Geotech

As you lower a coil to the ground the ground signal produces a stronger & stronger response because it upsets the induction balance of the coil. When you get down to the last inch or so the effect can actually reverse and the RX signal decreases a bit. To make things worse, in some coils it also causes a slight phase shift that messes up the GB. This can produce ground noise. Generally only a problem in bad ground. Concentric coils are the worst because the TX coil and the bucking coil respond differently to ground vs height. DD coils are better, 00 coils even better. Mono coils don't have the problem at all. You are right, the curvature of the TX coil means that the TX field density (and curvature) changes along the length. Larger deep targets will respond best at the center and you might find that the tiniest targets are only detectable near the ends. Moderate not-too-deep targets will have a more even response. Speaking of brooms, the Tesoro CleanSweep coil is the only DD-style coil I know of that has both a straight overlap and rectangular coils with no curvature. It should produce the most idealistic response you want. Some people hang on to a Tesoro just because of this coil. I have.

TG, the difference in overlaps is a trade-off between depth, coverage, and pinpointing. Yes, the straight overlap will be a bit more uniform in coverage but depth will still roll off as you get near the ends of the overlap. That is, it's not perfectly uniform all the way, end-to-end. The football overlap has less coverage but will get a little more depth at the center. It's front-to-back roll-off is more severe which can make pinpointing easier when X-ing the target. In severe soils, coils with harder radius changes will also exhibit a stronger lift-off effect. The Thunder will therefore be worse in that regard while a football overlap will be better. The double-0 SEF coil is even better as it has a very broad radius change.

If you have the 12" coil it floats like a cork. And because it's air-filled you only need to take it a few feet down for it to flex, crack the seam, and flood with salt water. Then it's a goner. The main case is pretty stout and as long as you take care of the Hexseals on the controls it will probably survive 100 ft. For a while White's made a diver version of the SMPI with a weighted loop. I had one but sold it.

Clad will work. Randomly dumped. I dug this up in 2008 so I haven't tried any detectors since then. It's very possible that a newer model might succeed.

Correct, lots of small eddy loops don't produce as strong a signal as one big eddy loop.

I used 220 US silver quarters (3 lbs) placed in a small plastic tub and buried 24" from the top of the tub to the surface of the ground. This was when I lived in NC and the soil was fairly mild.

I built a 1m x 1m frame coil for the TDI but it didn't work very well. Also made a 24" round coil for the TDI, it worked better but still didn't detect the cache.

Question: is there evidence that the connector leaks or has ever leaked? The story I get from the factory is that the connector is fully waterproof but after hunting in salt water people are removing the connector and allowing salt water to drip inside, which then causes the pins to corrode.

Vishay P16 It's IP67 but it may be that salt water is causing corrosion.

Vishay is the only company that makes this style of pot. I had almost no experience with them before the AQ, now I know better.

I can at least answer the battery question... there is a boost converter that creates 15V no matter what the battery level is at, and this 15V powers the whole detector. So nothing should change as the battery drops. I don't know what could be causing the whacko mode, I've seen it a couple of time, usually indoors where there is a lot of EMI. The AQ is cranked up really high. Yes, in some situations (esp combo salt & black sand) you have to turn up the delay or increase the ATS to deal with falsing. Either will decrease depth a little so try to keep them minimized. When I wrote the draft for the manual I had a "troubleshooting" section that had all this in it, but it got cut out for some reason.

35 years ago, a gem. Maybe the best you could buy. Today, not so much. It has less depth, poor target separation, and a traditional analog discriminator that costs even more depth. However, if you are hunting in a sea of bottlecaps for shallower targets it's perfect for that. It's a four-filter design that makes bottlecaps easy to hear, and the old analog ID meter was pretty solid. Another benefit is that it's a good detector to run a Bigfoot coil on.

Old White's, looks like a 6000di-Pro, circa 86-87. If it's in good working cond might fetch $100 or so.

Yes, 12 cells.

Alexandre (and I) has done all he can do for the AQ/Limited. I am told it will go back into limited production pretty soon, the current delay is with the re-vamped lower rod clevis. Things move very slowly here. We are both working on new PI developments with a continued emphasis on how slowly things move.

