Why are many stock coils 10 or 11 inches wide?

Searched for this before posting.

Seems over and over the common coil size is around 11 inches.

Wondering if there are any technical limitations or sweet spot reasons?

Regards

I think it has to do with depth bragging rights.

Chris

For years the average standard coil was a concentric coil about 8" in diameter. Companies competing for depth kept increasing the size to increase depth. soon, most all companies had reached their design limits so they started designing DD coils to help with separation. This allowed customers to go back to sites they had hammered with their early concentric coils and allowed them to recover targets missed with the concentrics. Then the companies began increasing recover speeds with the advent of much quicker digital electronics. The outdated analog detectors were on the way out. SMF simultaneous multi frequency detectors have been the latest achievements. The modern 11" coils seem to still hold the record for depth, separation, and manageable weight, so that's what most companies go for as standard fare.

An average coil size for most hunting situations. And then they get to sell accessory coils to everyone. But in the case of the Equinox I feel the 11 inch can do it all.

FloridaSon Wrote:
-------------------------------------------------------
> Searched for this before posting.
>
> Seems over and over the common coil size is around
> 11 inches.
>
> Wondering if there are any technical limitations o
> r sweet spot reasons?
>
> Regards

I think you are looking at the reason for increasing coil sizes? As they were in years past 8-10 inch's

Well it has to do with processing speed.

Processing speed allows for larger coils and more separation of targets. .

Smaller coils are best for smaller targets, larger coils are best for larger targets.
So if manufacturers want a machine to have 'all-round, general purpose' performance, an intermediate-sized coil of 8"-11" is the choice. Then the option of a smaller and a larger coil for you to purchase later are provided.

The physics of why small coils suit small targets etc is partly intuitive / obvious; if you're searching for cannon balls, a 5" coil is clearly not optimal; if you're after pin-head gold nuggets, a 24" coil is not going to do the job. But proper analysis of the physics is much more complicated, and it matters if you're talking PI or VLF, and it matters if the VLF is multi-freq, too.

While a larger coil will give a larger signal on targets, it also picks up a larger ground signal ( if it's a VLF) , which somewhat negates the large-coil benefits.
Then there's the nature of the targets themselves: large targets tend to be scarce, there's plenty of tiny stuff out there. Large targets by there nature are 'easy to detect', and even with a coil too small, you could find them. Whereas if targets are 'too small' for a large coil, you're going to miss them.
A large coil will see many more targets at once, which is undesirable in target-heavy ground, and can result in more masking. As a popular target buyer of a metal detector is 'the park hunter', and parks can be trashy, manufacturers are not going to make the stock coil too large.
In addition, larger coils are heavier, and people don't want heavy machines.
And larger coils pick up more EMI, which can render them no more effective than a 'standard' size coil.



Edited 1 time(s). Last edit at 12/10/2021 06:40PM by Pimento.

Pimento Wrote:
-------------------------------------------------------
> Smaller coils are best for smaller targets, larger
> coils are best for larger targets.
> So if manufacturers want a machine to have 'all-ro
> und, general purpose' performance, an intermediate
> -sized coil of 8"-11" is the choice. Then the opti
> on of a smaller and a larger coil for you to purch
> ase later are provided.
>
> The physics of why small coils suit small targets
> etc is partly intuitive / obvious; if you're searc
> hing for cannon balls, a 5" coil is clearly not op
> timal; if you're after pin-head gold nuggets, a 24
> " coil is not going to do the job. But proper anal
> ysis of the physics is much more complicated, and
> it matters if you're talking PI or VLF, and it mat
> ters if the VLF is multi-freq, too.
>
> While a larger coil will give a larger signal on t
> argets, it also picks up a larger ground signal (
> if it's a VLF) , which somewhat negates the large-
> coil benefits.
> Then there's the nature of the targets themselves:
> large targets tend to be scarce, there's plenty of
> tiny stuff out there. Large targets by there natur
> e are 'easy to detect', and even with a coil too s
> mall, you could find them. Whereas if targets are
> 'too small' for a large coil, you're going to miss
> them.
> In addition, larger coils are heavier, and people
> don't want heavy machines.
> And larger coils pick up more EMI, which can rende
> r them no more effective than a 'standard' size co
> il.

