A 100W PA for VLF use

BK Electronics in the UK make a range of MOSFET PA units with built-in toroidal power supplies. They have a 100W unit for £53, which is very reasonable for a built and tested unit. I believe these units work to 137kHz with a small modification and they would be ideal for 8.97kHz VLF use. There are versions up to 1kW, but these are quite expensive.

See http://www.bkelec.com/Modules/Amplifiers.htm

Easy mistakes

On Monday night I was looking for Uwe DJ8WX again on 8.97022kHz.  To my great surprise I copied nothing when G3ZJO and M0BMU were getting good copy. I'd copied Uwe several times before. After suspecting my PC sound card was faulty, I found out why I copied nothing tonight: on Sunday I'd retuned my loop ATU to copy M0BMU on 500kHz and I'd forgotten to retune it back to 8.97kHz. Although Spectrum Lab locked onto GBZ OK, the sensitivity was about 20dB down! We learn by our mistakes.

G3XIZ a good signal on 8.976662kHz VLF today

Chris G3XIZ has just started TXing again and is a decent signal here in 424uHz bandwidth with the signal up to 20dB over the noise (see picture). Eddie G3ZJO was also getting a good signal. At 1530z Chris dropped the power 10dB but unfortunatley I was unable to copy at this QRP level, whereas G3ZJO was still able to.

Back from holiday

There have been no posts this last week as I have been away on holiday in Devon in SW England staying at my brother's cottage. For once the weather was excellent with wall-to-wall sunshine every day. I did not take any radio gear with me this time but plan to take the FT817 when I go next time to do some pedestrian portable cliff-top QRP DXing. Last May I worked some excellent European DX on 6m with just the FT817 handheld.

Loop TX antennas at VLF

At first sight the idea of a 10m x 10m wire loop as a TX antenna at 8.97kHz VLF sounds a non-starter as a way of radiating any useful power. Having tossed the idea around with others though it appears that, although very inefficient, it would not be that much worse than a smallish Marconi vertical because these antennas need very large (and lossy) loading coils, height and good grounds to be effective. With 100W to such a small loop around 0.45uW could be radiated. Already G3XIZ has spanned 218km with just 2uW to his vertical, so for small gardens the VLF TX loop is worth a look.

G3XIZ and DJ8WX on 8.97kHz

The Spectrum Lab plot showing reception here again of these two stations around 8.97kHz VLF. Both were using back garden antennas. DJ8WX is around 645km away. G3XIZ about 45km.

GPS cannot always be relied upon

As if to remind us of the vulnerability of modern technology yet again:

The MoD has informed OFCOM of the following GPS jamming exercise

Dates: 19 – 26 May 2011.

Times: limited periods between 0700z 19 May to 100z 2 May 2011. 

Location:  The ground based radar jamming events will take place in the SW approaches against the remote radar head at Portreath 21-22 May, undertaken by F-18 Growler aircraft. Additionally there will be radar jamming against deployed targets in South West Wales throughout the period 19-26 May. Communications and airborne radar jamming will take place throughout the same period on the East Coast with aircraft operating within the North Sea Military Danger Areas. Communications and maritime unit radar jamming will take place in the SW approaches and Bristol Channel. Falcon aircraft will operate from Sea Level to FL240 unless jamming AEW aircraft where they may operate above FL245.

Note: Safety of life operations will take precedence over exercise activities at all times.

Emergency radio kit

The terrible Japanese earthquake, tsunami and nuclear crisis has reminded us all that modern 1st world technology cannot always be relied upon. The UK Foreign Office and the BBC have webpages suggesting essential items as an emergency "grab bag" to have ready in such circumstances. One of the attractions of simple QRP gear is its ability to work when nothing else may, for example using a small battery or solar cells with a compact CW rig that can be taken anywhere with a throw-out HF antenna. In the event of a sudden and unexpected disaster, would you and I be able to communicate still?

8.97kHz very sensitive spectrogram online

Paul Nicholson in Todmorden, Yorkshire UK has put on line his steerable and highly sensitive spectrogram for the VLF Dreamer's Band around 8.97kHz. Paul's system is super sensitive and detects just about anything going. His new spectrogram can be "steered" to a specific direction and the display clicked to show exact frequency and S/N levels. There is also a roll back function so that records from a few hours to days back can also be checked. Marvellous! See http://abelian.org/vlf/fbins.shtml

Great care needed when IDing VLF stations

A word of caution to those of us looking for very weak signals around 8-9kHz. DJ8WX was copied here overnight on 8.970022kHz without any doubt and my grabber was clearly able to see his close-down around 0700 today. The frequency and the trace timing corresponded perfectly. Likewise with G3XIZ and DK7FC/P some weeks ago.

This evening I was looking very carefully again at the Spectrum Lab screen from the last 24 hours in 424uHz BW and readjusting audio gain ranges, screen colour saturation and contrast. Out of the noise appeared another very very faint line, not at 8.970022 but at 8.96998kHz. I've attached the screen shot here (I have overlaid it with the time for clarity).

My immediate reaction was this must be Ossi OE5ODL as this is the frequency he uses. Then I checked his grabber and saw he was not operational overnight last night! So, what looked like a trace on his frequency cannot be him. It must instead be some artefact of SL or something else. We have to be very careful when seeing traces at very specific VLF frequencies: it may be an indication of a given VLF amateur station, but without some modulation or turning on/off of the carrier a doubt remains.

Sadly I don't think I did see Ossi today after all, but I shall keep looking.