Showing posts with label 8.76khz. Show all posts
Showing posts with label 8.76khz. Show all posts

14 Aug 2011

Best results yet on 8.76kHz earth mode

8.76kHz earth mode signal in 45mHz BW at 6km
This morning I slightly changed my VLF low input impedance tuned preamp by increasing the tuned circuit Q and gain of the first stage by around 6dB. After lunch I did a test at 6km from home to see what effect this had on the S/N of my 5W beacon. In a 45mHz bandwidth (previously 11mHz) I was getting a signal around 18dB S/N using Spectrum Lab on my 80cm square loop on the ground. With an earth electrode antenna the signal was marginally weaker and the noise level higher.

My signal is the steady carrier (with a 2min ID break) at 8.7605kHz. The large wobbly line is interference, but who or what is the signal at 8.754kHz? This is rock steady and appears to key on and off every 5 minutes. It looks like some sort of control signal, or is someone else near Cambridge testing?



15 Jul 2011

More VLF earth-mode tests

8.76kHz QRSS3 signal at 3.8km
Recently the water company repaired some pipes right outside our property in the road. I was informed the pipes are/were NOT metallic.

Today, just as a check, I tried 8.760kHz earth-mode running my usual 5W into 20m spaced earth electrodes just to see if the repair work made any difference to signal levels some kilometres away.

At my favourite roadside test site 3.8km from home I set up my RX and tried 2 different loops on the ground (a) my 30t 80cm square loop and (b) a large single turn loop.  Using QRSS3 the signal was copied on both antennas at the usual strength. This makes me think any "utilities assistance" is NOT purely from metal water pipes in the road.

My next planned test is to see how well an earth-mode signal propagates along one of the many fenland water channels using one TX electrode actually in the water and one on dry land. For RX I'll either use another earth electrode pair (one in and one out of the water) or a loop or E-field probe. The next couple of weeks are pretty busy with family matters, so this fenland river test may not take place until sometime in August.

4 Feb 2011

E-field probe test on 8.76kHz away from home

G3XBM E-field probe for 8-9kHz
With my 5W QRSS3 beacon on 8.76kHz I went out looking for my signal at the local National trust car park 1.6km from home. On the 80cm loop signals were as expected but on the E-field probe disappointing when compared with the results with G6ALB's signal at the home QTH a few evenings ago. Although I could detect my earth-mode signal using the EFP with it flat on the ground there was no copy with the probe handheld in the air. The e-field probe schematic is attached.

1 Feb 2011

VLF earth-mode success: G6ALB copied at 3km on 8.76kHz

Spectran trace of G6ALB on 8.76kHz at 3km
This evening G6ALB fired up his 8.760kHz earth-mode TX at his home QTH running QRSS3 and 40W into earth electrodes spaced 25m apart. We started listening outside his QTH with a colossal signal, moving to the next village (Reach - about 2km) where the signal was still good copy. Finally we parked up outside my QTH in Burwell exactly 3km from Andrew's TX. Signals picked up in the road with my 80cm portable loop into the tuned preamp were still strong - at least 20dB S/N on QRSS3 (see attached). For some reason I was unable to copy Andrew's signals on my own earth electrode pair: I may have had a fault and need to check why, as I was expecting this to give the best result. Andrew's signal was so strong on the 80cm loop that 12 wpm CW would have been copied. His keyer and 8.760kHz signal were generated using "Direct Digital Synthesis" from a 25 MHz clock using my Digilent Spartan S3 FPGA board.

This is the first time I've received another amateur by earth-mode (conduction). The result suggests much further is possible. Experiments continue!

13 Jan 2011

Further 8.76kHz VLF test transmissions today

8.76kHz earth mode tests - best DX 5.1km and 4.75km
Today being mild and dry (for the first time in almost 6 weeks) I decided to do a field test with my improved 8-9kHz tuned preamp and loop antenna. A 5W "earth mode" (through the ground) test signal was emitted on 8.760kHz from the Burwell QTH and I ventured out into the countryside armed with the 80cm loop and preamp to see where the signal could be copied. In addition to my usual test sites at 1.6 and 5.1km I tried other directions and sites today. The map attached shows where signals were received and where no copy was achieved. The signal does seem to travel along the metal water pipes, which must be absent to the east where I assume they must be plastic, at least in places.

