Receiver idea

Hi Harry
I am going to do a few experiments in VLF too soon. Right now I have been stuck trying to repair an old 70's organ. I finally figured out all the diode key switching and started making progress. I have all the instruments playing now except one and I think I am on top of the problem there too now. Not knowing much about music and never working on one if these it made it a pretty hard road but I am almost there now. This was one of the last analogue organs made so all instruments were made with filters. Like I always say you can't repair anything if you don't understand how it works. LOL
By the way Ivan,
It looks like that idea I had worked. You said that you thought open ferrite core would be better than toroids so I went with them. It runs at 500Hkhz and subtracting that I get a fairly steady variation from 0 to 10K. I divided the 230mh coil by 3 (230,153,75) and connected all 3 in series to the oscillator and placed them in a triangle under the magnet.

About a year ago I became interested in VLF. One of the experments I did was to take a 20m Dia. coil (10=pair telephone cable) loop and connect it to the MIC input of my compuiter. Using Adobe Audition I could shift frequencies down to the (my) audible range.

Arlanda Airport kicks out a load of garbage but at night and during thunder storms there are loads of interesting sounds to hear. I have heard the classic whistlers and even the "dawn chorus". The last was by placing the coil 10km in the woods with an FM link.

BR Harry - SM0VPO (Nerja, Spain)

Hi Jalex,
there should be at lest some natural signals in this range. Also some man-made signals (QRM) may be present. If you got interested in this range, a simplish E-probe and processing software seem to be quite easy start.

BR from Ivan

Hi Ivan
I was quite interested in the link you gave me. He made that look simple. I have never tried to receive anything in that frequency range as I don't know if there is any signal around my area that would be strong enough to be picked up easily.

Hi Harry
Yes I have but haven't done much experimenting this way. The one time I tried I found that the ambient light messed it up too much. Most of the links is have built just go from my garage to my house. About (30 meters). I have sent lots of different types of signals with pretty good success in this way.
Lately I have been interested in LF and have been experimenting in that range.

Have you ever thought about using infra-red?

You can modulate an IR diode with RF and re-create your own RF spectrum in a singe room. This will allow you to "transmit" using just about anything under 50MHz and all in the room can receive you, and each other :-)

BR Harry

Hi all,
an interesting article, including 8,97 KHz TX/RX experiments:
G3XBM
He received e.g. OE5ODL, 1046 km far !

BR from Ivan

Hi Thanks Ivan and Harry
I will play around with that more if I get some more time.

Hi all,
the AF signal is quite strong inside the loop, but decays outside of it quite quickly. Wireless headphone sets for language teaching used to be commercially manufactured in OK. The loop was placed around the classroom, it had usually 3 turns and connected to the external loudpeaker connector of a tape recorder. Every student had a headset with a small ferrite antenna and AF amplifier. As frequencies below 10 KHz were employed, no license was required.
BTW, transmitting / receiving experiments using slow CW and carrier about 8 KHz may be both exciting and frustrating. Again, no license is required in most countries.

BR from Ivan

One interesting additional comment is that for 100m you may NOT need any capacitance at all.

Back in 1968 I was in the RAF and the bath was about 50m from my barrack room. I connected my stereo amplifier (record player) to a loop of wire about 2m diameter made of 10-pair (plus earth) telephone cable to get a 21-turn loop.

The receiver was about 1000 turns on a ferrite rod (loopstick) BUT i extended the rod by adding two halves of a TV line-output transformer. My new ferrite was about 3cm square by about 30cm long with an 8mm Dia. bit in the middle for the coil. The ferrite coil I connected to an ordinary battery-powered AF amplifier.

This setup allowed me to listen to my music in the bathroom and the signal was so trong that it overcame all noticeable traces of 50Hz hum.

The down-side was that the corporal in the room beside mine was only able to hear my music on his hearing aid. In those days they had an external battery and a magnetic microphone.

So for lower frequencies you may not need any tuned windings for such a short distance.

So that was my little twopenneth. Hope it helps you.

Very best regards from Harry - SM0VPO (Al Rabita, Spain)

HI Ivan
Thanks for the info. Yes I have tried capacitive and resistance control. I also used an open core ferrite form but didn't get as much change as I wanted. I also tried hall sensors. They should have worked but I guess I didn't get them set up right. They worked but I didn't get a steady increase as the magnet moved over them. This could be the problem with what I am doing with the coils too but I haven't got it together yet.

What I have is a moving magnet isolated from the circuit by a 3 mm thick piece of abs plastic so the coil seemed like the best idea. I am only trying to get about a 500 hz shift

Hi Jalex,
you probably put the ferrite to saturation, where its differential permeability drops significantly. It is rather cruel use of toroids. Nevertheless, if you stay in the small signal region while making oscillations and provided that you get reproducible results moving the magnet to and fro, do not hesitate to use it.
AFAIK industrial sensors use open cores from permalloy and similar compositions. (Toroids are closed ones.)
Another possibility may be capacitive sensors, variable capacitors bound to the mechanical system. And even resistive sensors may be used at such low frequencies, using RC oscillator. A precise oscillator usable from several KHz to at least 25 MHz in one IC, controlled by one resistor, exists. I do not remember its type, but I can search it. It yields squre wave, so strong filtering would be required.

BR from Ivan

Hi Again Ivan
You got me thinking as usual. You said toroids don't vary too much around magnets. I was just wondering if you have a better idea on what to use for something like this. I just got this idea as one of them was laying in front of me on my bench and I was playing with it.

Hi Ivan
So far I have only ran tests and I am building the circuit now. I got a 20% drop in inductance with the magnet at 1/4 " away. They have a huge drop when the magnet is a few millimeters away. These are the toroids from fluorescent energy lamps. I don't think they are the quality of others that we see in consumer electronics. I plan on putting 3 in a delta configuration.

Hi Jalex,
"I want it to work below the AM band about 450Khz"
May be, but expect strong QRM from IF of AM receivers. And you may cause QRM to them. IMHO you should better go to say 400 kHz.

"Harry says there that I would need 550 turns on a loopstick. Can I get by with less if I use a bigger capacitor and what would that do to it's ability to pick up signal?"
You can use any resonant L/C combination, but expect sensitivity loss with less turns.

"The signal is from a basic oscillator where the frequency is controlled by magnets and toroid coils."
Toroid coils are almost insensitive to external magnetic fields. How do you mean to do the frequency control ?

"I have a FM modulated signal that I thought about demodulating and running it into a counter."
You have a signal comparable to SSB of a single AF sinewave. You need BFO to demodulate it. Alternatively, a sort of frequency discriminator will yield you DC proportional to the frequency shift.

"I don't want to sub-carrier it onto a higher frequency as I am trying to keep it simple. I would only need less than 100 foot of range."
OK

BR from Ivan

MW REFLEX RECEIVER
I want it to work below the AM band about 450Khz Harry says there that I would need 550 turns on a loopstick. Can I get by with less if I use a bigger capacitor and what would that do to it's ability to pick up signal?
The signal is from a basic oscillator where the frequency is controlled by magnets and toroid coils.
I have a FM modulated signal that I thought about demodulating and running it into a counter.
I don't want to sub-carrier it onto a higher frequency as I am trying to keep it simple.
I would only need less than 100 foot of range.




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