DIY Bigfoot coil?

[Steve Herschbach]

Hard to imagine nobody can commercially build a coil for the retail market and make money when they go for $400-$500 used on eBay.

[bklein]

Some random thoughts:
So, if I want to make one for my newly acquired Spectrum XLT, I could just go wind some wire in the stated figure8 vs oval pattern, and it would work somewhat. But I'm wondering about optimisation. It would be easiest to start with the TX coil. Pick a wire diameter and wind turns to equal the stock 9.5" coil inductance? I saw on a website somewhere that there are several discrete components in the stock coil for what purpose I'm not sure - but probably messes with the measured inductance reading taken for reference... So, would it be best to have some kind of remote reception of the TX signal and wind the coil for maximum signal?
Then starting on the RX coil should I immediately worry that the mass of RX coil near the TX would mess with the TX coil performance and perhaps I would need to add/remove turns? Should the TX be above, below, or outer perimeter of the RX?
So then I work that out with a rough approximation of the RX coil in place and then can tweak the RX coil. Again, best to match the impedance of the stock RX coil and add/subtract turns for optimization?
Placement of the RX figure 8 with the TX must be tricky. If biased to one side the TX rejection would be off. So I imagine you make the RX coil first outside the enclosure and tweak for best EMI cancellation then place it alongside the TX coil in the enclosure and move it around to best cancel the TX signal on the RX output...

[Polymer]

You had me laughing bklein. I had similar thoughts like your comment: "I surprised this hasn't come up before - either something everyone knows but me or some kind of taboo...".
I found it quite hard to get real information on the bigfoot too. Another "conspiracy" ... who knows.

I am wondering though. Since nobody seems willing to produce them in series anymore, why has pertinent information not been set free to let detectorists who want to experiment with this do just that?
I have a bigfoot coil now - in very good condition -. I am still learning how to adjust my detector optimally for that coil in the different terrains I have here.
There are some "issues" but have yet to discern if it's me or false settings in a particular terrain or both. I enjoy the bigfoot most of the time.

There is a picture of an open bigfoot coil in the ITMD book by Carl & George. I wish there was more to see but it's better than nothing. I assume the white rectangle inside is a capacitor. I am not ready to crack my bigfoot open yet.

Here's some values I found on the net for the bigfoot: Tx: 554uH & 1.81 Ohm
Rx: 43mH & 146.7 Ohm

I can only measure the dc resistance for now on mine: Tx: 1.6 Ohm
Rx: 120.6 Ohm

Perhaps we can get more puzzle pieces together in due time.

The argument of being to "work intensive" for production does not stand for me. There are many excellent hand-crafted products on the planet that command their price and have their niche.
It might not be a goodie for a corporate outfit, but for a person or group with a passion like the original builder, this could fly and be fun.

Enough rambling for now.

Have a great day!

[Qiaozhi]

There is a picture of an open bigfoot coil in the ITMD book by Carl & George. I wish there was more to see but it's better than nothing. I assume the white rectangle inside is a capacitor. I am not ready to crack my bigfoot open yet.

Have a look at the left-hand image in Fig. 5-22 on page 57. This is the configuration used in the "bigfoot" coils.

The earliest depiction of the figure 8 loop is in M. Jullig's January 1915 patent (attached), where it is shown as an "equivalent" diagram of an orthogonal configuration.

[Koala]

is the wire wound in a figure of eight or a circular loop made and twsted into a figure of eight or does it make no difference.

[Qiaozhi]

is the wire wound in a figure of eight or a circular loop made and twsted into a figure of eight or does it make no difference.

Personally, I suspect it's wound as a circular loop and then twisted, for the simple reason that adjusting the coil shape to achieve the correct balance would be easier if the turns are not interleaved at the crossover point.

[kt315]

pic of ciklon alias, from his site.

[Koala]

just noticed pic top of page 58 seem to show long coil ends used for nulling. More detail may be visible in the original photo

[Koala]

pic of ciklon alias, from his site.
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Thanks that's a great link kt315

Any chance of a translation please it's hard as it's a photo not text

[bklein]

What about the caps? I saw several discrete components between tx and rx coils of standard XLT coil. What's going on there?

[kt315]

thinking there are caps and resistor in RX. that is standart way for whites. nothing of whites patents gives full schematic of their coils btw. some kinda know how.
anybody has to put the coil under X-ray, then there will be clear pic.

[bklein]

I just bought a Bigfoot off Craigslist for my XLT. Perhaps I can get it xray'd at work.

[Polymer]

kt315: Thank You for the wiring and other info!

bklein: What a great idea - getting the bigfoot x-rayed ... Can't wait to see the pics, hope you can get it done at work ...

[bklein]

bigfoot xrays

Ok I got a guy to xray the coil.
Looks like it has variable caps, trimpots, or both along with discrete caps and some bizarre wiring....

[Altra]

Thanks for posting.