CW Skimmer (Stewart Cooper GM4AFF)

What is CW Skimmer?

CW Skimmer is a piece of software which can be used to decode CW. That’s it really. And I suppose most readers knew that already. However, what CW Skimmer can do that most humans can’t do is read multiple CW signals at the same time – up to 700 on a 3GHz P4. So how can we, as radio amateurs, use this ability to our advantage? Well, the goal, which I set myself when I first looked at CW Skimmer, was:

“I want to see a list of callsigns in my WinTest band map which have been generated by Skimmer”.

This sounds simple enough, but even this presented a number of technical challenges.

How does it work?

A basic set-up can be achieved by simply installing the software on your PC, removing any IF or AF filtering on your receiver and plugging your audio output into the PC sound card input. This will get you started. CW Skimmer looks at the audio coming out of your receiver and decodes all the CW signals that are within that audio pass-band. Although that pass-band is going to be limited to about 3 – 4 kHz, it is still interesting to tune your radio to a pile-up and watch what’s happening. This arrangement is fine for experimenting, but is of little practical use. In order to get the most from CW Skimmer you will need to get your soldering iron out.

Fig 1 – Basic CW Skimmer components

What do you need?

If Skimmer is going to achieve the goal I set at the start you will need access to your radio’s IF or use an independent SDR receiver dedicated to Skimmer. Access to the radio’s IF is not something that’s very common; the K3 comes with an IF output on the back if you have the KXV3A option. I realise that not everyone has a K3, but there are other radios with IF outputs such as the TS930, and yet more to which accessing the IF is well documented on the internet. There are also a variety of SDRs available that will fit the bill as stand-alone receivers. The cheapest is the SoftRock v9.0 which costs around $55 US and comes as a kit (see www.kb9yig.com or www.softrockradio. org). This will give you a 1.8 to 30MHz receiver which connects to the USB socket on your PC. There are many other SDR receivers available – see the resources list at end of this article. Sadly much of the information on SDR receivers on the internet has been written down, torn up, thrown on the floor, gathered up by monkeys and then published.

If you have a receiver with an accessible IF, you will need to feed this IF into an SDR receiver. If you have a K3, there is a SoftRock which is made specifically for this. The Softrock SDR is not a very easy thing to build, but if you have a decent temperature-controlled soldering iron and a steady hand, you will have no problems. I started from scratch. I bought a second-hand soldering station on Ebay, some 0.7mm solder and an illuminating magnifying glass! I had never done any surface mount soldering before, so I learnt as I went, but the thing worked first time, so if I can do it anyone can! The SDR is a small circuit board about 2” x 2”. It requires an input from the radio (buffered – see next paragraph) and the output goes to the PC sound card.

Between the radio IF and the SDR you should place a buffer amplifier to provide reverse isolation. Such an amplifier is the Z10000, which comes as a kit, or pre-built, and is available for $12 from Clifton Labs. (See www.cliftonlaboratories.com). The extra isolation is necessary because the Softrock has a rather strong leakage signal (around -40 dBm) out of its antenna input port. As a matter of good engineering practice, it's desirable to keep strong signals like this out of the receiver's IF chain.

Fig 2 – SDR receiver (right) and buffer amp (left) built into an old tobacco tin

I built the SDR and the buffer amp into an old tobacco tin, and take the supply voltage from the 12v socket on the K3 back panel. The RF input from the K3 IF is on the left, the 12v supply is at the bottom and the output to the sound card is on the top right. As you can see, I’m not the world’s best constructor!

Download CW Skimmer from www.dxatlas.com/CwSkimmer. It’s available as a free trial for 30 days and costs $75 if you want to buy it.

For Skimmer to work well, you will need a decent sound card. This will cost from around £30. Some high specification cards (PCI Express Sound Blaster X-Fi Titanium) can achieve a band-width of 196 kHz. This is over-kill and will probably cost a lot too. Most basic integral sound cards will do the job adequately and will give you a bandwidth of about 60 kHz. The more you pay the better the results.