Not unless it's sensitive to temperature. The problem was that "full cure" was a lot longer than expected and the 4x6 was especially sensitive to very minute shrinkage. But even the 10" DD used on the V3 was problematic, a lot of coils got replaced early on.

I'm certain there is EM energy getting radiated because we measure it when we get detectors FCC-certified. But it's primarily from the circuitry, not the coil or a target, and it's usually in the VHF & UHF bands and comes from switching noise. I suppose it's theoretically possible that fast transient eddies (as from a PI) could produce enough high energy EM radiation to be detectable. But detecting that radiation then requires a proper antenna which is going to be about as large as a TV aerial (the FCC testing labs often use a log-periodic array that looks suspiciously like a TV aerial). BTW, GPR does pretty much what you propose, except that they use a proper antenna and frequency for dealing with EM radiation. And then they respond to both scattering and dielectric absorption, meaning that a lot of non-metals will give it fits. There are also combo metal detector + GPR instruments used in land mine detection, Minelab has been involved in this effort. A few years ago there was a fellow on TNet who was claiming to be developing a new method of detection. I speculated that he was trying to pick up EM radiation just like you suggest. No entirely sure, and then he up & kicked the dust and left even the people who were loosely involved in the project not knowing what he was doing. I wrote a letter to his widow but never got a reply.

Yes, long-term epoxy shifts were the cause. The 4x6 was especially problematic because the whole thing was embedded in a single epoxy pour. We were baking coils for a week and then letting them age for 3 months, and every coil was re-tested before shipment. Yet there were still returns. Fortunately the 4x6 is not hard to open and very easy to re-null.

On the TX side, fewer turns & lower inductance is a benefit. But in a mono coil that same winding is also the RX coil and the induced target signal is proportional to turns. As it turns out, in a traditional PI detector the 300uH coils we use is roughly the right balance between TX field strength, RX signal strength, and decay speed. However, there are other non-traditional methods where 300uH is not the right answer. In some cases more turns are better, in some cases fewer. Coin shrinkers work the same way, except for the side effect that the coil literally explodes every time. I figure this to be a good way to optimize a PI design: increase the cap discharge energy until the coil explodes, then back off slightly. 😉 Realistically, a 300uH coil can be very lightweight. Most VLF detectors have TX coils with higher inductance, and then add an RX coil. Many PI coils are heavy because they were made heavy, not because they needed to be. To answer your last question, yes, there are ways to design a system with low inductance & high current. It requires using a low drive voltage off a buck regulator but also using a high-voltage kick-start circuit to achieve a reasonable rise time. Similar to what the GPZ does. This would obsolete the massive stock of 300uH coils we currently have, which is a reason companies & customers are reluctant to any changes. To the original question: PI weight is dominated by batteries and the TX circuit. Pulsing amps of current in a total-loss system means things get hot, and heat requires a way to dissipate. Ergo big parts and heat sinks. Again, there are other ways to do PI that might get around this problem. Also, if you open up a CTX there is probably a half-pound of toroid ferrites inside for EMI suppression. I would assume the same in some of the Minelab PIs.

The last-gen models weren't analog, a micro generated the audio. The Golden uMax went out-of-production because they lost the micro code. You'd have to go back to the Bandido 2 uMax for an all-analog Tesoro.

Sounds like the D2 is creating a fake threshold. The V3 did the same thing and I never liked it.

Get one quickly, won't be long before Minelab files for patent infringement.

I also have 2 of the Cache probes, a 1st-gen and a last-gen. If you remember the old AH Pro detectors from the late 70s, it's the same guy (AH = Allen Hametta). I think he's through building them. They use the same off-resonance technique as the AH Pro models did. The probe is a stainless steel tube with a stainless tip so, oddly enough, the (mono) coil is completely enclosed in metal. It's a good concept for pristine lawns since the probe can literally be stuck in the ground, no need to dig a hole.

Oh, I see. Yes, someone grabbed the wrong image.

Thanks, I now see that Registered US Mail is commonly used for precious metals and can be insured up to $25000. I'll contact Provident and a few others.