New 481THz QRSS3 beacon ready for action

Completed 481THz beacon electronics

This afternoon I finished off the electronics build of the new dual frequency optical beacon TX for 481THz (red light) over-the-horizon (NLOS) tests. In the end I opted for QRSS3 on a choice of 2 sub-carrier frequencies selected by toggle switch together with the option of a continuous sub-carrier transmission on either frequency to help beam alignment. I have not incorporated an FSK facility at present. All that remains is to align my sighting scope with the optics so that I can use this to help with aiming. Currently the beam appears off-centre in the spotting scope cross-hairs.

As I now have the option of a lower sub-carrier frequency, I need to revisit the improved RX to see if I can better optimise sensitivity at the lower frequency. Theoretically the detector should be several dB more sensitive at a lower sub-carrier frequency which should help with NLOS tests where signals are weak.

The beacon is powered by a 19.5V 4.5A ex-Dell laptop SMPSU, although I only need around 300-400mA with the current LED, which is exceedingly bright.  In the picture above you can see the BACK of the LED and that is pretty bright. At least with the PSU the whole beacon is self-contained.

Progress on the new 481THz beacon TX

Today I made a start on the new optical QRSS3 beacon. This time, the circuit has some improvements: a continuous sub-carrier option and a choice of audio subcarrier frequency. Also, the frequencies are now derived from an HF crystal so stability will be excellent.

The part completed new 481THz QRSS3 beacon

The oscillator/divider is a 4060 IC and this is enabled by the output of a K1EL keyer IC programmed to send "XBM" in QRSS3.  The square wave output from the 4060 feeds the gate of an IRF510 FET which switches the 280000mcd, 10mm diameter, LED in 100mm optics.

A possible refinement will be to pull the crystal using the keyer output signal so that I have a continuous carrier but FSK keyed. This would mean I'd always have a signal to aim at, but with FSK QRSS3 CW on it. I'm not sure how much pull I'd get after dividing down if I just changed one of the capacitors loading the crystal. I shall have to experiment and see. Even as little as 5Hz would be enough, but that would need 50kHz shift at crystal frequency! A better way may be to key the frequency out of the 4060, so mark is, say 550Hz and space 1100Hz or vice versa. Plenty to try tomorrow.

Frustrating day at both ends of the spectrum

Today I tried two experiments. Firstly a continuation of my tests with the loop and E-field probe out in the fens on 8.977kHz and then tonight, another over-the-horizon test on 481THz.

Failure 1 at VLF: whereas in the past I've had a decent signal at my test site at Tubney Fen 3.5km from home on the loop antenna on 8.977kHz, today I could copy nothing. There was some strong interference and I thought that may have been the problem, so I moved on to a second test site 5km away where again I usually get a good signal. Again nothing. In the past this second site has given me signals so strong that I could copy 10wpm CW from home on 1kHz by earth mode. Now around the village and in nearby Swaffham Prior the signal levels were (as far as I can recall) similar to past levels, so what has changed? One theory is that the wet winter has saturated the fenland soil so conductivity is much higher, resulting in much higher attenuation at VLF.  Another possibility is some utility has changed - a change of pipe type or a re-routing of an electricity cable? My TX and RX equipment is largely the same as in previous tests.

Optical QRM on the 820Hz sub-carrier frequency

Failure 2 at 481THz: I tried a third attempt at my over-the-horizon QRSS3 reception of my signal on a 820Hz sub-carrier. This has been successful in the past. The 100mm lens TX was carefully aligned just to the right of our local windmill on "the hill" and I traveled to what should have been a direct line path (but over the horizon so the signal has to be scattered) about 3.5km away. Optical conditions looked good with clear visibility of street lights in Burwell. Well, I am pretty certain that I was able to hear my signal by ear in the headphones keying away slowly, but because of a lead failure (later fixed) and then a strong interfering signal very close to the 820Hz sub-carrier, I failed to see my signal with Spectran and make a screen capture. I think this QRM signal is related to new street lighting as panning the horizon brought up this interference on most street lights at around 3km range. The solution will be to choose a different sub-carrier tone frequency. BTW, it is fun to hear the strobe lights from aircraft: these are very strong and can be copied well off the direct path by scattering.

