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.  

New VLF Beacon TX

Today I completed the build of my new VLF beacon transmitter. The beacon puts out 5W into a 50 ohm load, which is very close to the resistance of my 20m spaced earth electrodes. The beacon operates at either 8.970kHz or 4.485kHz with (a) continuous carrier, (b) 10wpm CW or (c) QRSS3.

VLF Earth-mode Beacon Transmitter

The main changes were to separate the keyer and the PA, mount the whole unit in a larger metal box and to use a 3C90 matching transformer with a fixed turns ratio having measured the ground resistance, which does not change greatly. On a continuous carrier soak test the case temperature only rises about 10 deg C, so frequency drift in the oscillator divider frequency source should be only around 0.1Hz at final frequency.

Having got the beacon finished, the next stage is to start work on some improved receiving kit and to see what sort of range can be achieved through the ground. My best DX so far with the original beacon was 6km, but this is certainly not the limit.

iPod Touch 4g - resetting

This evening I decided to do a "return to factory reset" on my Apple iPod Touch 4g as I'd run out of storage space on the 8GB machine and the battery life, despite regularly closing open apps, was getting very poor. I'd put this down to the age (2 years) and almost daily use draining the LiIon batteries.

So, I backed up the photos and videos, ensured the apps were safe on the main PC and pressed the "reset". Then I loaded iOS6.0 and all the apps back on the iPod. First impressions are that the battery life has just about doubled and appears to be almost back to what it was 2 years ago. I wasn't expecting this. To be honest, it feels like a brand new machine.

Although I don't buy that many modern gadgets the purchase of the iPod Touch 4g 2 years ago was one of my best ever purchases. It allows me to keep in touch with my emails, BBC, family by Skype and FaceTime video, WSPR spots monitoring, take photos and videos, Facebook, etc wherever there is a WiFi connection.  It slips in my pocket and is an almost constant companion.

My whole family are Apple converts!

Latest sunspot news

The latest from NASA:

The current prediction for Sunspot Cycle 24 gives a smoothed sunspot number maximum of about 75 in the Fall of 2013. The smoothed sunspot number (for 2012/02) is already nearly 67 due to the strong peak in late 2011 so the official maximum will be at least this high. We are currently well over three years into Cycle 24. The current predicted and observed size makes this the smallest sunspot cycle since Cycle 14 which had a maximum of 64.2 in February of 1906.

 The latest prediction graph from http://solarscience.msfc.nasa.gov/images/ssn_predict_l.gif

Charity jazz concert, Oxford Sunday Oct 7th

Final reminder: Tim Lapthorn Trio (jazz) at the Jacqueline du Pre Music Building, St Hilda's College, Oxford this Sunday at 1pm. Concert is raising funds for multiple sclerosis research.  Tickets from http://www.oxfordplayhouse.com/ticketsoxford/index.aspx?catid=24

or on the door. Good music, good cause. Please come if you can to support it.

Tim in one of his many appearances at Ronnie Scott's in London

My son Tim is a world class professional jazz pianist - see www.timlapthorn.com/ - and he is launching his new CD called "Transport" available later this month. Proceeds from the concert will got to the MS Society. 

If you cannot attend and want to make a donation to MS, please email me. Samples of his music available on his website.

Over-the-horizon on 480THz

My order for 3 off 12W red LEDs known as PT54 Phlatlights has arrived. These are VERY VERY bright LEDs mounted onto substantial heatsinks which emit a strong signal on 480THz. They were originally intended for portable projection systems. One of mine is going into my QRO optical beacon running continuous carrier, CW, DFCW, QRSS3 or QRSS30 to use in further over the horizon (non line-of-sight) beaconing tests using the scatter from particles in clear air and the reflection off the base of clouds. Last winter my own simple beacon running at around 300mW was detected several km over the horizon using QRSS3. G4HJW's optical beacon using a Phlatlight LED was detectable in my village some 8-9km over the horizon. In neither case could the red glow from the TX be detected by eye.  This week I also received some SFH213 10 degree half angle PIN photodiodes which should be considerably more sensitive than my BPW34 detectors currently in use. Estimates suggest at least 6dB more sensitivity.

