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Geotech last won the day on December 9 2016

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  1. Air hots are pretty good (nickel @ 3+" @ med sens) and should be the same in ground. It has a frequency offset adjust (16 levels) so you can minimize interference.
  2. Bigfoot Coils On Ebay

    After Jim died Jimmy Normandi bought all his Bigfoot inventory and had Roy Van Epps building them in the old defunct Discovery factory. I went there to visit Roy one day, what an eerie place, like a ghost factory. Roy and a helper were building Bigfeet that day, he said only about half of what he built would work right. Health problems forced him to stop, no idea who got all the left over stuff. There was also a big batch of stuff stored at Whites for building coaxial Eliminator coils (also Karbowski). Like the Bigfoot, those were also unique and useful.
  3. I've been up to the Aho mines and the ruins of their cabin, very similar mine tunnels. Forestry has been installing bat grates on most of the tunnels to keep people out, but someone has come behind them and cut holes in the grates to crawl in. They're quite dangerous, the last time I was in Aho #1 a big slab of ceiling was cleaving off. When it falls, it'll crush anyone under it.
  4. Most detectors also have a high-pass filter on the preamp to knock down 60Hz before the demods, and the higher the operating frequency the more the 60Hz gets attenuated. So this might account for why high frequency detectors might be quieter. Personally, I've found just about every detector I've used to be subject to 60Hz EMI. That is, I don't recall any particular detector that is immune.
  5. 60Hz ain't always 60Hz. The power frequency is regulated to have a very high accuracy over the long-term, so that old-fashioned motor-driven clocks keep good time. But in the short term, frequency can vary by a decent part of a percent or so. That's why you can have little interference, then a few hours later a lot. Metal detector frequencies are usually chosen so that multiples of 60Hz fall outside the target filter bandwidth. But if 60Hz is really 60.15Hz (1/4%) then at 12kHz the apparent shift is 30Hz which is enough to fall completely inside the target filter. The higher the operating frequency, the higher the apparent shift will be, which means that higher frequency detectors will be more sensitive to 60Hz variation.
  6. New White's D2 Coil Design For 2017

    The D2 is epoxy-potted, but not completely filled. So weight can vary a lot depending on how much epoxy was used. And that can vary a lot depending on who is running the epoxy machine, it's all a manual process.
  7. Dealer Communications Or Lack Thereof

    At PriorJob is found out that all official communications between the company and dealers had to be done via snail-mail because many of them didn't own a computer. I asked, do they all own a telephone? Should we play it safe and use the telegraph instead? I suggested that we require all dealers to have a computer and to understand and use email, but that didn't happen. Not necessarily. While many dealers are detectorists, some are not (inherited the business, or just a side-line), and many of the dealers are old-timers who prefer simple machines. Hand them a modern LCD-and-button interface and they don't know what to do. These are the ones most likely to still be using the telegraph, see above. As detectors get more and newer features and have graphical displays with menus, the dealers have no idea how to run them, so they're less likely to try and sell those models.
  8. Minelab And Customer Communications

    A little late on the topic... I've known and worked with a whole lot of engineers. The cliches are largely true, and Dilbert paints a fairly accurate picture of the engineering mind. When I tell people that I'm an engineer, I often see that sad look in their eyes that says, "Oh, bless your heart." My partner had that look when I first told her; now she says, "Ya know, for an engineer, you're pretty self-aware." Right out of college I worked at NASA-KSC. Only for 5 months, but ever since then my resume says "Rocket Scientist -- NASA" as a joke. Another engineer got annoyed at my joke because he thought I was trying to be a braggart. He didn't get it. Engineers often don't.
  9. Is Fisher Next??

    There are several ways to do a simultaneous VLF/PI. The method I did at White's is called truncated half-sine, I got a patent on it if you want the details. There are other time-domain TX waveforms that will give similar results. And there are other waveforms that are neither VLF nor PI, but perform as if they were both. At some point, we'll have to come up with new terminology for this. Probably someone's marketing dept will kindly step in and do that for us. Modulating a sine wave on a pulse would make for a difficult TX circuit design but I'm sure it could be figured out. It reminded me that I was loosely involved with a third VLF/PI effort, a design that my book co-author had conjured up. He sent me a prototype (I still have it) and it worked, but had flaws. He figured out how to fix the flaws, but then never got the time to implement the fixes. I should call him.
  10. BFO detectors were popular in the 60's & 70's as the detecting hobby emerged. They were super-easy & cheap to build, a simple design can be made with only 3 transistors. Anyone could become a detector manufacturer. Garrett probably had the best designs, and they were about the last to continue making them, up til the early 80's. BFO is not good on depth, in any category. That's why no one makes them any more. The only thing BFO did that was semi-unique was respond well to mineralization, something you normally don't want, unless you are tracking black sand deposits. But you can actually do that with some VLF models, if you know how. Besides having the best BFO, Garrett also made the best assortment of coils. Here's an old photo of my vintage collection, in the middle is a Garrett BFO with the 24x24" square coil. It's easy to build a custom coil, it's just a single coil of wire, I think the inductance is around 500uH. Shielding is important because it's very sensitive to ground capacitance.
  11. Yes, in radar impedance matching is critical. There are no opamps to make life easy!
  12. Impedance matching is done on RF systems where circuits are difficult to design with low/high impedances, so they are normally designed around 50 or 200 ohm impedances. At sub-MHz, it's easy to design around opamps which don't need impedance matching. For the RX coil, the incoming magnetic field induces a voltage on the coil, so all we need to do is slap it on a non-inverting opamp. For the TX coil, yeah, you can stick it on a 3904. But usually it's resonated with a capacitor at the desired frequency. This creates a sinusoid, plus recycles energy. Not for impedance matching.
  13. And metal detectors don't radiate electromagnetic energy by design, too. Only a local magnetic field, just like a transformer. We do our best to suppress the electric field with shielding.
  14. That's just it, they don't. A detector coil is no more an antenna than the transformer hanging on a power pole.
  15. Detector coils are not antennae, they are designed as a transformer. There is no EM propagation, only a local magnetic field. The coils don't have to be impedance-matched to anything. Most RX coils are just connected to a high-impedance preamp, with some token load resistor (typically 10k). Some designs throw in a few extra components to reduce EMI.