Re: filters and contests and interference question/opinions

Posted by Bill W4ZV on Mar 26, 2012; 1:19am
URL: http://elecraft.85.s1.nabble.com/filters-and-contests-and-interference-question-opinions-tp7403392p7404470.html

goldtr8 wrote

Currently I have a 2.7, and 2.1 filter plus the narrow ones for digital.

My brain is starting to tell me to stop using the 2.1 and get either a
1.8 or even a 1.5 or possibly both.   My thoughts are that the dsp would
work much better if some of the interference can be directly blocked out
by the filter.   These thoughts are guided by past readings on the list
that this is the case.

What is the collective wisdom of these filter changes I am thinking
about.   I am curious about others who may have tried these and if they
kept them or if they thought it made no difference at all.

The answer is...it depends.  :-)

I believe you're fairly new to radio and contesting.  Therefore I'm assuming you mostly tune for DX stations and call them rather than running a pileup yourself, which requires good antennas and high power on SSB.  For DXing or Search and Pounce (S&P) contesting, you normally have plenty of time to tune in stations carefully before calling.  In this case narrow filters (either XFIL or DSP) will work even with bandwidths down to 1.5 kHz.  BUT...this assumes you have time to carefully tune in the station you want to call.

In the case of someone running a pileup at fairly high rates (e.g. 150-200 per hour), it's a different situation.  You want to copy callers correctly the first time without any tuning, send your exchange and get his exchange in the space of 15 seconds and go on to the next one.  The problem with extremely narrow SSB filters is that many callers will be slightly off frequency (e.g. 100 Hz) which renders them unintelligible when using narrow filters.  With a little wider filter, your ears can still copy the off-frequency guys without needing to touch the VFO.  

I had a 1.8k filter in the CQ WW last October and gave up using it because of this problem...and a 1.5k would be even worse.  Instead I found myself using a DSP setting of 2.0-2.1k with the stock 2.7k XFIL, so after the contest I traded my 1.8k XFIL for a 2.1k.  Unfortunately conditions on my favorite SSB band (10m) have been so poor that I opted not to enter the ARRL DX SSB or CQ WPX SSB and really haven't had a chance to try it under fire, but I believe it will be better than the 1.8k for my purposes.

As Don said earlier, the major advantage of a narrow XFIL is to prevent AGC pumping from strong stations nearby.  In extremely crowded contests this can be a major problem.  However a narrower filter WILL NOT remove the splatter from nearby stations...nothing can do that short of using phased antennas to null their signal.

73,  Bill