My next immediate priority is to change the optical beacon TX so that I can use a range of different sub-carrier frequencies all derived from an HF crystal divided down with a 4060 divider. This will also allow me to run a continuous signal which will help with alignment and I can avoid QRM by moving the HF crystal frequency if needed. I could also arrange DFCW modulation by FSK keying the HF crystal: this will allow a continuous signal for audible alignment yet be detectable with software packages like Spectran in QRSS3.

As regards the VLF earth-mode tests, for now I am going to draw these to a halt and will try again in a month or so when I hope the fen soil conductivity has reduced. If things are unchanged then I suspect that something in the utilities metalwork out in the fens has (permanently) changed.

Projects update

It is some time since I summarised my projects list and progress. This is the current "ideas list" for the coming months. As we are in the middle of a house renovation/move project over the next 3-6 months it is quite likely that my amateur building work will have to go slow.

• VLF earth-mode - optimising the E-field probe and loop antennas for mobile coverage tests
• 481THz Optical NLOS tests - testing new RX and then trying the 10dB higher powered PhlatLED TX and seeing how far over the horizon I can reach.
• WISPY 10m WSPR beacon - combining the TX and RX boards into a full WSPR/PSK31 transceiver. Separate modules made and tested but yet to combine.
• Tenbox - I still need to finish either the AM transceiver or modify the design to a DSB transceiver.
• 2m horizontal omni antenna - I need a simple horizontally polarised omni antenna for the new QTH. I'm tempted to organise a pair of stacked big-wheels with around 5dBd gain.
• Simple test equipment for the shack at the new QTH - I want to build a number of pieces of simple test gear such as a frequency counter and simple spectrum analyser 
• Rebuild of the 10m Homebase-10 halo - the wooden frame structure is showing signs of age, so a fibre-glass support may be less visible and more durable 

Now, knowing the way things work with me, I 'll wake up one morning and think of something completely different to work on. That is the fun of simple homebrew amateur radio experimentation: no-one tells you what to do, unlike when at work, and each day is one filled with opportunities.

Double or single peak for cycle 24?

At the moment the jury is still out on this. We've seen activity slide since the peak around Nov 2011 but there may be signs that the trend is upwards again leading to a second peak as has happened in a few recent sunspot cycles. This NASA video gives some insight into the thinking.

More analysis with Spectrum Lab software

Spectran is a very easy package to use but it has its limitations when analysing weak audio signals. A more powerful package is DL4YHF's  Spectrum Laboratory which is an extremely powerful piece of software. The problem is that if I've not used it for months it takes me hours to remember how to drive it. A secret is to store ".usr" configuration files so one can go back instantly to a set of settings e.g. a given frequency to analyse with a specified bandpass filter and bandwidth.

Faint line at 8.9775kHz visible at greater range (43mHz bandwidth)

Anyway, to cut a long story, this evening I set up some config files to allow me to look again at the recordings of my 8.9775kHz VLF earth-mode signals made during recent drive around tests locally. Although the original analysis was done at 180mHz bandwidth I can now replay the recordings and look in bandwidths down to 34mHz. What happens then is that signals present but too weak to see now appear above the noise floor as a faint line. The net result is that the signals can be detected in some of the "in between" locations that were not apparent from the 180mHz (wider!) bandwidth analysis.

I intend to use Spectrum Laboratory and narrow bandwidths for the over-the-horizon 481THz optical tests if Spectran proves not up to the job. However, when bandwidths are very narrow there is a time-lag before the trace appears on the screen. This is less than ideal when trying to align weak optical signals with beamwidths measured in a degree or less. What is needed is an accurate beam heading and then allow the trace to build on the screen. I don't have enough experience yet to know how much beamwidth spread a cloud or free space dust scattered optical signal gets. When looking for G4HJW's signal over the horizon in clear skies last year the alignment was quite critical, but his signal was audible in headphones so the optics could be peaked by ear before analysing with Spectran or Spectrum Lab. I had more luck with my own QRSS3 signal over the horizon on a shorter path, managing to align by eye on visible landmarks enough to see the trace on the PC and then peak it.

