[CQ-Contest] Skimmers etc

Bill Tippett btippett at alum.mit.edu
Wed Jun 4 12:30:20 EDT 2008

On Tue, Jun 3, 2008 at 5:20 PM, <Jimk8mr at aol.com> wrote:
>   Contesting will remain a test of running.  Skimmers will just make it
> easier for experienced contesters to do even more running, while waiting for
> the Skimmer to find the occasional multiplier requiring a second radio or
> second VFO qsy.  So the big guns will be even more monopolizing of their run
> frequencies, making it harder for a newcomer or less well equipped veteran
> to find a place to run.

Having been closely observing my own Skimmer setup for about 6 weeks now I
tend to agree with you.  Here's my take on what Skimmer does very well in

1.  For the casual op, find and identify *stronger* signals (that identify
at least twice).  As you stated above, this will benefit relatively stronger
stations CQ-ing.  If there is one BIG plus of Skimmer, it will penalize
those folks who seldom identify (you know who you are)!

2.  For the more serious op who is running, find and identify *stronger*
stations calling CQ.  This would benefit finding new multipliers or unworked
stations, especially in slow contest periods like SS on Sunday.

3.  Detect (but not decode) *weaker* signals.  Skimmer's waterfall is
similar to PowerSDR or other SDRs.  This can be a very useful way to quickly
scan a "dead" band like 160 in poor conditions or like 10m when it's opening
or closing.  Using Skimmer this way on 160 in WPX was very effective in
scanning the band for weak DX.  Skimmer never confirmed a single DX call but
clicking on its decoder dots was an effective way to send my K3 to
frequencies of interest and then use my ears actually copy the signals.  If
you study some of the plots on Skimmer's website, you'll see a number of
weaker signals on the waterfalls that have decoder dots active but no call
signs listed.  I believe I could have easily copied every one of those based
on my experience with Skimmer thus far.

Here's what Skimmer does not do so well:

1.  Decode *weaker* stations answering your CQs.  For the *weaker* signals
I'm accustomed to copying in most DX contests (I don't do SS), it would not
work very well.  Most stations replying only send their calls once, not
twice as currently required by Skimmer.  Also with typical low band QRN/QSB,
Skimmer does not work very well (...at least not yet).  It would be
interesting to run a Dayton pileup with QSB and QRN added like we actually
have on the low bands.  From what I've observed so far, Skimmer's or
PowerSDR's waterfall mode is barely detecting signals I can easily copy Q5
by ear (e.g. about -107 dBm or 1uV).  Neither even detects -117 dBm signals
which are still Q5 by ear (and the MDS of most rigs goes ~10 dB below
that).  I'm still trying to determine if I have something set incorrectly
but both Skimmer and PowerSDR behave alike with weak signals using
calibrated output levels from LP-PAN and my sound card.

2.  For the same reasons, Skimmer may not find all the *weaker* multipliers
calling CQ.  It's waterfall may detect the presence of *some* but it will
not decode them.  I proved this to myself in WPX on 160.  However, a
waterfall (Skimmer's or PowerSDR's) is an excellent front-end for quickly
directing your rig/ears to signals of interest.  Of course a waterfall-only
display is perfectly legal under any interpretation of current rules since
it's not doing any decoding.

What is *stronger* and *weaker* above?  I'm still trying to quantify that
since it depends on relative levels of QRN and QSB.  From what I've observed
so far, I believe Skimmer begins to have problems at about -107 dBm (1 uV)
on a normal night on the low bands.  160 during WPX was relatively quiet for
this time of year (probably more typical of early spring or late fall) but
the the waterfalls of both Skimmer and PowerSDR were barely detecting
signals at the ~1uV level in QRN of about the same strength (and all being
detected were readable by ear).

BTW Skimmer's Softrock-IF mode (the one recommended for a receiver with a
wideband buffered IF output like the K3) is limited to 24 kHz.  This is an
internal limitation of Skimmer and not due to the K3's IF output or the
LP-PAN's I-Q detector.  I can use PowerSDR or Winrad waterfalls with the
same K3/LP-PAN hardware at full 192 kHz bandwidth.  If you want the full
bandwidth of Skimmer you may need a separate Softrock, SDR-IQ, etc.

Of course every limitation I've mentioned above is subject to change as Alex
improves the software, which no doubt will happen.

73,  Bill  W4ZV

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