Coil Inductance Coil to Control Box (Surf PI)

[multieagle]

There is a great deal of information covering construction of a coil for the Silverdog Surf 1.2. It seems an average around 300uH works best. My question resolves around the increased induction caused by the coax connection to the control housing. I recently constructed an 11.25" coil, with 24AWG, Teflon coating, silvered 7-strand conductor. With copper shield it measured 305uH, with 1.5Ω. I had on hand a spare vintage guitar amp cable, rubber, 95% copper shield, rubber & poly wrapped 21-strand copper conduction which I used for connecting the coil to the system. When I measured induction at cable end, it came in at 368uH, and remained 1.5Ω.

I have been searching to find if there is an average acceptable increase between coil and control box, but thus far have been unable to find reference. So, can anyone suggest an acceptable average induction increase at coax end? Or does it even matter?

TIA

[Silver Dollar]

The big problem is added capacitance which slows the coil down so you miss small gold.
Some people use twisted pair instead of coax or low capacitance coax.

[baum7154]

There is a great deal of information covering construction of a coil for the Silverdog Surf 1.2. It seems an average around 300uH works best. My question resolves around the increased induction caused by the coax connection to the control housing. I recently constructed an 11.25" coil, with 24AWG, Teflon coating, silvered 7-strand conductor. With copper shield it measured 305uH, with 1.5Ω. I had on hand a spare vintage guitar amp cable, rubber, 95% copper shield, rubber & poly wrapped 21-strand copper conduction which I used for connecting the coil to the system. When I measured induction at cable end, it came in at 368uH, and remained 1.5Ω.



I have been searching to find if there is an average acceptable increase between coil and control box, but thus far have been unable to find reference. So, can anyone suggest an acceptable average induction increase at coax end? Or does it even matter?

TIA

I have used the same Teflon insulated 24 AWG but 19 strand wire for a 335uh spider coil and used a continuation of the coil wire twisted at a rate of 3 turns per inch in place to the traditional coax feed line. My feed line is 33 inches long. The twisted pair adds about 1pf of capacitance per inch. Inductance of this 600 Volt insulted teflon wire twisted pair is 1.3uh for a 63" long sample.

I would guess that guitar cable added a lot of capacitance but it is hard to figure a 63uh increase in inductance. Your copper shield will inhibit operation at short delays necessary to see small gold. The time constant of copper is quite slow and as a result it is probably being 'seen' by the detector long after small gold responses have faded. If you are not operating at very short delays i.e. below 18 to 20us it may not be an issue. For 'fast' coil shielding graphite/varnish is the most successful material.

I would try to replace the guitar cable with just about any coax and re-measure the inductance. If you can get RG62-B/U coax it is only 13.5 pf/ft and a 3 foot piece of it is only 0.254uh.

Best of luck,

Dan

[multieagle]

Thanks for the info. Unfortunately I have already potted the coils. Cutting and splicing new leads would probably make it worse. Dan, will experiment with your suggestions next coil.

Gord

[baum7154]

Sorry to hear it is all 'locked up'. Are you searching on the beach or over dry land? Do you intend to seek small gold i.e. 10 grains or 1/2 gram or so? If you do then you should be careful what the coil is potted with, as most potting adds a lot of capacitance and slows coils down too. As far as shielding goes there is a rule of thumb that says "if your detector can see a sample of your shielding material (with the detector set wide open on delay and sensitivity) don't use it." Some coil geometries may not require additional shielding. See the 'CHANCE PI COIL' photos and construction details I have posted in this forum.

Good luck on the next coil.

Dan

[multieagle]

Dan, quick question. Do you connect the cable directly to the PCB, or to an external connector?

[baum7154]

I connect the PC board to a bulkhead mount connector. A mating connector is put on the end of the feed line. That way you can easily change coils with their feed lines permanently attached to them. No hard and fast rules here though, direct connection of the feed line to the PC board should insure low contact resistance.

Dan

[multieagle]

Thanks. JFYI, I just tested the guitar cable with my pin-pointer and it does not detect it. A pleasant surprise. I had a 66" length lying around, and measured it at 1.7uH and 0.1Ω A 36" length of 24awg 19 strand Teflon wire: 0.992uH and 0.2Ω. However, the shielded coax I have used in past sings out loud and clear. Gotta get rid of that stuff!

[baum7154]

So did the additional 63uh of inductance come from tightening of your coil bundle when it was encapsulated? It does happen.

[baum7154]

Thanks. JFYI, I just tested the guitar cable with my pin-pointer and it does not detect it. A pleasant surprise. I had a 66" length lying around, and measured it at 1.7uH and 0.1Ω A 36" length of 24awg 19 strand Teflon wire: 0.992uH and 0.2Ω. However, the shielded coax I have used in past sings out loud and clear. Gotta get rid of that stuff!

You can use shielded feed line, but coax generally adds a lot of capacitance slowing down the coil. It is the shield material over the coil that should not be seen by the detector as it will act like a significant target if it is seen and you will not be able to see real targets until that shield has decayed. That is why slow decaying copper foil is a bad material. Even aluminum foil that decays about as fast as gold gets in the way of seeing something as fast decaying as small gold. Another significant factor is the thickness of the material. For example aluminum pop can metal decays about 4 times slower than regular thin aluminum foil because it is about 4 times thicker. We make standard detection targets out of pop can and aluminum foil for testing sensitivity to small gold at short delays. See the TARGET TIME CONSTANTS thread in the Tech Forum.

Good Luck

Dan

[GoldSeeker]

Why do people Use RG58? That cable has more capacitance because it is 50 ohm coax. You will get better results with RG59, which is 75 ohm coax, lower capacitance. But it may not be low enough for the problem people seem to have.

[baum7154]

RG58 is very bad for contributing capacitance if you are building for a PI detector for small gold. It is actually better to just use PTFE/Teflon 600 Volt insulated twisted pair stranded wire as it is just about 1pf per inch of length. RD62B/U is 93 ohms and 13.5pf per foot. Generally the higher the impedance the lower the capacitance of coax, but watch out some may not be.

Dan

[GoldSeeker]

But everybody can get TV coax (RG59).

[sinclairuser]

i found guitar cable works best and its nice and supple.
but it has never increased my uh as much as in the op.

[multieagle]

Sorry, I should have responded sooner. Found had been using a faulty multimeter. I purchased a computer powered induction meter and retested. It is now measuring 310uH. Dahhh! So, would have to agree, for this coil, vintage guitar cable is excellent.