The Crossbow Classic PI

[TowySalmon]

I am open to offers for the coil.

[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.

[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]

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

[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.

[surfdetector]

Thanks for finding it.

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

[6666]

Yes, we need the patent number indeed,

Does this look like the correct one?
https://patentimages.storage.googlea...17015730A1.pdf

Thanks for finding it.

[Qiaozhi]

Yes, we need the patent number indeed,

Does this look like the correct one?
https://patentimages.storage.googlea...17015730A1.pdf

That looks like the correct patent, but I notice it mentions TX and RX coils.
If I remember correctly, the one I looked at was wired for use as a mono with both loops in series and anti-phase.
It was very strange because it still reacted as if it was a DD with the sensitive part in the crossover area between the two loops..

[surfdetector]

Was there a patent number on the coil ? thanks.

Yes, we need the patent number indeed,

Does this look like the correct one?

https://patentimages.storage.googleapis.com/95/c1/54/17e56dd1fd86cc/WO2017015730A1.pdf

[surfdetector]

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.
It would be interesting to experiment with this configuration.

Yes, I am wont to resurrecting old threads. I believe this one looks like a good project.

[6666]

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.
It would be interesting to experiment with this configuration.

Was there a patent number on the coil ? thanks.

[Qiaozhi]

@Qiaozhi Would this coil require nulling prior to having the series connections made?

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.
It would be interesting to experiment with this configuration.

[surfdetector]

It turns out that the Detech SEF coil is rather unusual. Initially it was assumed that the coil was a DD type, which was quite reasonable as Detech advertise it as such. However, the cover of the box says:
-----------------------------------------------------------------------------
The double elliptical coil with Symmetric Electro-magnetic Field
is the best performance coil.
Compared to the Double D coils it has the following advantages:
- better ground balance
- more stable operation
- deeper in the ground
- better pinpointing
- more accurate target identification
-----------------------------------------------------------------------------
So clearly it's not a DD coil after all.

The coil plug wiring is as follows:
L1 263uH (6R6) coil between pins 1 and 2
L2 706uH (7R0) between pins 2 and 3
pins 1 and 5 connected together
pins 3 and 4 connected together

Obviously, with pins 1 and 5 shorted together, the coil plug wiring is connecting the coil as a mono with L1 and L2 in series. Effectively this means the detector "sees" a mono coil with 439uH, requiring a 330R damping resistor between pins 4 and 5.
What's interesting is that a mono coil is sensitive from edge to edge, whereas the SEF coil is sensitive across the overlapping section of the two loops. Hence it reacts like a DD coil, but with the advantages of a mono.

In a way, this setup is similar to a White's Dual Field, but in a Butterfly (DD) format. Although, in this case, it was possible to critically damp the coil with one resistor.

@Qiaozhi Would this coil require nulling prior to having the series connections made?

[SVEN1]

Closed down the Mirage/Crossbow forum as there was really no activity for a couple years or so. So I posted the particulars here about the CrossBow.
Regarding the Mirage, my last version I was happy with and did not need to go any further with it. Elliot took over and made up his version(s) of the Mirage with
further improvements, contact him for a PCB etc.

[MartyJ1963]

Hi Alec,,,I haven't built a Crossbow yet,,It's on my 'To build' list as is the HammerHead so I can complete my collection of all the Hammerhead variants. I have all the Crossbow gerbers and I intend ordering some boards in the future. It's the Hammerhead I'm stuck with as I'm unsure which version to make. The Mirage is my favourite up till now,,I've built 8 units so far and 2 for my friends. Funny thing is I've only ever been out detecting about 4-5 times as I don't drve and the nearest beach is 2 hours away,,lol,,so I just keep building!,,lol,,Thanks, Marty