Coil advice please

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Hi All,

I live in the UK and use a GP3500 in conjunction with other detectors to search for artefacts... bronze age, roman, saxon, medieval and so on.

So far Ive had tremendous success at using the GP3500 on heavily worked sites where conventional detectors are turning up nothing because of the depths involved.

I currently use the bog standard 12" coil and also Commander 15" x 12" DD and Commander 18" DD coils.

Question is regarding the Commander 18" mono. I understand itll go deeper than the 18" DD equivalent but approximately how much deeper... 1 or 2 inches perhaps on the same field?

Also say I get a good deep signal on something like a small silver medieval coin using the 18" mono, will the machine still give me a non-ferrous audio signal, as in "tone rising, then falling" as a DD coil would?

I know theres no disc offered with a mono coil but Im willing to do some needless digging in order to get a bit extra depth and to get to more objects on certain sites.

Many thanks for any advice offered,


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Hello Nick,

Thanks for joining the forums and posting. The depth questions always seem to come up when people start to compare DD to Mono. There is no doubt the Mono will get a bit more depth, but I think you're on the right track when you say 1-2 inches. I wouldn't think even under the best conditions you would get more than 3-4 inches, if that.

If you're looking for something really light and will get a bit more depth over the 18-inch DD, you might consider the Coiltek UFO. This coil is a 24x12 inch elliptical Mono that is extremely lightweight due to the "spider-web" design. It's also completely waterproof, also dubbed one of the "All Terrain Searchcoils." You would get more coverage per swing due to the length (24-inches) and at least 1/3 the weight of the Minelab Commander 18-inch round.

There are coils even larger if you're looking for maximum depth penetration.

The major disadvantage is the fact you can't use discrimination with a mono coil.

As for the tone questions, you will get just the normal tone you mentioned until you get close enough to discriminate the target out. None of the Minelab's will discriminate at true detection depth, but I'm sure you know that. You may get a faint whisper type signal, but you probably won't get an accurate discrimination until you're fairly close to the target.

I'm sure I didn't answer everything; so feel free to post back.

Talk with you soon,

Rob Allison

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The standard answer from the reading is a mono gives about a 20 percent

increase in depth over a DD of same size. Robs estimate is about spot on.

Minelab owners forums may be of some help as there are alot of British

finds and users on that site. Nice stuff to.

As to the tones of non ferrous signals at depth.... It'll probly be a low to high

signal simular to ferrous returns or ground noise. Thats a guess based

on the detectors operation~ deep signals use a seperate circuit in the detector

and its response is the Woo Whee. A suggestion if you can afford it is

the new NF XP coil series. They feature a switch that changes the deep signal

response to the Wee Whoo via changing the phase relation of the coil.

Technical but basically it would give you what your asking after if you desire

the high tone response on deep stuff.

Those coils are DD's as well and have other advantages when it comes to interference.

My two cents. Welcome and good luck. Rob gives good advice :D


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