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High voltage amateur radio - VLF

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Paul Nicholson

Sat Jul 15 2017, 05:42PM

Registered Member #47220 Joined: Fri May 30 2014, 12:20AM
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Posts: 4

There are no amateur radio allocations below 9kHz but in some countries you are allowed to transmit. In the UK you need a 'Notice of Variation' to your licence. In most European countries, and I think also in the US, you can just go ahead and transmit. It depends on how each national authority interprets the rules for the 'unallocated' frequencies below 9kHz.

A few hundred watts from an audio amp, a step-up impedance match to the base of a large loading coil (much like a CW Tesla coil), which puts a few tens of kV onto a long wire antenna. Some 10uW to 100uW is radiated.

What use is 10uW ERP? Actually a little goes a long way at VLF because the path attenuation is very low. 10uW will get you 1000 km and 100uW will cross the Atlantic, west-to-east.

You need a GPS to generate the audio signal to transmit - the frequency must be very precise and stable because ultra narrow bandwidths are used.

Tempted? If you're a radio ham and a HV/Tesla enthusiast, you're half way there already. Re-purpose an old Tesla coil?

Listed here are some of the distance records achieved so far

Uspring

Sat Jul 15 2017, 07:40PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

WOW!!! 0.05fT is _very_ close to nothing. Congratulations.

Some questions from a amateur radio noob:
1) What do you do about noise, e.g. line frequency etc? You must do some serious filtering in order not to choke your amp.
2) What kind of antennas do you use on the transmitting side? It looks much like a matter of efficiency producing 100uW output from hundreds of watts input. Are simple long lines or coils more effective?
3) Same question on the receiving side. Also, is it an issue to have much sensitivity there, since noise coming from the antenna will also be larger?

Paul Nicholson

Sun Jul 16 2017, 07:14AM

Registered Member #47220 Joined: Fri May 30 2014, 12:20AM
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Posts: 4

What do you do about noise, e.g. line frequency etc?

DSP filtering using a PC. The 'receiver' itself is just a simple audio preamp, but one designed for optimum noise performance with its antenna, which is placed as far as possible from houses and power lines and carefully isolated. The broad band preamp output is digitised by a PC soundcard and software does the rest.

What kind of antennas do you use on the transmitting side?

The wavelength is so long that whatever antenna you use approximates a vertical Hertzian dipole. Thus it's simply a matter of getting as much wire as you can, as high up as you can, and then putting as many volts onto it as you can muster. This is done using a loading coil to resonate with the antenna capacitance at the operating frequency.

Your ERP is proportional to the square of your antenna current, and the square of your effective height.

Same question on the receiving side.

Receiving antennas are quite different, usually small E-field probes or magnetic loops. You might have 100mV of 50Hz or 60Hz on your E-field probe, with the wanted signal at 20nV. DSP removes the mains and its harmonics and some sidebands. That leaves you able to see the natural radio background noise at about 10uV/metre or 30fT in 1Hz bandwidth. Most of that noise is sferics (distant lightning) which software also removes, leaving you with about 1uV/metre or 3fT of noise remaining in 1Hz. The rx noise floor itself is lower than 50nV/m or 0.15fT in 1Hz. Further gain is then obtained by using very narrow bandwidth transmissions, eg 100uHz bandwidth leaves you with a natural noise floor of about 10nV/m or 0.03 fT.

Messages are sent using coherent BPSK and strong forward error correction.Eg EbNaut

Uspring

Sun Jul 16 2017, 04:00PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Couldn't you also use a coil antenna (magnetic dipole)? Possibly corona is the major source of loss for a high voltage antenna. The energy loss in a coil can probably be kept small by investing in copper wire.

Paul Nicholson

Tue Jul 18 2017, 05:45AM

Registered Member #47220 Joined: Fri May 30 2014, 12:20AM
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Couldn't you also use a coil antenna (magnetic dipole)?