16 Dec 2010

Long duration VLF test on 8.7608kHz yesterday

Yesterday I ran my 5W 8.7608kHz beacon into the earth electrode antenna on a quasi-continuous basis with transmissions of 30-60 minutes interspersed with 30-60 minute breaks. The tests lasted from 0900-2200z. Several stations between 25-70km were looking for my signals and Paul Nicholson up in Yorkshire has still to analyse his recorded files. So far, no station has detected any sign of the signal, which was not surprising considering the radiated ERP would have been in the pW level.  I was hoping that a few stations 10-20km  from JO02dg might have been looking as these would have had some chance of copying something. So, an unsuccessful test this time.

Paul Nicholson (Todmorden) advises me to use a GPS locked source so that the TX frequency is known with great precision. Then he, and others, can look for statistically significant weak signals at precisely the times and frequency of transmission. My HF crystal and divider source would not have been sufficiently stable to allow this sort of "digging in the noise" over any great time period.  For QRSS3, the current stability is fine. For the record, these were my transmission times: 0840-1000z, 1029-1229z, 1330-1500z,1531-1600z, 1630-1800z, 1830-1900z,1930-2207z on 15.12.10.

10 Dec 2010

How far is possible with earth mode on the Dreamer's Band?

A further observation from the test today: with 17dB above the noise signals at 5.1km using QRSS3 there is plenty of (theoretical) range still to be had with utilities assisted earth mode and just 5W.

Assuming 18dB loss every time distance is doubled (an inverse cubed attenuation rate for induction/conduction - is this right?) then almost 10km should be just possible, assuming the utilities that are aiding propagation are still there in the ground. Going from QRSS3 to QRSS30 should give another 8-10dB (see http://www.qsl.net/on7yd/136narro.htm#QRSS). Increase power from 5W to 100W gains another 13dB, so QRSS30 and 100W could give a range of some 20km by utilities assisted earth mode in favourable locations.

So, in some locations using the utilities buried in the ground all around us could offer some quite decent ranges with an earth electrode "antenna" at the TX end that requires minimal matching and no large loading coils, just a step-up transformer.

What is less clear to me is what exactly is the signal being carried along? Water pipes (often these are plastic now), overhead mains cables, gas pipes (usually non-metallic), phone lines, rivers even?

8.76kHz tests with loop + earth electrodes on RX

Today I did a test at 5.1km to compare reception of my 8.76kHz 5W transmission using (a) an 80cm loop antenna and (b) a small earth electrode antenna (14m spacing) using a variety of preamps.  Although quite solid reception (17dB S/N in 0.18Hz BW) was achieved using the loop laying flat on the ground, there was absolutely nothing detected using the earth electrodes, which was both surprising and disappointing. I used the very same preamp in both cases and also tried the lo-Z input tuned preamp with the earth electrodes that successfully copied DK7FC at 648km last weekend.

Several possibilities for the failure with the earth electrodes:
  • The electrode spacing was far too small
  • The barbecue skewers used as electrodes were too short and not making decent soil contact
  • The soil was too wet, so the pick-up was lower than with dry, low conductivity soil. The test site is in the fens where the soil is a dark peat.
Next step is to repeat the test up on the chalk uplands to the south of the QTH where soil conductivity will be lower and to try a larger earth electrode spacing on receive.

1 Dec 2010

8.76kHz field testing in the snow at 1.5km

With a temperature around 0 deg C I did a field trip today TXing on 8.76kHz VLF to test two things: (1) to check the rebuilt 4-5W TX based on the TDA2003 could be copied at moderate range and (b) to try out the new tuned preamp with earth electrode receive antennas and see how these compared with a loop. The usual first test site, a National Trust car park about 1.5km from home was chosen. Today it was totally deserted with more sensible people curled up in front of warm fires. Using the 80cm receiving loop or with the earth electrode antenna (electrodes placed about 13m apart) decent
copy was achieved in QRSS3, using Spectrum Lab this time rather than Spectran on the PC.  For the earth electrodes I just used barbecue skewers pushed in about 10cm into the very cold ground. Reception was also possible with the two earth electrodes just 2m apart. The first picture shows the signal received with a loop. The second is with the earth electrode antenna on receive. The ground was covered in a thin layer of wet snow and results when the soil, and soil surface, are dry should be better on earth mode conduction using earth electrodes at each end. The roads in the Fens were too icy to try reception at a more distant point 5-6km away. Also, I need a much longer baseline earth electrode pair in the car in case this is needed at greater range.

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.

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.

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.