As Skimmer is often used in contesting, the radio used for Skimmer also needs to be connected to the contest logging software (unless of course, you can afford to have a radio dedicated to the Skimmer operation). This would apply to both single op (including SO2R) and multi-operator environments. The problem is that Skimmer needs to know what frequency the radio is on, so it needs a CAT connection to the computer; but the contest logging software also needs to know what frequency the radio is on! The solution is to install a port sharing application such as VSPE (Virtual Serial Port Emulation). This allows the logging software and CW Skimmer to connect to the same serial com port (i.e. the radio). (See www.eterlogic.com)

Fig 3 – 160m screen shot taken during CQ WW CW 2009 at GM0F

Fig 4 – 80m screen shot taken during CQ WW CW 2009 at GM0F

The screen shots above show Skimmer at work in CQ WW CW 2009. The left section is a waterfall of what is being ‘heard’. The middle part of the screen shows the raw callsigns and other decoded fragments being heard. Note that CQ and/or 599 is added to a call if it was heard being sent. The list on the right shows a validated list of callsigns heard. The strength of validation is controllable on a sliding scale; at its most aggressive, calls can be validated against your master.dta database, or simply passed unchecked at the other extreme.

What I’ve not covered here…

There is little point in me showing how to install and set up CW Skimmer here – it is all explained well on the DX Atlas web site. Personally, I found it difficult to set up the offset which corresponds to the difference between the K3 IF frequency and the oscillator frequency of the SDR. This needs to be set so that the frequency displayed in Skimmer corresponds with the actual frequency of the stations heard. I also make no apology for avoiding the “quadrature I/Q data streams” gobbledegook – it’s all stereo to me!

Some things you may not have known Skimmer can do:

- You can ‘record’ an entire swathe of radio spectrum to your hard drive as a WAV file, then go back and ‘watch’ it later – an RF time machine. Now, I wonder if anyone has ever tried re-transmitting it - that could liven up a dead band!

- Skimmer can behave like a cluster server and push out DX spots to the network in a multi-op contest environment.

- Skimmer can be left to monitor a band like 10m and warn of openings.

- Not only can Skimmer display all the stations in a pile-up, but it can also spot which station is sending 599 (or 5NN), so you can see who the DX station is working.

- The Reverse Beacon Network (www.reversebeacon.net) is a network of Skimmer users who feed what they receive into a network, which is available for all to see on the internet. You call CQ and then see where you got heard. Great for checking propagation!

How to configure WinTest for contesting:

Download and install VSPE (Com Port Sharing)

Configure VSPE to provide a shared port for your K3.

Run WinTest – connect the radio to the com port configured in VSPE.

Like-wise, connect Skimmer to the same com port configured in VSPE.

Configure Skimmer so that the Telnet server is running.

Start Skimmer.

Run WTTelnetX and connect to the Skimmer telnet server.

Configure WinTest to ‘communicate’ with WTTelnetX.

Conclusion

Although Skimmer probably won’t make much difference to day-to-day ham radio, I expect increasing use of it by dx-peditions as it can pull out calls from a pileup easily, making for simple (less tiring) point-and-shoot operation. I have deliberately avoided getting involved in the argument concerning whether the use of CW Skimmer gives a contest operator an ‘unfair’ advantage. If the use of Skimmer places an operator in a different category, then it’s only our mistrust of others that is a problem. Skimmer does provide an operator with some advantages over, say, the DX Cluster, because what you see on the screen is what is actually being heard at your station. But to be honest, by Sunday afternoon in a 48 hour contest the majority of calls that are being captured by Skimmer have already been worked. It is no substitute for a good pair of ears. It does however provide a good indication of whether a band is open or not, and it really does help in VHF CW contesting, where signals are weak and sparse. How long before SSB Skimmer arrives?...

Resources

CW Skimmer download and information - http://www.dxatlas.com/CwSkimmer/

Intro to CW Skimmer - http://www.dxatlas.com/CwSkimmer/Files/Skimmerintro.pdf

Elecraft K3 – http://www.elecraft.com

SoftRock - http://groups.yahoo.com/group/softrock40/

RF Space - http://www.rfspace.com/RFSPACE/Home.html

Quicksilver - http://www.philcovington.com/QuickSilver/

HPSDR - http://openhpsdr.org/

MicroTelecom - http://www.microtelecom.it/perseus/

FlexRadio - http://www.flex-radio.com/

SDR Kits - http://www.sdr-kits.net/

Reverse beacon Network - http://www.reversebeacon.net/