VLF ready, hopefully!

M0RVB

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.

An Airspy HF+ Discovery SDR

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 port). As my current use for both is VLF work the ranges are ideal.

Swapping the antennas between the two backs up my previous findings that the mini whip rolls off below 20kHz, rather unfortunate given SAQ is on 17.2kHz. The response of the YouLoop is flat across the spectrum which is not surprising given it is passive.

The complete setup is as follows:

  • The Airspy is plugged into the Windows PC via USB.
  • The RSPdx is plugged into the Linux PC by USB.
  • The mini whip is powered by a bias-tee and connects to the Heros VLF converter which converts the received signals up to 4kHz.
  • Access to the Airspy is via SDR# on the Windows PC with output via headphones plugged directly into the PC.
  • Access to the RSPdx is via SDRconnect server on the Linux PC and SDRconnect as a client on the Mac mini with output via the speakers in the attached display

Both setups receive the MSF (60kHz) and DCF77 (77.5kHz) time signals perfectly well. There was some very annoying interference on the AIrspy setup but this was traced to connectors. I had reused coax that was originally connected to the HF random wire and had a number of clip-on ferrites. Removing those resulted in me discovering the patch lead I was using was solid cored and the BNC connector had worked loose, plus there was a vibration issue affecting signal strength. Removing the coax from the BNC patch panel and attaching the Airspy directly to it cured the interference and shows the Airspy + YouLoop to be a superior setup displaying nice clean signals across the band. Of course, comparing these two setups in this was is extremely subjective and that was not my aim. The aim here is to hopefully copy the SAQ 100 year anniversary broadcast on 1st December 2024.