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Conundrum

Sun Dec 15 2013, 03:48PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Just found this, check out the guy's homemade AM antennas.

Finally, a use for all those dead toroid and AM antenna bits I couldn't bear to throw away.

I think this guy is onto something, a lot of research into wireless power transfer assumes that a small antenna simply cannot work but in fact if it is properly made the efficiency is actually higher than conventional knowledge predicts.

Also a worthwhile method of boosting the efficiency of an AM detector is the use of LED based varicaps, as 10 blue LEDs in inverse parallel can have as much as a 500pF dC/dV and thus allow a very simple microcontroller based circuit to tune from 30 to 600 kHz.

-A

Ash Small

Sun Dec 15 2013, 05:33PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Have you tried this out yet, Andre?

Proud Mary

Mon Dec 16 2013, 01:17AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I was a member of an online ELF group for a few years until it started inflating with 'national security' and Area 51-type nonsense.

I remain enthusiastic about the 8.9 kHz experimental amateur band, where contacts over several hundred kilometres can very occasionally be made.

That said, the best online resource by far for the low frequency experimenter is the excellent "Radio Waves Below 22 kHz."

There is a whole section on practical designs for VLF/ULF antennae.

There has been a lot of VLF interest in so-called 'negative impedance active antennas' and 'negative impedance converters' over the last ten years, and I will be constructing one as a winter project - when the weather rules out getting drunk in the garden and painting my nails. These are perfect for use with ferrite rods. A search for negative impedance active antenna will produce a good crop of papers and patents dealing with everything from VLF to UHF.

Lastly, one of the most popular forms of frequency selection at VLF and ULF is the gyrator which does away with the need for huge variable capacitors or their varicap analogues. In the gyrator, a capacitance - or really the effect of a capacitance - is multiplied by an amplifier, as we see in this simple but fully tested receiver design:

,d.d2k

Conundrum

Mon Dec 16 2013, 07:38AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Interesting PM, thanks for sharing.
Not heard of a negative impedance before, how does this work?
Is it similar to negative differential resistance?

EDIT:

Took 2 seconds to find these on Google Skynet.

Proud Mary

Mon Dec 16 2013, 11:53AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

For someone who wants to get into VLF and ELF at minimum outlay, by far the easiest and quickest way in, is to use your computer soundcard in conjunction with a free audio spectrum analyser like Spectrum Lab

You can be monitoring ELF today!

The biggest problem with ELF/VLF receivers is man-made noise, hard to get away from at these frequencies. Directional antennas like ferrite rods can help reduce some local noise.

BigBad

Mon Dec 16 2013, 03:23PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Conundrum wrote ...

I think this guy is onto something, a lot of research into wireless power transfer assumes that a small antenna simply cannot work but in fact if it is properly made the efficiency is actually higher than conventional knowledge predicts.

Nah.

I wondered about that as well, but ELF detectors have to power the antenna from an external power source.

It turns out that antennas below quarter wave are increasingly lossy.

It doesn't matter much for a detector; because you can use positive feedback to cancel out the losses, or amplification, but for wireless power it slaughters link performance, you can't do either of those two things.

At least, this is with standard antenna materials like copper and aluminium. All bets are off if you've got superconducting detectors, they may well work at very small sizes relative to the wavelength particularly at ELF.

Proud Mary

Mon Dec 16 2013, 04:09PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Induction coils - huge solenoids usually buried in the ground - are one of the best antenna types at ULF - for getting reliable signals from the Schumann Resonances for example:

Conundrum is right to the extent that interesting work can be done at VLF with short E-field probes, and that one doesn't need a Soviet-style 100 km long wire antenna to discover much that is interesting at the very low frequencies.

Below about 10 kHz, the Earth antenna produces surprisingly good results - two copper rods banged into the ground as far apart as possible - 50 metres is enough but 100 metres is better. (The transmitting version of this is called a ground dipole or horizontal electric dipole.)

Lastly, there is the tree antenna. Two nails banged into the trunk of a big tree a couple of metres apart will produce a small but detectable ELF output. (This really does work in practice, and a few peer-reviewed papers have been devoted to it.)

Scott Fusare

Mon Dec 16 2013, 04:55PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

I have had success in detecting the Schumann resonances with a relatively small E-Field probe. It was not without challenges as I had to pack up and drive all the kit to a nearby location (riverside park) that was "quite" as compared to my home.

Keeping the probe relatively small (2 meter long telescoping whip) was also key as I had nearby medium wave transmitters as well as NAA pounding the front end. The big downside is the need to keep the input impedance in the 10s to 100s of GigaOhm range. Still easier than winding giant coils IMO...

Proud Mary

Mon Dec 16 2013, 06:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Scott Fusare wrote ...

I have had success in detecting the Schumann resonances with a relatively small E-Field probe. It was not without challenges as I had to pack up and drive all the kit to a nearby location (riverside park) that was "quite" as compared to my home.

Keeping the probe relatively small (2 meter long telescoping whip) was also key as I had nearby MW transmitters as well as NAA pounding the front end. The big downside is the need to keep the input impedance in the 10s to 100s of GigaOhm range. Still easier than winding giant coils IMO...

Well done on bringing in the Schumann's, Scott!

I live a few miles from a power station with acres of HV switching gear and giant pylons straddling the countryside, so VLF and below is challenging here. A portable rig you can take out into the boondocks is the only way to go here.

Scott Fusare

Mon Dec 16 2013, 06:53PM

Registered Member #531 Joined: Sat Feb 17 2007, 10:51AM
Location: Burlington, Vermont
Posts: 125

Proud Mary wrote ...

Well done on bringing in the Schumann's, Scott!

Thanks Mary!

Truth be told, I think I was just incredibly lucky to find an acceptable monitoring location on my first try. Even as quite as it was I had to run the data collections early in the morning. Passing cars, bicycles and even insects were detectable. Nothing like playing with a 100 Gohm input impedance system to see how alive the world is from an electrical sense, charge separation going on all around :)

My sympathies for your noisy location. During my natural radio listening days I found I could hear the big HV power lines for many a kilometer. As the Schumann site was not good for whistler reception (something I never understood) I used to drag everything 100 KM away into the nearby national forest area to find electrical peace and quiet.

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