VLF Transmission Using Soundcard

sru2

I intend to deal with this in the digital domain. I could look at the phase differences, test for modulation and eliminate the intermodulation that way.

Do you see any issues with this?

sophiecentaur

You are free to try, of course, but do you really have a tried and tested strategy? Just saying that you will use the "digital domain" could mean absolutely anything. Is there a suitable algorithm that you can use which will do all you require? I'm not sure.
Have you done anything towards working out the SNR you will need for this processing and the data rate you can achieve? That's the bottom line.

sru2

Do you have experience with DSP? I am writing software that should be able to handle this.

I started on this tonight. I hooked up a 500G roll of magnet wire to my sound card and took a look at the background noise of the system.

This first diagram shows the complete background noise for 0-20KHz.



To test the magnet wire, I brought an electric razor close to the wire (about 5cm) and slowly withdrew it (to about 30cm). The signal drop off was rapid as the distance increased.



I started to look for additional sources of noise and I found that I could create noise by refreshing Firefox.



Moving a laser mouse around also produces detectable noise.



Playing sounds through a Java application did not produce any noise, but playing an MP3 in Windows Media Player did. It would tend to suggest that the sound card was not the source of the interference, but I have yet to isolate the source. What is interesting is that this signal has a electronic signature revealing what song is being played and specifically what part of the song is being listened to. This should be of value to anyone interested in TEMPEST.



Finally the same song played through Youtube also reveals a unique signature that betrays information from your PC.



All of this is done unamplified in any way. The keyboard also produces unique signals, but it was too faint to be of much use in a small picture on this forum.

So noise at the receiver does not appear to be much of an issue. The next real question is how much energy does the receiver detect from a nearby transmitting source.

It looks like the MDS will be around -80dBm above 2Khz and that there is a 5dBm difference in noise between day and night. Given this, it may be best to amplify the transmitter, rather than the receiver.

sophiecentaur

As you don't know the level of signal that you expect to receive, how can you say this? It's the ratio of signal to noise power that determines the performance. You do not know what level of signal to expect yet.

This is the basis of all communications engineering. Amplifying a signal that already has noise or interference added is very little use. You need to use as much TX power as you can afford.
I really do urge you to establish just how much signal you can get across the distance you need. Remember, your interfering sources may be right next to the receiver but your wanted source may be 100X as far away.

sru2

I moved the magnet wire roll out of range of the magnetic fields of my equipment. I am now receiving VLF and ELF stations, from the US, Germany, Britain and Russia. This includes extreme narrow-band Tacamo broadcasts.



If I am receiving this, some of which is 3000+ kilometers away, I should be able to hear a broadcast within 20m very clearly.

sophiecentaur

Do you know what Tx powers these foreign stations are using or what their field strength is at your receiver? Their field strength drops off at a much lower rate than your near field source will.
All I can say is that, without measuring signal level from the transmitting equipment, at the distance of interest, it could be wasting a lot of effort on other things. You can't lose by trying it out. If it's really as trivial a problem as you think then you may as well just do it early on. I cannot think of an example of radio communications design in which the actual signal levels were not considered very early on in the process. Perhaps you already know the carrier to noise ratio you need for a given error rate, using your proposed system - in which case, your interference measurements will tell you just what level you will need.
Your application may be a very specific one but how can you guarantee that there will be no local sources of interference when you use your system in an arbitrary location?

sru2

sophiecentaur, my signal generators arrive today, so I should be able to get some tests done sometime tomorrow.

I have one question about safety though and I was hoping you could clarify it for me.

I have been thinking about this near-field transmission. Am I right in saying that if I generate 3Khz sine and push it into an antenna two components will be generated? Firstly, an EM field localized to the antenna (near-field) and a free standing electromagnetic wave (radiated).

Let's leave how much is radiated to one side for the moment.

This time-varying magnetic field (near-field) will extend about 100Km at 3Khz. At close proximity the line of force are densely concentrated so induction is optimal. At 90Km the lines of force have diverged enough to make induction very weak.

Assuming a near ideal antenna exists within 100Km range (for 3Khz) and that it is directly connected to ground, at what distance from the transmitter does it become safe to begin broadcasting without this antenna causing a short? Could this antenna draw to much of a load? Would I need to make it high impedance or introduce a quick blow fuse?

Is there a formula for the divergence and the load it could draw?

sophiecentaur

The sig generators should give you some fun! Good luck with them; they should give you a really good feel for what's going on.

I'm not sure what your concern is about the effect of a distant structure. How could something so far away possibly couple with your transmit antenna enough to worry you? You will have a hard enough job getting usable signals across your small gap with low power sources so the effect of your hypothetical 'parasitic' cannot be a problem. Such structures can be a nuisance to receivers in their vicinity because they can modify the fields locally to them and cause a shadow / hole in the reception near them. Steel framed buildings / conurbations can play hell with the mf ground wave reception on rare occasions but that's another matter.

