The Crossbow Classic PI

[surfdetector]

Thanks for finding it.

I wasn't impressed with the content. Certainly not as informative as the Whites Dual Filed Coil patent.

[Carl-NC]

I was examining this coil for someone else who wanted to use it on a Crossbow detector. That post was from Nov 2017, and I cannot remember whether the two loops were balanced, although I suspect not.

The SEF is a "Double-0" coil as opposed to a Double-D. The only difference is that the DD has tight corner radii and the 00 is a perfect ellipse. If you make the ellipses round you have an OO coil as used on the Nexus (Alexander Graham Bell used an OO coil on one of his designs). So all these coils are very closely related. All the SEF coils I've seen are inductively balanced.

The advantage with 00 vs DD is that the tight radii on a DD give rise to lift-off effect. Lift-off is even worse on a concentric. And the OO coil is even better than the 00 coil on lift-off; in fact, you can't get any better than a round coil.

Yes, we need the patent number indeed,
Does this look like the correct one?
https://patentimages.storage.googlea...17015730A1.pdf

That's for their flat-spiral DD coil. IMO there's nothing really novel in the patent, old techniques that have been around for a while. They used to advertise the SEF coil as Patent Pending but I never saw a patent app for it.

[MartyJ1963]

Hi Carl, I've never heard of 'Lift-off' effect, could you explain what it is please? Thanks, regards, Marty.

[Carl-NC]

Hi Carl, I've never heard of 'Lift-off' effect, could you explain what it is please? Thanks, regards, Marty.

Consider a concentric coil... there is a large outer TX coil and a small bucking TX coil next to the RX coil. IB is achieved when the RX coil sees equal (but opposite) fields from the two TX coils. As you lower the coil to mineralized ground the field from the large coil begins to get distorted and this upsets the IB. As the coil gets even closer to the ground the field from the smaller coil also gets distorted and that helps restore IB. You can hear this effect when bobbing a coil over highly mineralized ground and it can make for noisy operation.

A DD coil does a better job, except for those tight radii along the overlap area. Again, as the coil is lowered the broad radius portion of the field is distorted first, then the field of the tight radii when even closer. However, the tight radii field is a much smaller percentage of the total IB so the effect is much less than concentric.

An 00 coil (SEF, ellipses) is even better than DD, and an OO coil (Nexus, circles) should be the best. The OO coil has a uniform radius and therefore a uniform field so there should be zero lift-off effect.

Lift-off is a term widely used in non-destructive (eddy current) testing.

[MartyJ1963]

Hiya Carl,,,,Thanks for the explanation. I'm stilling learning about VLF Coils and this has helped me a lot. Thanks again, regards, Marty.

[TowySalmon]

I am open to offers for the coil.

[surfdetector]

I am open to offers for the coil.

I think on this forum that we are more interested in making our own clones

[MartinB]

Hi Marty,
Did you ever do one of the Crossbow machines or just the Mirage? I was thinking about getting a few Crossbow boards made as the Gerber files are here.
Cheers

Hi Surfdetector,
I am in the process of ordering 5 boards as per the files in post #18 of this thread.

[surfdetector]

Hi Surfdetector,
I am in the process of ordering 5 boards as per the files in post #18 of this thread.

Hello Martin,

I had a successful build of the Crossbow Classic and I did a series of YouTube videos of my build here that you might find to be of interest:

https://youtube.com/playlist?list=PL...25R5R4iut8Y_s1

[MartyJ1963]

Hiya Martin. Here are the Gerbers for the Crossbow pcbs that Alec used successfully. I don't know if the ones in post #18 are different to these but these are proven. Have a successful Build! I'll be starting a new Crossbow thread hopefully soon. George has kindly given me his consent to write a Crossbow Build Guide that he will review for me before I ask his permission to post it. It will be in accordance with a lot of information from Alec's top class videos and photo's etc that I will be taking as I build along to add to the guide. Hopefully it won't take too long!,,Lol Regards, Marty.

[6666]

Hiya Martin. Here are the Gerbers for the Crossbow pcbs that Alec used successfully. I don't know if the ones in post #18 are different to these but these are proven. Have a successful Build! I'll be starting a new Crossbow thread hopefully soon. George has kindly given me his consent to write a Crossbow Build Guide that he will review for me before I ask his permission to post it. It will be in accordance with a lot of information from Alec's top class videos and photo's etc that I will be taking as I build along to add to the guide. Hopefully it won't take too long!,,Lol Regards, Marty.

[MartinB]

Hello Martin,

I had a successful build of the Crossbow Classic and I did a series of YouTube videos of my build here that you might find to be of interest:

https://youtube.com/playlist?list=PL...25R5R4iut8Y_s1

Thanks for the heads up. I will take a look
Boards are now in production a should be with me in a couple of weeks

[MartinB]

Hiya Martin. Here are the Gerbers for the Crossbow pcbs that Alec used successfully. I don't know if the ones in post #18 are different to these but these are proven. Have a successful Build! I'll be starting a new Crossbow thread hopefully soon. George has kindly given me his consent to write a Crossbow Build Guide that he will review for me before I ask his permission to post it. It will be in accordance with a lot of information from Alec's top class videos and photo's etc that I will be taking as I build along to add to the guide. Hopefully it won't take too long!,,Lol Regards, Marty.

Hi Marty
Thanks for the gerber files. When I get time, i will have a look and do a comparison. Good luck with the manual

[MartyJ1963]

@Martin, Thanks. The Gerbers I posted were the one's obtained a couple of years ago when I was a member of the Mirage and Crossbow forum (Area51). The Crossbow and the Mirage Projects were the first detectors that attracted me to joining Geotech in 2016. My first build was the Surf longboard because I wasn't experienced enough back then to build the Crossbow. Keep us posted on your build. Regards, Marty.

[surfdetector]

TowySalmon would you mind measuring the outside dimension of one of the coils ellipses please
as per attachment you will see the arrows point to the measurement I'm asking about thanks.
[ATTACH]41132[/ATTACH]

@6666, Did you ever do any experimentation with this type of coil? Thanks.