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.

Another approach to 472kHz WSPR

An old work colleague Richard G4KPX has been doing more experiments with indoor loop antennas. Recently he has been using an indoor loop for 472kHz with an ERP of around 1mW. I just checked the WSPR database and see he has achieved some amazingly good results with 49 unique spots with best report from Sweden at over 1300km! Well done Richard.

G4KPX's WSPR results over 500km with just an indoor loop TX antenna

Optical NLOS test - next time pack everything!

This evening I set off to my Landwade location where I hoped to check the non line-of-sight (NLOS) signal level from my 481THz, red LED, QRSS3 beacon TXing from home. This time I'd carefully aligned the TX and adjusted the RX optical alignment in daylight, so everything should have been spot on.

481THz RX with iPod Touch 4g running SpectrumView

Then I realised I'd left a vital lead at home that allowed me to connect the optical RX to the laptop. Next time I need a checklist as it is a 10 mile round trip to the test site.

Instead, I tried to copy the signal using the optical RX fed into my iPod Touch with SpectrumView software, but the bandwidth was too wide to allow me to find the weak signal. I hope to repeat the test in the next week (with the laptop and Spectran set to 0.34Hz bandwidth), but this time with EVERYTHING packed for the test.

I was very annoyed with myself for forgetting this audio lead as it prevented a meaningful test being done. I also need to find a different NLOS test site that is less far to drive to but still at about the same distance (3.6km).

The limitations of my cheap tripod are also apparent: I need a much sturdier one that has a compass attached and much smoother pan and tilt.

8.97kHz earth-mode tests with mag-mounted E-field probe

Today I started some tests at 8.97kHz with my mag-mounted E-field probe (EFP) on the car roof. As usual, the transmitter was my 5W QRSS3 beacon feeding the 20m spaced earth-electrode antenna in the garden.

The logical starting point was to drive to my usual "strong" test site 1.6km from home and compare signal levels on the usual 80cm square RX loop with that on the EFP.  For reasons I have yet to understand the signal was NOT copied, even on the loop! What I did see was what looked like an FSK telemetry signal, possibly from overhead power lines close to 8.97kHz. I've never seen this before. Anyone know what it is?

8.97kHz 5W earth-mode signal at 0.4km on E-field probe

I then drove to my new QTH (due to be occupied in about 4-5 months time) which is 0.4km from the TX and sat in the car in the drive with the iPod Touch 4g running SpectrumView software connected to the EFP. The antenna was a short 19 inch whip. Signals were copied quite well (see photo showing "3" from my callsign). The bandwidth on SpectrumView cannot be narrowed enough to optimally receive QRSS3 and results would be better with the PC set to 0.34Hz bandwidth.

Tomorrow I'll have to find out why I couldn't see my earth-mode signal at the usual test site on either antenna. I'm also going to do some /M reception at 8.97kHz using the E-field probe and laptop with Spectran set to 0.34Hz or 0.17Hz bandwidth. With a continuous signal I'll be able to log the signal level as I drive around the village and nearby.