Shielding Woes

[baum7154]

Running a quick calculation based on your 305uh and 800khz SRF your damping resistor should be 766 ohms. Something does not click.

If I run your specs at 402 ohm critical damping and 305uh your SRF should be about 419khz.

Dan

[baum7154]

How did you determine the the shield added 80pf of capacitance? It is plausible. DO you have an inductance / capacitance meter?

Dan

[Dai]

Yes,now you mention it, i was using/trying damping resistor of around 750ish ohms, that was a while ago just after I built the coil. When I first wound the coil it measured just under 300uH,about 296 i think, after putting on the first binding (insulation tape) it went up a bit as the windings were compressed. The 305uH reading was the last measurment I took before puting on the high density foam spacer ( a strip of camping ground mat,spiral wound), tapping this up with another layer of insulation tape followed by a thin layer of fiberglass tissue and resin to make it rigid enough to prevent flexing. I then applied the first coat of shield 3-4 days later once fully cured. It is more than probable this has compressed the windings further but I have not checkd inductance latley.
The coil to shield capacitance was measured with the Peak Atlas lcr40, as was the inductance measurments. I shall try and get a photo of the measurement tomorow/later.

[Dai]

The damping resistors were chosen partly by viewing the scope and by detecting coins, rings etc to give the greatest detection distance.

Dave

[Dai]

I am expecting my replacment sig gen to arrive next week sometime, so will conduct further up to date tests, my old one was far to unstable and drifting to be of much use, one of the reasons it was very rarely used. I do expec the SRF to be way off though,although your calc at 800KHz did work with the 750-60 ohms I was trying at the time. What I dont understand though, regardles of coil spec's SRF etc is why my hand is now being detected by moving in front of coil and it wasnt before,and when I touch the coil/shield it produces a large signal.

[Dai]

Bed time now, thanks for your help so far Dan, I shall resume tomorow.

Regards
Dave

[golfnut]

I made a surf and had a simple unsheilded coil and i found small roman with it.
When I made a credible larger coil with an ally sheild it all went noisey and unmanageable. ?