Pex is cheap and comes pre-coiled,

3/8" pex is 0.360" ID.

1/2" is 0.485" ID

Fill with "sea water" using distilled water and sea salt to proper concentration.

Use for AM and/or Shortwave coil.

Sea water is HIGHLY conductive and interacts with radio waves.

I need some suggestions for the size of the coil and how many turns, I intend to test it.

I'm within 2 miles of a couple radio stations, @ ~1100 and ~1400 so sensitivity issues will be minimal.

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    $\begingroup$ HI! Welcome. This is a question and answer site, but you seem to not have asked a question, but given some sort of an answer. Did you maybe mean to post this as an answer to an existing question? $\endgroup$ – Marcus Müller Dec 18 '18 at 11:38
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    $\begingroup$ Original post clarified. I Want to build one. $\endgroup$ – Aerothorn Dec 18 '18 at 22:07
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    $\begingroup$ The conductivity of even thin copper wire is far higher than seawater in your plastic tubing. $\endgroup$ – Mike Waters Dec 19 '18 at 2:39
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    $\begingroup$ In order for anyone to answer your question about the dimensions, we first need to see a schematic with the values of your components, especially the tuning capacitor(s). $\endgroup$ – Mike Waters Dec 19 '18 at 16:40
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    $\begingroup$ I'm still learning the web site, but schematic is at 10:37 youtube.com/watch?v=0-PParSmwtE . I'm using a 47k resistor in parallel with earphones instead of the 0.001 pF capacitor, 0-1000 homemade variable capacitor, germanium diode. I'm willing to amp it if the signal is not strong enough. It's intended as a physics demonstration. Selectivity is not an issue, I just want to get the local station(s), which are very close. If both station come in, that fine also. $\endgroup$ – Aerothorn Dec 20 '18 at 18:04

Sea water is not that conductive: about 5 S/m. By comparison, the conductivity of copper is about 60000000 S/m, so seven orders of magnitude more conductive.

The low conductivity of sea water will effectively appear as a resistance in series with the coil. This will increase losses in the coil, reducing sensitivity.

But worse, it will decrease the Q factor of the LC filter, reducing selectivity. The resulting "radio" would be incapable of tuning to an individual station. Perhaps this isn't too much of a concern, since given the sensitivity issues you will have to be very close to a transmitter to hear anything, at which point that single transmitter is probably the strongest source of RF around.

I guess technically we could call this a "radio", though by this definition you could attach just about anything conductive to a diode and call it a radio. For any practical application, copper (or really, most metals) is a better choice: it's smaller, cheaper, electrically superior, non-corrosive, and solid at room temperature.

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    $\begingroup$ I think he just wants to try it for fun, practical or not, and wants to know the dimensions of the seawater coil. Also, one thing missing from that link to determine the actual resistance is the diameter of the meter. $\endgroup$ – Mike Waters Dec 19 '18 at 15:07
  • $\begingroup$ The dimensions of the coil don't really matter, since the resistance is so high the resulting filter will have practically zero selectivity. $\endgroup$ – Phil Frost - W8II Dec 19 '18 at 16:24
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    $\begingroup$ fondriest.com/environmental-measurements/parameters/… show conductivity up to 80,000. this one shows max conductivity at ~0.5M for KCl and NaCl htracyhall.org/ocr/HTH-Archives/Cabinet%207/… (if I'm reading it correctly) $\endgroup$ – Aerothorn Dec 20 '18 at 18:26
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    $\begingroup$ And this myweb.wit.edu/sandinic/Research/… shows a great chart, up to 240,000 with NaCl, and 580,000 with HCl, 410,000 with NaOH $\endgroup$ – Aerothorn Dec 20 '18 at 18:34
  • $\begingroup$ @Aerothorn Units matter. 1 μS/cm = 0.0001 S/m. And your question states seawater, and I assume normal atmospheric pressure and temperature, while the papers you reference are showing conductivity for all kinds of other solutions. There are solutions with higher conductivity than seawater, especially at extreme temperatures and pressures, but copper is still much better. $\endgroup$ – Phil Frost - W8II Dec 20 '18 at 19:04

Can I create a functional crystal radio using a plastic coil filled with seawater as the inductor?

Yes, of course. Matter of fact, you can make a crystal radio without any inductor, too. Connect the antenna, diode, Hi-Z earphones, and ground wire, all in series.

The coil/capacitor is to tune the radio, filtering out all other stations except for the one station that is tuned in. I would recommend a capacitor in parallel with the coil, to properly tune. Crystal radios are very forgiving to build. Especially with a big outdoor antenna, and a 50,000 watt blowtorch station a few miles away. You might even be able to hook up a small speaker. Go for it.


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