28 Nov 2010

New front-end for 8.76kHz VLF earth mode tests

Today I completed the design of a new "front-end" for my tests on 8.76kHz when using earth electrodes as the RX antenna. Up to now I've used an 80cm loop/preamp at the far end of the test, usually with the loop on the ground for maximum signal pick-up. Now I want to compare results with an electrode pair antenna at the RX, laid across the road to form a pickup loop within the ground around any buried pipework and cables. The electrode pair is about 30-100 ohms, so I needed a low input impedance. I decided on a grounded gate FET amplifier with a simple LC tank circuit in the drain tuned to 8.76kHz fed via an emitter follower into the PC sound card. It has sufficient selectivity to reject 50Hz and lower harmonics and reject the stronger VLF and LF stations which could cause intermod and overload. I've also added a small LC lowpass filter on the input to the FET. This will be field tested at a site 5.3kms from the home QTH as soon as the weather improves: it is currently below freezing day and night!

25 Nov 2010

Receiver/grabber improvements on VLF

Just received a special build of Spectran from Alberto I2PHD that keeps the same filename for captured grabber images. This means I can now use Spectran with my VLF grabber. This is fired up from time to time when people are doing VLF tests. It will also allow me to look for my own signal at home when I am out transmitting in the Fens locally some kms from home. I find Spectran easier to use than Spectrum Lab, which is a clever program but complicated for someone like me to configure.

I have also rebuilt the 4-5W beacon TX on 8.76kHz and given it a 1 hour continuous soak test. There is a video about this on my YouTube channel.

My next job is to work on a permanent VLF E-field probe outdoors and to make improvements to the receiving system for field use. I need a better preamp/filter that can also be used with earth electrode receiving antennas as well as the loop and E-field whips.

22 Nov 2010

Looking for G3XIZ on VLF this evening

G3XIZ is transmitting with 20W to his Marconi vertical on 8.9719kHz this evening from 2000-2200z. I am looking out for his signal with my loop into both Spectrum Lab and Spectran software. You can look on my VLF grabber to see if he is visible at all over here in JO02dg. At the moment all that is visible is a wobbly 50Hz related signal and my 8.760kHz QRSS3 beacon running on a dummy load. UPDATE: nothing copied and Chris had to close because of an antenna fault at 2100z.

21 Nov 2010

VLF Grabber operational capturing 8-9kHz in JO02dg

I'm setting up a VLF grabber at my home QTH to take periodic screenshots of the 8-9kHz Dreamers Band and make these available online. At the moment it is taking input from my earth electrode "antenna" into Spectrum Lab and is very much a work in progress and needs a lot of improvements on the RX and antenna front. It won't be ON unless I'm in the shack and don't need the PC for other VLF stuff.  

The resulting screen grab every few minutes appears at
https://dl.dropbox.com/u/15047843/xbm_grab.jpg . 

VLF Grabber

Today I intend to make a VLF grabber to receive sub9kHz signals and automatically upload the screen grabs to the Internet. My aim is to use Dropbox as the place where the grabs will we stored and visible.

19 Nov 2010

6 Spanish stations get access to 501-504kHz

The Spanish authorities have granted limited access to 501-504kHz (100Hz bandwidth, 5W) to 6 stations (EA1AY,  EA2HB,  EA3CC,  EA3WX,   EA4BVZ, and  EA5DY) until the end of May 2011,

16 Nov 2010

iPod Touch 4g

Recently I upgraded my pocket PC - I used a Dell Axim for 7 years - to a new iPod Touch 4g purchased at a good price from Amazon. It really is an amazing piece of kit in a tiny size with an incredibly pin-sharp screen which is so good that individual pixels cannot be seen: the resolution is better than my TV. I've already downloaded the Echolink app (free) so I now also have a 2.3GHz pocket ham transceiver too. I hope to use it for some other ham radio applications including datamodes and VLF reception.

If you want to see what is inside the iPod Touch 4g look here for a tear-down analysis: there is a lot in this small package!

14 Nov 2010

First Norwegian VLF amateur transmission

From Jan LA3EQ:
"First succesfull transmission on 8.760kHz using WSPR mode in Norway using 40 meter longwire and only speaker output from the pc soundboard as a transmitter and a 12volt to 220v transformer as impeadence "antenna matching" transformer..  Several WSPR spots of -2dB to -7 dB. distaince only 50 meters, but it is a start. Next try will be with earth dipole and 10 watts and an active e-probe antenna on receive."