Ummmm.....maybe coils of old. I can tell you the 15" Minelab coil for the Equinox will find mind blowing small items. MAYBE better than the 11" coil. To me coil size is now about separation and trash content of a site.

When I said it matters if the machine is multi-freq, I should've elaborated a bit:
Using multi-frequencies allows the machine to work out the ground signal level much better than a single-freq machine, which lets it 'eliminate' ground signal much more effectively. A large coil will pick up a stronger ground signal - this would normally be detrimental to performance on a single-freq machine, but multi-freqs are less affected by this, allowing more gain, and more depth. This is probably why the Eqx has a seemingly larger-than-standard stock coil, they can make an 11" coil work as well as a 7"-8" .

And regarding the genuine Minelab 15" coil: it's not a 15" coil, it's 15 long x 12 wide. And it's the width that determines most of a coil's characteristics, including depth, and how it sees different sized targets. So .. as it's 12" versus 11", it should retain most of the performance of the stock 11" coil.

And it matters whether it's a PI machine because ( for a constant coil inductance ) PI coils pick up the same level of ground signal regardless of their size. So a larger coil is not hampered by increased ground pickup, it's just how it sees the target that differs.

For a circular mono PI coil, it's possible to calculate what signal a target gives, and hence work out what the optimum size search-coil is for a specific target. Sadly, it's much harder for a DD / OO coil arrangement, ( even on a PI ) and VLF's are harder still ... there was a discussion thread over on Geotech1 forum about this topic, and no-one could come to any useful conclusions about 'optimum' coil size. No surprises, though, when you've got VLF guys discussing things with PI guys, nugget-hunters debating with coin-shooters etc.

Another thing to consider: Some modern machines have 'smart' coils ( eg. Equinox / XTerra ) that can identify themselves to the 'brains' , and the detector behaviour is adjusted accordingly. So the response-speed may be sped up when a small coil is fitted, for example. And the gain may be recalibrated, so depth gauges stay true. Also, coil pre-amplifiers are more common now, so it's possible the coils have different gains built into them for different coil sizes. All this makes comparing one coil to another pretty tricky.

midalake Wrote:
-------------------------------------------------------
>
>
> Ummmm.....maybe coils of old. I can tell you the 1
> 5" Minelab coil for the Equinox will find mind blo
> wing small items. MAYBE better than the 11" coil.
>


But does it go deeper on coin-sized targets ? I'll bet that the stock 11" and that 15" probably do about the same, for coin-sized objects. Eh ?

About the only machine I've seen where an enormous 15" coil goes deeper for coin-sized targets, is a Sov./Wot combo. But that's a rare exception. And it comes at a cost of warbly performance, difficulty pinpointing, probably lousy for target separation ghost-townsy purposes. But for the wide open beach or fields, It does get coins deeper.

Aside from fuke anomalies like that, an 11 -ish inch coil is about the "point of diminishing returns" for coin-depth. Hence the reason most "stock" coils have 11" or so as their standard-stock.

((( We're trying to change this. Reinvent the wheel. ---Rid the snow-tires and put slicks on it--- )))

Thanks everyone, enjoyed the discussion. Good points.

Tom, hope those racing slicks are shipping by next year, I am ready for an upgrade…
Best regards

Reminds me when I used to be a young fireghter, an old chief was teaching history. Someone asked why standard (at the time) fire hose size was 2-1/2 inches diameter?

Bottom line: Firehose in the 1800s was leather, 2-1/2 inches gave the most efficient use of a cow hide.

8" coil used be be standard. It seems after the Equinox came out, 11" coils became the standard.

I don't like 11" coils either. I would be happy with an 8" or 9" and 5.5x9 coils. Having a choice when you buy would be nice.

Rick..... your request is going to become commonplace.

Good to hear.