On Tue, Jul 4, 2023 at 11:54 AM, Jim Lux wrote:
PVC pipe has two potential problems as a coil form (based on experience of
folks building tesla coils, so 100-500 kHz):
1) it's hygroscopic so the dielectric loss varies with the water content
I asked Google AI whether PVC is hygroscopic. Here, in part, is what it said
Polyvinyl chloride (PVC) is a hygroscopic powder material. Hygroscopic substances can take and hold moisture from the surroundings.
PVC is non-hygroscopic, meaning it does not absorb moisture internally into the pellet. However, moisture can be collected on the surface of the pellet.
Other non-hygroscopic polymers include polypropylene, polystyrene, and polyethylene.
I thought that was an interesting distinction. Google AI generates very useful summaries, but I don't fully trust it yet, so I searched onward. I found two more references that said PVC is not hygroscopic.
I'm sure your remark about Tesla coils is based on practical experience. Any idea why it might differ from what these references say?
By the way, I've collected dielectric data for years for use in my coil inductance and Q calculator. The PVC dissipation factor it uses is 0.016 at 1 MHz and 0.0055 at 3 GHz. (This is why testing PVC in a microwave oven does not indicate its HF properties.). My coil inductance and Q calculator logarithmically interpolates these values. PVC is not too bad at HF, but much better dielectrics are readily available. Try any round plastic container from your refrigerator or kitchen cabinet. It is likely to be one of the non-hygroscopic polymers Google mentions. Read the recycling code on the bottom to determine just what it's made of.
Brian