When working out the fields due to a radiator, you get several terms. One (/some) of them starts of with high values but are subject to high drop-off (1/r² and 1/r³) and there is on with a 1/r term which is the one that survives at great distance. The terms add together at all distances so there is a smooth transition between the regions. Where 1/r dominates is the one referred to as far field. Look at the Wiki pages about near and far field.
is just stating the above in an arm waving 'geometrical' way.

sru2

I was thinking the same thing. I thought it would be best to double check first.

sru2

If I have a transmitter with 5W, which produces an ERP of 50W, where does the additional power come from?

Is the gain supplied by a separate power source?

vk6kro

An transmitter plus an antenna would have an ERP of 50 watts with 5 watts into the antenna, not just a transmitter.

What it means is that in the same direction, you would need 50 watts to a dipole (used in its best direction) to give the same signal strength as the 5 watts produces with this antenna. So, the antenna has a gain of 10 dB.

There is still only 5 watts total, so you couldn't fry an egg with it or anything like that.

You could try reading this article:
http://www.w8ji.com/radiation_and_fields.htm
especially the big formula near the bottom quoted from Terman's book.

Look at the effect of frequency on radiation. Notice that frequency and wavelength are both used in the equation.

sophiecentaur

ERP (Equivalent Radiated Power) is a term used for transmitting installations and includes the power of the transmitter and the directivity of the antenna.
Antennae are never omnidirectional. They all have nulls in some direction and 'maxs' in others. A simple dipole will have a maximum radiated power around the 'equator' and zero at the poles.
Your 5W transmitting system will be putting most of its 5W in one general direction so that, on the main beam, you have the equivalent of a 50W transmitter (Equivalent Radiated Power). It is said to have 10dBi gain - that is to say 10 dB of gain relative to an isotropic antenna.

You won't be getting this sort of gain from a VLF antenna of any practical size.

[Edit: Rats, you got there before me. I seem to remember ERP being relative to an isotropic radiator and not a dipole? It's been a long time though. There's about 2dB of difference.]

vk6kro

I think it is EiRP for isotropic.

I checked with Wikipedia and they said that it was always for a dipole in the USA for FM transmitters. I had to look it up, though, because it isn't something you use every day.

Easier to explain than an isotropic antenna. I'm still looking for one of those.

sru2

Thanks for that guys, its been very helpful.

I have been focused on extending my visible bandwidth, lowering noise and removing mains hum for the last few days.

I figured out that the drivers with a sound card restrict the bandwidth to the audible range, up to 27Khz, before applying a high pass filter. By moving to ASIO to talk to the card directly on the line-in, rather than the microphone, I was able to get a visible bandwidth of 48Khz with a sampling rate of 96Ksps.

Applying a tracking filter I was able to strip most of the harmonics from the mains signals. That said, in doing so, I discovered secondary transmissions layered under the harmonics. Beginning at 150Hz, they are spaced around 300Hz apart. Given that I am running half a million samples through the FFT with a 96Ksps input, I am getting one pixel added to the spectrogram every six seconds. So, its difficult to tell if they are modulated, but distinct gaps are appearing so I assume so. I really need a good ADC and bandpass filters.

I am still running unamplified, but I am starting to see very weak signals emerge around 20Hz, 80Hz, etc. It looks like submarine comms. I'm also getting a lot of lightening strikes being detected. My noise floor is around -130dB at the minute, but that seems to be the card itself, as far as I can see I am dragging no noise in from the receiver whatsoever. This seems like a good approach, as I increase sensitivity I am able to eliminate noise as I go.

With respect to noise, have a look at the following diagram. This is around 26Khz, but I am seeing similar signals from around 1Khz. As I progress up the bands, I find random squiggles, but they appear to be confined to areas where the hum filter has reduced the dB. I think the automatic gain control is revealing them. As we approach the mid-20Khz, the signals start looking like the picture, rather than being confined to the edges of the purple bands that you can see (that's the hum filter).



Any idea what these signals are?

As a final question, I was thinking about noise reduction at the receiver. I know that one way to reduce thermal noise is to cool the antenna, cables and sampler. I was thinking of another way. If I suspend the antenna in a magnetic field, then it should apply a force to the electrons reducing their thermal movement. This should, in theory, reduce the noise floor and the intensity of that field will dictate by how much. The idea is that incoming signals of weaker intensities will then be detectable.

Has anyone tried this? What was your experience?

the_emi_guy

Force on moving charge within magnetic field is perpendicular to both the field and the direction of motion, F = q(v x B). It will not slow it down or create drag, just change its direction (look up cyclotron frequency).
Using a preamp with low noise figure will insure that the electronic noise will be primarily thermal noise (which you really can't get rid of without serious expense).

sru2

Ok, I found this statement:

To me, that implies if I have a single bar magnet, the particles will flow along the lines of force in the direction opposite to their charge. Correct?

Now what happens if I have a second magnet, reversed, parallel to the first magnet?

That is, they are being accelerated in one direction by the first magnet, then accelerated in the opposite by the second.

If the forces are equal, then I should be able to "pinch" the particles and hold them still. That "pinch" will be proportional to the field strength.

Now as I understand it, this should drag down the signal strength right across the bandwidth, but will it also lower the thermal noise floor and, in doing so, increase sensitivity to weaker signals?

sophiecentaur

Ah yes, of course - EiRP!!
Thanks for stirring my memory.