Mobile 8.977kHz VLF loop tests started

Today I started to do my tests on 8.977kHz using my 5W earth mode transmitter at home but using a mobile loop antenna on the car connected to my PC via a tuned preamp. The idea is to be able to drive around and measure signal levels with Spectran software whilst actually on the move.

30t 80cm loop mounted behind the car

The loop was mounted behind the car in such a way that it would detect any ground propagated signals. The loop is about 10cm off the ground.

A drive test to Swaffham Bulbeck was carried out and signals were detected more or less continuously out to 3.5km from home before they disappeared in the noise. Bandwidth used was 0.18Hz with a continuous carrier. Although coverage was as I anticipated, signal levels were not as great as when the loop was actually laid directly on the ground. At one of my usual test sites 3.5km out in the fens there was no copy with this loop arrangement yet there was a decent signal copied with the loop on the ground a few days earlier. I need to do some direct comparisons between the loop on the ground, the loop mobile mounted 10cm above the ground and with the E-field probe on the car roof. Initial indications are that the difference between the EFP and the mobile loop is probably no more that 5-6dB.

Signal received with Spectran and the mobile loop

Revised UK Frequency Allocation Chart

From the OFCOM email newsletter today:

UK Frequency Allocation Table

Ofcom has published a revised UK Frequency Allocation Table. This details how various frequency bands are used in the UK, and which bodies are responsible for planning and managing them – including frequencies assigned to individual users or installations at particular locations. It also shows the internationally agreed spectrum allocations of the International Telecommunication Union. 

The table shows frequencies below 8.3kHz are unallocated in the UK but there are some footnotes in the ITU frequency allocation table that require administrations to ensure no harmful interference to services above 9kHz and to notify other administrations about research below 9kHz.

My understanding is therefore that below 8.3kHz the UK administration "does not care" what happens as long as interference to allocated services is avoided. This is my interpretation and not a legal statement.

VLF earth-mode mystery deepens

This afternoon I did a larger coverage test with 5W  8.977kHz earth-mode, driving in several directions locally in the car with the roof mag-mounted E-field probe RX antenna and with a PC in the car monitoring the signal. I drove for several kilometres in different directions recording where the signal could, and could not, be copied. The best reception distance with the E-field probe was 3.2km.

The map shows the results. Yellow shows where I drove and red shows where there was signal present and recorded on Spectran. I have recordings of the whole trip which I will more carefully analyse later.

5W 8.977kHz earth-mode coverage using EFP RX antenna

The interesting thing is that the signal could be copied in 4 local villages (Burwell, Reach, Exning and Swaffham Prior) but there was almost zero coverage once outside of these villages. This is NOT the case when looking with a magnetic loop RX antenna, where the signal can be copied more extensively in the rural areas at even greater distances.

It would appear that the E-field signal needs to be strong above ground to be copied with the EFP and this only happens where there are buildings i.e. in the villages. It suggests I'm detecting the signal from cables or pipes in houses.In more rural areas the signal is weaker above ground and not detected, at least not with 5W TX.

Mobile on 8.97kHz VLF

Route taken from A to F (about 4km)

Today I did a fascinating experiment on 8.977kHz VLF using my 5W earth mode transmitter and a mag-mounted E-field probe and laptop running Spectran in the car. Basically I did a "drive around" test to see where the signal could and could not be copied.

Signal strength on 4km run between 2 villages

A continuous carrier was transmitted and I continuously monitored the received signal in the car. The drive was from the middle of the next village (Swaffham Prior), out through to the main road, then back along the main road to Burwell, around part of the village and then back home to the TX location. The signal was visible in Swaffham Prior at 5-10dB S/N, then disappears and returns on approaching Burwell where it is up to 40dB/S/N in 0.18Hz bandwidth. Within Burwell it is almost solid copy. The red timing ticks are every 30 seconds.

What I am detecting (I think) is the local E-field from the VLF signal in the ground, no doubt aided by local utilities. What puzzles me is why there is NO copy in between the 2 villages when there are, I think, pipes and cables in the road.

In the coming days this test is worth repeating locally in other directions and further afield. Fascinating to think a 5W VLF signal injected into the ground can be copied on a 19 inch whip on the car roof like this.