VLF ⁽⁵⁾

Here is a quick and dirty comparison between the Behringer UMC204HD + PA0RDT mini whip and the Airspy HF+ Discovery and YouLoop looking at a similar amount of spectrum from 0 up to 96kHz or so. First the UMC204HD: And the Airspy: This was not done in any scientific fashion, it's just to show that the soundcard method works ok but is not as good as the Airspy. No surprises there and this is no criticism of either method. I can see the utility of the soundcard method when coupled with the vlfrx-tools package as all manner of analyses become possible. I have only just started reading about that.

Another new toy arrived today. A Behringer UMC204HD "2x4, 24-Bit/192 kHz USB audio/MIDI interface". I had been reading about the use of soudcards for VLF reception and the raves about the UMC202HD - the 204 model has the addition of a MIDI interface as well which may be useful and one came up at the right price (ok, I missed several of the 202 models on eBay so grabbed this 204 one buy-it-now!) I did experiment previously with the built-in soundcard on the Linux PC but got very poor results, quite possibly due to a lack of perseverence (fiddling) on my part. Anyway, I took the advice of the many raves. And it works!I installed the excellent vlfrx-tools package plus an additional package vtlivesgram. Running vtcard and vtcat piped into vtlivesgram results in a very useful display: The trace above shows peaks where I see them using the Airspy HF+ Discovery / YouLoop combination which provided my first ever reception of SAQ. The results are limited when compared to the Airspy but that is to be expected. Here, the Behringer input is simply the output from a PA0RDT mini whip in the loft, plugged directly in via a 1/4" jack.

Very clear reception of the SAQ 100 year anniversary broadcast today, 1st December 2024. This is the first time I received SAQ despite previous attempts. I had two setups running in the hope to at least see something. SDRconnect was running on the Linux box connected via a Heros upconverter and a mini whip in the loft, and SDR# was running on the Windows 10 PC connected to the Airspy HF+ Discovery and a YouLoop antenna in the loft. The YouLoop was oriented towards SAQ at about 62 degrees from here. The mini whip saw nothing but as I mentioned previously it looks to have a serious drop off below about 20kHz. But the Airspy + YouLoop combination worked flawlessly. I saw the carrier appear at aroudn 09:45 UTC as they were switching everything on, followed by a loop of VVV SAQ. Then, at 10:00 UTC came the anniversary broadcast itself begining with CQ de SAQ. It was quite neat to see the gentleman on the key at SAQ and see the morse via the SDR. You can see the morse trail from SAQ on the image above, not a great deal above the noise floor but sufficient to read…

I now have two new toys, an Airspy HF+ Discovery SDR, and an Airspy YouLoop antenna. Plus I already have a SDRplay RSPdx which was purchased some time ago. The initial aim of these is to hopefully receive signals from SAQ’s 100 year anniversary broadcast. The YouLoop is in the loft and connected to some RG213 coax back to the shack. That coax was feeding the random wire for HF but as I now use the external wire the coax was pulled back and shortened. It is entirely passive so does not require any voltage supply and has a range of 10kHz to 30MHz plus VHF up to 300MHz. There is also an IP35 mini whip which was acquired some years ago and as an active device needs a supply which is achieved by a bias-tee fed from the 12V shack PSU. That antenna has a range of 10kHz to 30MHz and is fed via some RG58. The YouLoop is pointing, quite by chance at the MSF 60kHz time signal transmitter. The IP35 is largely omnidirectional. The Airspy covers 0.5kHz to 31MHz and 64MHz to 260MHz whereas the RSPdx covers 1kHz to 2GHz with no gaps (200MHz on the BNC…

I am making a better effort at picking up the SAQ broadcast due on the 1st of December. I now have a VLF mini whip (no idea where it came from now!) up in the loft fed via a bias-tee and connected to a Heros VLF converter which takes 0 to 500kHz and converts it to 4,000 to 4,500kHz. Although the loft is a poor location for this it is a lot better than in the shack. the Heros is connected to an RSPdx on it's BNC port and this is connected to the Linux PC which runs SDRconnect as a server. Reception is via SDRconnect on the Mac. So far, so good, I can see the MSF and DCF77 time signals on 60kHz and 70.5kHz respectively plus a ferw other peaks, at least some of which correspond with known transmitters. Getting the settings right is a fiddle. The above photo has the base set low and the red line is where SAQ should be. But there is quite a rolloff there so hopefully the signal will be sufficiently strong to receive. The antenna is supposed to work down to 5kHz and the Heros is specified as <5 to 500kHz.