Next SAQ (17.2kHz CW) test transmission

Next Sunday, this historic alternator TX is transmitting for Alexanderson Day on July 4th. I have a QSL card somewhere from them for a transmission many years ago. This is a good chance to test your VLF system.

See https://alexander.n.se/category/news/?lang=en .

See https://sites.google.com/site/sm6lkm/saqrx/ .

Improved Software VLF Receiver

SWL Roland's enhanced SM6LKM software VLF receiver

An SWL called Roland from Germany has produced an enhanced version of SM6KLM's software VLF receiver, originally designed to allow reception of SAQ on 17.2kHz CW. The new version has:

•  44.1k/48k/88.2k/96k/176.4k/192k sample rate support
• More CW filters, SSB filter and AM filter added
• Spectrum display for audio frequency
• File-I/O for *.wav format (16bit)
• Left/Right channel select
• RMS signal level bar for audio level
• Muting (M) key
• Time+Date display, UTC or local time

This now makes the "enhanced" SM6KLM software receiver a superb tool for VLF use. Information and downloads are available at Roland's website https://sites.google.com/site/swljo30tb/

To use the receiver all that is needed is a VLF signal feeding into the mic input of the PC. Be sure you know what you are doing: the usual safe thing to do is to put a couple of back-to-back diodes across the DC isolated VLF input to prevent damage to the sound card. With an E-field probe antenna this receiver is capable of receiving many VLF transmitters from around the world. I shall be using it in future to monitor my 8.97kHz earth-mode CW beacon when out in the field testing.  

Optical receiver noise floor measurements

A PC with some simple free software makes a very useful piece of test gear.

For example, today I wanted to check the noise floor of my optical receiver head between 0-20kHz in total darkness and in daylight and compare this with the noisefloor of the PC with the head turned off. SM6LKM's excellent little software receiver designed to listen to SAQ's transmissions on 17.2kHz was ideal. This tunes from 0-22kHz. Here are the results (see plots) which show noise goes up by about 5dB in the daylight, although noise at 0-2kHz seems to go down in sunlight. By connecting the optical receiver head to the PC I was able to use this little package to see the noise floor changes.  G4JNT thinks the FET is possibly being biassed to a less optimum position, or it is saturating, in daylight. By changing the bias I may be able to get a lower noisefloor and better sensitivity. That I shall try tomorrow.

Simple VLF Receiver

SM6LKM's excellent PC based VLF receiver

In testing my Chirpy 10m transceiver, I used the simple PC based VLF receiver by SM6LKM as a bolt-on SDR. This little VLF receiver works very well as a stand-alone VLF receiver that tunes the 0-22kHz frequency range. Simply by sliding the cursor to any frequency it produces an audible signal just as you would get by tuning a conventional receiver.  Bandwidths are adjustable down to around 300Hz. For my video showing this in action listening to VLF signals (via my loop and small preamp) see http://www.youtube.com/watch?v=TDxRhQYg7lQ. With the SM6LKM VLF receiver you can clearly hear the Omega beacons around 11-14kHz, SAQ when transmitting on 17.2kHz, and various MSK military signals around 18-22kHz. It has also proved useful when carrying out earth-mode (through the ground) VLF communication tests.

Soundcard based VLF receiver

SM6LKM has a very neat sound card based VLF receiver ideal for listening to sub-9kHz ham signals and SAQ on 17.2kHz. On his website you can download the files and instructions.