Monday, November 12, 2012

Introducing the OH6BG Skimmer Station

My Skimmer station started operations in May 2011. My principal goal with this station at that time was to collect real-life data to evaluate the quality of VOACAP propagation predictions. The skimmer also serves as a real-time propagation monitor, especially for Nordic radio amateurs and other interested parties. To see all the data skimmer outputs in real-time, 24/7, go to my Nordic Real-Time Propagation page. I also send all CQ spots from the skimmer to the Reverse Beacon Network.

The OH6BG Skimmer setup from bottom up: a laptop computer, the SRL QS1R Receiver and the Z10043S preamp.


If you want to set up a skimmer station of your own, you will need a minimum of four things:
  1. SDR (software-defined radio)
  2. Antennas
  3. Computer
  4. Skimmer software
1. The SDR receiver

As I wanted to monitor up to 7 bands simultaneously (ranging from 1.8 MHz to 28 MHz), my choice was the SRL QS1R receiver from Software Radio Laboratory LLC in Ohio, USA.

Some of its features are:
  • Frequency Range (BNC LPF Input): 15 kHz to 62 MHz
  • Input Impedance: 50 ohms
  • Clipping RF Level: +9 dBm
  • Maximum Display Bandwidth: 4 MHz
  • ADC Sampling Clock: 125 MHz (1 - 130 MHz with external encode input)
  • I/Q Image Rejection: >110 dB
  • Voltage: 5 - 6 VDC, 2A fused, reverse polarity protected
  • Current Draw: 500 mA (typ.)
  • Dimensions: 160 x 100 mm (3.299" x 3.940") (board size)
The service from Software Radio Laboratory LLC is excellent, and they do deliver worldwide. You can pay easily via PayPal.

2. Antennas

I have only a modest antenna setup to cover everything from 1.8 MHz to 28 MHz. For 160, 80 and 40 meters, I use a cross-dipole tuned to 80 meters and 40 meters, with the feedpoint @ 10 meters  above ground. Reception on 160 meters suffers quite heavily from this untuned antenna.

For 14 MHz - 28 MHz (incl. WARC bands) I have a Mosley TA-53-M, a five-band, three-element Yagi, with the feedpoint at 13 meters above ground. Currently, the Yagi beams at 275 degrees.

3. Computer

An SDR definitely requires a computer, and my choice was, first of all, a laptop computer with an Intel i7 processor. In my limited experience, laptops are very good because they do not generate harmful noise which could affect the reception of weak signals on HF. Secondly, I wanted to invest in a powerful computer so that the CPU load could be kept at a minimum during normal conditions. I would presume this would also generate less heat - but I can be wrong. During big contests such as CQ WW CW a powerful computer can then decode thousands of CW signals without choking up.

4. Skimmer software

The ultimate skimmer software is Skimmer Server from Alex VE3NEA. Alex also offers CW Skimmer but that is for skimming one band only.  Skimmer Server can handle up to 7 bands simultaneously with the QS1R receiver. The typical bandwidth I use is 96 kHz per band, although I could use 192 kHz. I do not want to put unnecessary stress on the computer in normal conditions.

If you want to be part of a global monitoring network, the Reverse Beacon Network (RBN), that collects all real-time skimmer spots from all corners of the globe, you will also need software (Aggregator) that connects to the Skimmer Server and sends CQ spots to the RBN.

These were the minimum requirements for skimming purposes. If you want to use your QS1R Receiver as a normal receiver, you will need listening software for it, too. I would recommend three software which are all free:

  • SDRMAX V, developed by Software Radio Laboratory LLC
  • HDSDR, available for free download at
  • SDR-RADIO v.1.5, available for free at - this software does not allow you to control or listen to your QS1R, but it offers an excellent choice for playing back your recordings you have made with the QS1R
For recording bands, I recommend a set of free software developed by the OL5Q team. Take a look at their software offering at Their recording software is called CWSL (CW Skimmer Listener). This requires that a few other pieces of software are installed, too - but they are all available at the same address. Just for a reference, recording 6 bands for 48 hours takes approximately 900 GB of disk space. These recordings can be played back e.g. with the HDSDR and SDR-RADIO software.

5. Pre-amp

This last item, a pre-amp, is not absolutely necessary but I would wholeheartedly recommend one, anyway, after using such thing myself only for a few days now. I had been running the skimmer station over a year without a pre-amplifier, i.e. a device that amplifies the signal (and the noise, too!) from your antenna before going to the receiver. But I felt something was still missing. I figured that a pre-amp may bring advantages in my case as the man-made noise level at my QTH is low. And a high-quality pre-amp seems to indeed make a difference!

I ordered my pre-amp from Clifton Laboratories via PayPal, and their customer service is perfect. They offered me a new model, the Z10043S with a gain of 11 dB from approximately 45 kHz to 45 MHz (I have not seen the exact spec yet). This product is not yet listed on their website but they claim that "The Z10043S is identical in gain and a bit better in 2nd and 3rd order intermodulation performance compared with the '42A [Z10042A]."

Please note that the Z10043S does not have reverse voltage protection, which is why I succeeded in blowing up the voltage regulator. These are surface mount components so you will need special equipment to replace them. The lesson learned? Always, ALWAYS, check the polarity of power cables, no matter what! Do NOT trust ANY markings the cables may have.

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