Together, the QRO optical beacon running from home, focussed with a 100mm lens (gain about 24-30dB), and a portable high sensitivity 100mm lens based detector should allow plenty of scope for innovative NLOS tests on dark winter evenings.

Just realised: 5W out (for example) into a 30dB gain lens "antenna" is equivalent to 5kW of light power in the beam. That is SOME bright light. Clearly great care is needed in siting and aiming such a system to ensure safety.

New soldering iron needed

My old Weller soldering iron station (an old Pye Telecom chuck-out from about 1980) is on its very last legs - it is physically cracked and really does need replacing. You can tell how old it is by the mains wire colours! It was possibly the one I had on my bench when I started work in 1970.

I'm looking at the Maplin soldering iron range for a replacement, which look good value. Most of my work is with discrete parts but increasingly some SMA parts are being used too. Up to now I have been using up my stock of tin-lead solder, but am happy to move to lead free.

So, please may I have your recommendation on what soldering iron to go for?

I don't mind spending a bit more if by doing so I get a more reliable soldering station. Clearly replacement tips must be available inexpensively.

3C90 cores at 8.97kHz

The output of my TDA2003 based VLF earth-mode transmitter is around 5W into 4 ohms. The new semi-permanent earth mode "antenna" just installed measured at around 50-60 ohms resistive at 8.97kHz.  I used an AC potential divider technique to check this. Today I wound a small 3C90 based transformer using the advice I got from various people yesterday and it works very well, matching the TDA2003 perfectly to the earth electrode pair. I managed to destroy my K1EL message keyer's 5V regulator (and the keyer IC too!) so a rebuild is required before I go out into the field again to do some RX measurements. All being well these new tests will start in the next few days.

My SFH213 PIN photodiodes arrived today and my 12W Phlatlight LEDs are due next week. I'd better crack on with the new VLF earth-mode tests before these arrive as I will want to try some over-the-horizon 481THz tests with the more powerful optical transmitter and more sensitive detectors.

More on LF transformers

Following on from the earlier blog entry about using a 3C90 core for a VLF and LF transformer, I got this reply from Jim M0BMU last night on the RSGB LF Yahoo group. I post it here as it contains some useful additional information. See also the mini-Ring Core Calculator from DL5SWB at http://dl5swb.de/ .

"Dear Roger, Andy, LF Group,

>> Four turns minimum for 137kHz 25 Watts. 60 or so for 9kHz
> Yes these values look quite practical ones.

...But now the inductance of the winding and AL value of the core do become important. (BTW, the value of 2000 is the relative permeability of the 3C90 material. The "inductance factor" AL, the "inductance per turn-squared", is a different number which depends on the shape and size of the core as well as the permeability.) AL for this core is given as 2690nH nominally. With a four turn winding, the resulting L is about 43uH, with a reactance of only 37ohms at 137k. In a 50 ohm circuit, this will cetainly mess things up a bit. As a general rule, you would probably like the reactance of the 50ohm winding to be at least 250ohms at the operating frequency. This requires an inductance of more than 290uH, so a winding of 11 turns minimum will be needed for a 50ohm impedance level.

This is a typical result when using a core that is much larger than what is  required by power handling considerations - the number of turns needed to keep the flux down to an acceptable level becomes so small that the inductance becomes the deciding factor. It also obviously makes it tricky to match to low impedances, which is often what you are trying to do in a PA or
loop-matching transformer - you may well find that you end up with windings of less than 1 turn! In these cases the inductance or the required turns ratio becomes the determining factors. In the more normal situation where you are trying to design a transformer with an economically-sized core for a given power level, the inductance is usually large enough not to be an issue, as Andy stated.

At 9kHz however, the 60turn winding is quite reasonable from the inductance point of view, giving 9.7mH and about 550ohm reactance. Also, the core losses would be lower at 9kHz, so you could allow a higher flux density and reduce the number of turns (or increase the power level, which might be better!)

Cheers, Jim Moritz
73 de M0BMU"

Azores Islands

This evening I switched on the FT817 not expecting to work anyone on 10m when I heard CU7AA, Faial Island in the Azores archipelago, calling CQ on 28.520MHz. A quick single call and he came back to me with a 57 report on SSB. Although I've worked the Azores several times, including on 6m QRP, this is the first time I've worked Faial Island I think