a more detailed can this be done (improving coil design)

[strike it rich]

hi after posting can this be done it seems I confused a couple of people with what I was trying to get across my fault bad explanation so here go's.

As i see it modern Md's work using radio waves they go up down left and right. What I was trying to ask is if the top of the coil was covered with a material that reflected the waves back down and inside the coil was the same material spinning thus the radio waves could only go down added to this they get channelled into a small concise area would this have the effect of increasing the strenght of the waves thus increasing depth.

I know metal detectors have come a long way and that coils have too what iam proprosing is a totally new radical design for a coil your thoughts please

of course iam willing to build a one some day but before i waste my time and money i would like to bounce the general idea off you guys and gals.

 

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[Rob in KS]

What you are talking about is a reflector. Think of the coil as an antenna. You want to focus the energy from the coil in one direction. Thats what directional antennas are all about. Look up a corner reflector antenna or a parabolic dish. The main problem will be the frequencies your talking about. I think most detectors run in the kilohertz range. That makes the wavelength HUGE. The reflector would have to be a certain fraction of a wavelength from the coil so that the waves from the coil and the reflector reinforce each other.

You can learn more by reading up on antenna theory.
The Radio Amateurs Handbook would be a good start.

 

[gallileo60]

I stated alot of this on the other post....It does seem as tho you might be able to reflect some of the energy downward..Im not sure how you would do this..I have built several directional antennas (Bought a few too) Beams, Yaggis, quads, Just dont know how you would do this without size being a factor..Im sure some of the high dollar (read 30,000, or so) detectors do something along this line...I bet there are some smart guys out there that have some ideas...Then again I dont know how much like an antenna the coils are, we know they are very small for the frequincy they operate on, so they must be a fraction of the wave length...

 

[SoCalDesertFox]

I really hate to resurrect/necro a thread, but I do know of methods to do this. We use them in other induction applications (in my case, induction lighting.)

You know that flexible magnetic stuff they make fridge card magnets from? A layer of that on the back of the coil itself helps to direct EMF downwards for better detection ranges. We do this for better efficiency in induction lights. Any diamagnetic material coating on the inside of the top cover of the coil will also serve a similar function.

There are also some exotic coil geometries which can generate some freaky field dispersion patterns. I've seen a couple that generate a near-zero flux in the inside, almost all on the outside, used as internal induction electrodes.

 

[flgliderpilot]

Well the problem I see is that metal detectors are not radio transmitters. They are induction coils, basically a loosely coupled AC transformer. A magnetic field is created around the coil (a very very weak alternating field) which is induced in a receiver coil. The coil is coupled in a way so that the Tx and RX signals cancel one another. When metal is introduced into the field, it alters the tuning of the Rx coil breaking the tune and thus you hear a tone.

There is no "transmission" to be reflected. Its not a radio transmitter nor is the coil an antenna, its an inductor of an RC circuit.

 

[Terry Soloman]

If you want to increase depth, increase power to the magnetic field. Ever wonder why a Minelab GPX 5000 is so darn heavy?

 

[flgliderpilot]

If you want to increase depth, increase power to the magnetic field. Ever wonder why a Minelab GPX 5000 is so darn heavy?

Yes in addition to coil size, or configuration Frequency is important also.

If strike_it_rich is interested in how metal detectors work I highly recommend the book "Inside the Metal Detector". The second edition is available and it covers BFO, VLF, PI, and other detectors in detail both in theory and the circuitry, schematics, and PCB layouts to build your own detectors.

They do not get into VLF multifrequency machines but they do describe the theory. By the time you get to this section, assuming you've built all of the other detectors and understand them, you should be able to venture alone into that area.

It's the best book available on the topic. The first edition had a section on LRL, but they state in edition 2 that the section was removed because it was misleading. The second edition only contains detectors which will work. If you are interested in LRL the 1st edition is probably the one to get although it's hard to find and expensive, and the LRLs do not actually work but are designed to fool you into thinking they might (from what I understand from the 2nd edition).


BTW you won't strike it rich detecting But we all try.

 

[DannyB1954]

Here is an interesting PDF I found online discussing the history and design of metal detector coils.
http://www.deeptech-bg.com/search_coils.pdf
There are links in it if you want to view the patent document.