In theory yes, and the radiated power is proportional to the square of the loop current, loop area squared, and number of turns squared. For some reason though, nobody uses magnetic loops for transmission at VLF, amateur and military transmitters all seems to excite the E-field. ELF transmitters that use a ground loop may be the exception here.

Perhaps it boils down to cost of copper - the copper in the loading coil versus the copper in the magnetic antenna.

Uspring

Tue Jul 18 2017, 10:44AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

An electric dipole radiating 100uW at 10kHz requires a moment of 2.4e-4 Cm, e.g. a 10m high antenna of 100pF at 240kV. An equivalent magnetic dipole has 70000 Am^2, e.g. a 1000m^2 loop carrying 70A. The former you have done, the latter seems doable. Does polarisation matter?

Dr. Slack

Wed Jul 19 2017, 11:43AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Some observations/questions, comments invited.

Surely what's good for the transmitter is also good for the receiver? Or is it the case that environmental noise exceeds practical device noise, so antenna/LNA matching is moot?

A coil laying on the ground would radiate omni vertical magnetic, so therefore horizontal electric. Any coupling to vertical electric receivers would be poor to non-existent. Is there any difference to propagation with vertical or horizontal electric, I'm thinking ground resistance.

A vertical coil would radiate a polar pattern of horizontal magnetic, so vertical electric and OK for receiver polarisation, but not omni.

Sulaiman

Wed Jul 19 2017, 02:04PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

A quick calculation;

The largest vertical rectangular loop that could fit my property is 20m long x 15m tall (bye-law)
using 0.84" od x 0.54" id copper tubing
giving about 92 uH inductance with 6 mOhm dc resistance
reactance at 9 kHz = j5.2 Ohm ... Qu = 867

Let the resonant current in the loop be 867 Arms (about 65 C temp rise)
the voltage will be 4.5 kV rms, VAR in the loop = 3.9 MVAR !!!
(that's quite a large 3.4 uF MMC)
maximum input power = 4.5 kWrms. (into 4.5 kOhm, or a tap)
Estimated cost: More than my total amateur radio spending to date.

Assuming I use 240 V 13 A ac source, maybe 2.25 kW rms PA output

Unfortunately I do not know how to calculate far-field-strength

Would a rectangular loop 20m long x 15m high with 433 Arms give a better transmit signal ?

Loaded Q would be almost the same as the unloaded Q due to the very low bandwidth and radiated power,
(some power will be lost to heating the ground, nearby trees etc.)
such a high Q antenna could be good as a receive antenna but I suspect that a smaller but electrostatically shielded, balanced magnetic antenna, would perform better.
Especially if alignment with the transmit antenna is adjusted for minimum coupling.

P.S. I can't imagine the vertical that would not create lots of corona with 240 kVrms at 9 kHz

Uspring

Thu Jul 20 2017, 11:38AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Or is it the case that environmental noise exceeds practical device noise, so antenna/LNA matching is moot?

It looks like this. The OP mentions a 1uV/m noise after removing spherics in a band of 1Hz and a rx noise floor of 50nV/m.

Would a rectangular loop 20m long x 15m high with 433 Arms give a better transmit signal ?

The dipole moment is Ipeak*area, so about 180000 Am^2. Transmitted power is then about 700uW. That is neglecting effects from the ground around it. Equations for radiated dipole power can be found here:

Sulaiman

Thu Jul 20 2017, 12:27PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Uspring wrote ...

Would a rectangular loop 20m long x 15m high with 433 Arms give a better transmit signal ?

Equations for radiated dipole power can be found here:

You just resurected my dislike of electromagnetic field theory !

I hated this topic, div, grad, curl ... brain damage caused by thinking in four dimensions + time in the early '70's ... I'm not doing that again.
(the few remaining grey cells were pharmaceutically degraded in the late '70's)

This is why we (I) need younger members - with quicker brains :)

Thanks for the answer

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