7 kHz roofing filters 
> installed in my K3.  I mostly use CW and usually found the DSP filtering 
> perfectly adequate for my narrow bandwidth needs.  But after having a 
> few QSO's disrupted by nearby strong QRM, I decided to try a narrower 
> roofing filter.  So I ordered the 500 Hz 5-pole filter from Elecraft.
> 
> It arrived, I installed it, and configured it (including the frequency 
> offset).  To set the gain, I used the birdie at 49.380 MHz and 
> Spectrogram to make sure that the audio signal amplitude with the 500 Hz 
> filter active was the same as with the 2.7 kHz filter active.  This 
> required 6 dB of gain on the 500 Hz filter.  However, I noticed that 
> boosting the filter gain by 6 dB also boosted the noise floor by 6 dB.  
> In other words, the S/N ratio at 500 Hz bandwidth was 6 dB worse than at 
> 550 Hz bandwidth.  After thinking about it for a moment, I realized it 
> made perfect sense.  I checked with Gary at Elecraft and he said that in 
> general, the narrower roofing filters will have greater loss than the 
> wider ones, and nothing he said made me think that 6 dB of loss for a 
> 500 Hz filter was unusual.
> 
> I tuned around and found some strong signals and did some A/B 
> comparisons with the 500 Hz filter on and off.  The filter definitely 
> works as advertised for dealing with strong signals on a nearby frequency.
> 
> I then sought out some weak stations, some of those 'RST 219' guys.  
> These stations virtually disappeared into the noise at a DSP BW=0.20 
> with the 500 Hz filter enabled, but were somewhat copyable with the 2.7 
> kHz roofing filter at the same DSP BW.  In the absence of strong QRM, 
> the wider roofing filter gives me better performance because of its 
> lower loss.
> 
> But does it make any practical difference?  I encounter weak signal 
> conditions (usually due to QSB) more often than I encounter s9+30 
> signals 500 Hz away.  So given the current situation, I'll probably keep 
> the 500 Hz filter turned off most of the time. I wish there was an easy 
> way (i.e. one button press) to enable or disable a roofing filter so 
> that I could engage it only when really needed.
> 
> I've heard that the 8-pole filters tend to have a little less loss than 
> the 5-pole ones, but I'm undecided as to whether to try that route.   
> Any voices of experience?
> 
> 73
> -- 
> Joe KB8AP
> 
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Hello Joe,

You raise an interesting viewpoint.  Most HF operators seem concerned with 
filtering out strong nearby signals, which is where the narrower roofing filters 
really come into their own.  However, if you look at the charts on the Elecraft 
web page, you can see the attenuation introduced through the use of these filters.

That is the main reason why I ordered the 6 KHz wide filter with my K3.  I use 
that wide filter setting for all my EME contacts, to make sure I get the most out 
of the antenna.  And when I am looking for that weakest CW signal on multi-hop Es, 
I try to use a wide filter setting and on the radio, and use a narrow audio filter 
(such as the one in Spectran) with my headphones.  In fact, that was one of the 
great things that came with the K3 - the ability to very finely tune the incoming 
CW signal to move it right into very narrow passband of the Spectran audio filter.

GL and VY 73, Lance



-- 
Lance Collister, W7GJ (ex: WN3GPL, WA3GPL, WA1JXN, WA1JXN/C6A, ZF2OC/ZF8)
P.O. Box 73
Frenchtown, MT  59834  USA
QTH: DN27UB
TEL: (406) 626-5728   URL: http://www.bigskyspaces.com/w7gj
2m DXCC #11, 6m DXCC #815