VLF kite transmission by DF6NM

DF6NM's signal in the UK
Marcus DF6NM carried out another successful VLF test yesterday on 8.97kHz (200uW ERP) and 6.47kHz (50uW ERP) with reception by Paul Nicolson in Todmorden UK.  Marcus is the second German VLF amateur to be copied at this distance.

12 Nov 2010

Inactive week radiowise

This week I've been very inactive on the radio because we have had our grandson's 3rd birthday party here with his French grandparents (neither of whom speak English) staying along with his mum and dad. Still, a lovely time was had and he got lots of nice presents. By the way, he now knows the difference between square waves and sine waves and loves going "oo oo", which means fiddling with the audio oscillator and scope.

7 Nov 2010

Looking for G3XIZ on 8.9719kHz VLF

Looking for G3XIZ on 8.9719kHz - maybe a faint trace??
Between 2000 and 2200z this evening G3XIZ has been transmitting on 8.9719kHz VLF with his Marconi vertical and about 20W RF into it. I've been monitoring with my earth electrode antenna, a preamp/filter and a PC running Spectran on 0.17Hz resolution, but don't think I've seen any sign of him.

5 Nov 2010

Real QRP on 8.760kHz today

Weak trace at 1.5km from the 320mW TX by earth mode
320mW 8.760kHz QRSS3 transmitter
Having blown up my 4W transmitter yesterday I built a smaller QRSS3 beacon TX today that puts out 320mW into 50 ohms. I connected this to the earth electrode antenna and went out with the loop and E-field probe and PC to see how it performed. The signal was copied weakly but solidly at 1.5km away at the local National Trust carpark (see picture from Spectran showing the XBM part of may callsign). An attempt at 5.1km was unsuccessful although faint traces of the line at 8.760kHz may have been visible.

How to be stupid ...follow me

Just managed to blow up my 8.76kHz TX by putting the supply the wrong way round: took out the 4060, 5-Vreg and several electrolytics.The TDA2002 was also damaged. Rebuilt the keyer and freq gen part OK but now need to source a new audio PA IC or module. Now I have a reverse protection diode across the pins! You learn the hard way.

In the meantime I may try even lower power with my trusty 2N3904/3906 1W PA and see if this can be seen at the NT carpark (1.5km) and at Lord's Ground Farm (5.1km). Also, as this PA is efficient and runs cold for ever I could try 1 day on, 1 day off, QRSSSSS and see if more distant people could copy it using very narrow bandwidth reception as per DK7FC's tests.

4 Nov 2010


It's time to remind people of the wonderful circuits and ideas at Mike Rainey AA1TJ's site. Always full of amazingly simple designs that really work. See  http://www.aa1tj.com/radio.html

3 Nov 2010

UK first transmission on the Dreamer's Band (8.760kHz)

Signal received at 5.1km from home QTH on 8.760kHz VLF
This morning 0915-1115 GMT I transmitted a QRSS3 beacon signal on 8.760kHz under the terms of my NoV recently received.  I think this is a first in the UK, legally at least.

8.76kHz TX
TX was 4W from a TDA2002 audio IC matched into earth electrodes 20m apart. The beacon signal was provided by a K1EL keyer chip and the frequency reference by dividing down an HF crystal by 512 times in a 4060 divider IC. Reception was possible 5.1km away from the transmitter location, detecting the signal with an 80cm loop fed into an E-field probe (Hi-Z input) into Spectran running on a WinXP PC. Best reception was always with the loop flat on the ground suggesting the main mode of propagation is utilities assisted earth mode. Comparing results on 8.76kHz with those at 838Hz earlier in the summer, my first impressions are that signal levels are some 6dB weaker, but more careful tests will be needed.

2 Nov 2010

500kHz interference

In the last few days several stations across Western Europe have noticed a strong wideband signal that is severely impacting reception between 498-502kHz. This appears to be a NATO test transmission for a marine data system that may be introduced in the near future.

8.75kHz VLF transmitter ready

My small 4W, crystal stabilised, 8.750kHz VLF transmitter is now ready to test having fitted the crystal and 4060 divider this morning. All being well I'll test this Wednesday or Thursday when I've optimised my soundcard settings on the PC used at the RX end.

1 Nov 2010

Preparations for Dreamer's Band Tests

This evening I modified my loop antenna and VLF preamp to better optimise them for use at 8-9kHz and also modified my E-field probe antenna. So, on the RX side I think I'm ready with the hardware. By Wednesday I hope to have the 4060 divider behind the HF crystal to give me a stable 8.750kHz source for the TX. If the weather is reasonable I should be out testing in the Fens locally by Wednesday or Thursday.