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Alex,
I have built 2 coils so far.. One just testing on the bench, the other I have used outside. I don't think size is so important other than I get about a 20% performance gain with a larger coil. Both coils are right around 6.0mH for TX and 6.5mH for RX. I have come across several methods of determining L without an inductance meter and they all agree.
My smaller coil measures 18 x 10mm for one "D". My larger one is just the standard 252 x 137mm for one "D".
For winding, I have a coil winder with both a top and a bottom... and spacers in the middle, along with pegs for the basic shape going through top and bottom. Now, here is the neat thing. My wire comes off a spool and goes though a container filled with varnish. It goes IN a small pinhole on one side and OUT a pinhole on the other. so.. my wire is "sticky" when it goes on the form. I am using .25MM wire 30 guage I think.
When my coil comes off the form several hours later, it almost has a cross section that is square, and needs no string to bind everything together. Then, I can easily remove a few turns to tweek the inductance a little.
Then, one layer of vinyl tape, then a layer of copper foil tape (so I can solder a ground wire to it). I find that a USB 2 cable works fine.. That's it! I get around 30mm for a 2.5mm brass coin.
Don
I have built 2 coils so far.. One just testing on the bench, the other I have used outside. I don't think size is so important other than I get about a 20% performance gain with a larger coil. Both coils are right around 6.0mH for TX and 6.5mH for RX. I have come across several methods of determining L without an inductance meter and they all agree.
My smaller coil measures 18 x 10mm for one "D". My larger one is just the standard 252 x 137mm for one "D".
For winding, I have a coil winder with both a top and a bottom... and spacers in the middle, along with pegs for the basic shape going through top and bottom. Now, here is the neat thing. My wire comes off a spool and goes though a container filled with varnish. It goes IN a small pinhole on one side and OUT a pinhole on the other. so.. my wire is "sticky" when it goes on the form. I am using .25MM wire 30 guage I think.
When my coil comes off the form several hours later, it almost has a cross section that is square, and needs no string to bind everything together. Then, I can easily remove a few turns to tweek the inductance a little.
Then, one layer of vinyl tape, then a layer of copper foil tape (so I can solder a ground wire to it). I find that a USB 2 cable works fine.. That's it! I get around 30mm for a 2.5mm brass coin.
Don
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