Air-core coil design

So I wanted to make a coil earlier this evening.

The initial requirements were:

Voltage across coil of one to several microvolts.

Load resistance/impedance unknown, but probably around 50 ohms or so.

Indoor, room-temperature use. RH range maybe 20% to 95%.

Used with fixed equpiment; not required to be portable.

Light enough to be easily hung on a wall.

Able to be manufactured in one hour with materials on hand.

Minimum useful lifetime of approximately 4 hours; more is welcome but not worth spending money on.

Frequency range approximately 0.5 to 1.7 MHz; the frequency of most immediate interest is about 0.6 MHz, but the others may be interesting as well.

The previous design included a simple transmission line, which did the job, but was also suspectible to external interference. Previously, I had been able to shut down the interference source when I needed to use the transmission line. I had also relocated the transmission line away from the inteference source, which helped a little, but still led to unwanted signal pickup. Tonight I wanted to run the interference source at the same time as I used the device that the transmission line fed, with minimum interference.

I Googled around and came up with an existing design that was optimized for portability and moisture resistance. Since I did not require much of either quantity, I discarded most of the mechanical aspects of that design, but kept the electrical ones. I ended up with four turns of

22 AWG stranded wire, in a square loop, 12" on a side. The lead wires came out in the center of one edge. (If you swing the other way, that's about 0.20 mm^2 wire, and a square 305 mm on a side).

For the wire, I selected some 4-conductor 22 AWG stranded cable, used for wiring sensors to a building alarm system. I have several chunks of this cable, left over from an alarm install at a previous workplace in about 2004; the chunk I used had "server room motion" written on it.

For the coil former, I re-used a piece of cardboard packaging from Yum! Brands, Inc. It fit the 12" square dimensions of the coil, and also had flaps on the side that served as a mounting bracket.

To hold the coil in place, I used some Scotch[R] Magic[tm] Invisible Tape, because it was on hand. Arguably, this is the only item in the BOM I spent money on.

I connected it to the circuit using the same transmission line I was previously using. It is parallel line; twisted pair would probably be a little better.

I operated the circuit and held the coil up in the air, turning it until it produced the maximum amplitude with minimum interference. (The desired signal output was in the audio range, so I could listen for this as I moved the coil.) When I found a good spot, I attached the coil former to the wall with more Scotch tape, and used tape to hold it at the optimal angle.

Sustainability report: Most components, except for the tape, are salvaged or re-used from other products. This helps compensate for the short design lifetime.

The coil itself is likely Pb-free, as the wires are not tinned, and I used mechanical connections instead of solder. I *think* the transmission line is Pb-free as well, but one of the conductors might be tinned a little. The wire used is old enough that it is probably not halogen-free or low-halogen. I dunno about conflict minerals.

The copper wire and coil former can both be recycled using locally available channels. The wire insulation is not separately recyclable; it probably goes into the copper stream and gets discarded later. The tape is also not marked for recycling; it either becomes landfill, or goes into the coil former stream and gets discarded later.

As built and installed, it looks like this:

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This was succesful in allowing me to listen to the Royals game on

0.61 Mhz, while watching it on the local Fox TV station (590-596 MHz), and avoiding the interference from the microprocessor and display in the TV set that plagued the previous transmission-line solution.

The reason I did not use the TV audio were 1) it lags reality by about

15 seconds and 2) I don't like the TV announcers.

The only thing I'm somewhat concerned with is the life spec... the tape that holds it to the wall seems to want to slowly peel away over the course of a few hours. The coil itself will be fine, but I may have to revisit the mounting.

Matt Roberds

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  • Wire does not create any voltage. One would need a difference of metals for a thermocouple, and certainly that does not need a coil. You said nothing concerning using this coil to pick up AC signals, nothing about frequencies involved, nothing about orientation WRT signal sources(s), nothing about distance between the coil and the source(s). Therefore that question has no valid answer.
  • Wire has no RH sensitivity; use it in the desert or under water. Most decent insulation also does not care.
  • "Lifetime"? AFAIK electronic equipment of any type does not vaporize below 200C.
  • Good for starters in answering multitude of questions cited above.

  • Why the heck use a transmission line, unless that is the ONLY wire you had?
  • Am guessing you use 4-conductor to get those 4 turns; why not a single wore looped 4 times? Cheaper.
  • "Interference". Every person's junk is some-one else's gold so to speak. Line frequency hum could be desired signal, or be "interference". The higher frequency "bzzz" could be desired signal and the humbe the interference. Or...Schumann waves could be the desired signal and all else be the interference. NO CLUES.
  • OH, _design_ "lifetime". Subjective POV.

  • Idiot! These RoHS and REACH "rules" are for the big boys that make and SELL stuff in volume.
  • And you can also ignore WEE.
  • NOW, finally more specific info is given.
  • That is not a "specification". Use different tape and maybe find that removal will also peel the paint off the wall.
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Robert Baer

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