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Concentric and DD coils

A

Anonymous

Guest
Last night before I went to sleep I was thinking about DD coils and Concentric coils. Correct me if I am wrong. instead of a bucking coil for the concentric could you feed a signal from the tx circuit to the rec circuit that is 180 out of phase and adjust the amplitude to get the null you are seeking in the bucking coil effect. Another thought is if you could adjust the phase of this signal, what would be the effect.Or is the timing more critical than I am thinking. The second thing I thought about is everyone makes the DD in the shape of a D. Would the effect be the same if both coils were circular and overlaped or does the area between the overlaps create the best area for detecting? and the 2 sides of the d coils have to be parallel? I have not tried this just been wondering about it.
Just thinking outside of the coil or is that the box.
 
Hi Ray,
The DD shape is just a convenient one for squeezing two coils into a circular shell and getting the right overlap for an electrical balance. They could just as well be two circular coils, two square coils, or two rectangular coils, or any other shape.
The response is squashed in one direction by the D shape, but is not like a flat shovel as some advertising would have us believe. The best response is when the object is equidistant between the two coils and at a distance that is equal or greater than the coil separation. i.e. electrical centre TX to electrical centre RX. Get closer than this, and you start to get multiple positive and negative responses, which in the case of mineralised ground, gives a useful measure of ground cancellation.
To get nearer to a shovel shape response, you would need to overlap two rectangular coils that have at least a 3:1 ratio in their side dimensions.
Eric.
 
Eric,
Why are the coil manufactures not building Rectangle shaped coils with your 3:1 dimension ratios and maybe you could slightly round off each corner?
I presume this shape would give greater depth and sensitivity along with the more straight edge blade search pattern as you say if in a DD.
Does this Rectangular shape give a mono coil the same advantages over the Round and Elipticals?
Regards,
Gary
 
Hi Gary,
I can only assume it is because the public have been conditioned to expect a round coil on a metal detector. I made a detector with a rectangular coil back in the '70s. It didn't sell because it did not look right. I have since made quite a few rectangular "specials" for people who want their performance rather than their looks.
A rectangular coil certainly gives better ground coverage, with a more "blade like" field pattern, and very good depth. Pinpointing is a bit more difficult and you need to do a cross scan.
Yes, rounding the ends would give it a bit more aesthetic appeal.
Eric.
 
Thanks Eric for the response. This is the first I have heard about a 3:1 ratio. I must have missed it in earlier posts from way back. I will keep it in mind when building a coil.
Ray(NM)
 
There I coined a new phrase, W-DD coils "W" for wide. Its too bad people are conditioned "round" because wide double D coils are the answer to many problems associated with larger coils in the 14 to 20 inch range.
In air tests the larger the coil the deeper it goes and in the case of DD coils depth has a lot more to do with how wide the coil is than how long it is.
That said air tests mean nothing, drop a monster 20 inch DD coil on the ground and let it take a monster coil size bite of ground mineralization and there goes the depth, thats problem #1.
Problem #2 is weight, a coil that size is going to be heavy, there's just no getting around that. Sure you can use microspheres to lighten it a bit but larger coils require a greater level of rigidity. Its not much use if the average person can't swing it for more than an hour or two.
Problem #3 is ergonomics, how do you work a large coil between the trash targets? When you hit the 18-22 inch range how do you keep from stepping on it!
I learned the above first hand after building an 18 inch, 20 inch, and 22 inch standard double D coil for my Explorer. The 22 inch nearly snapped the lower rod by the way. <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt=":D">
But being a depth junky I was not easily discouraged, I stepped back and looked at my 18 inch coil...where can I remove some weight...I chopped off the front and rear 3 inches of the coil.
As Eric mentioined the front and rear of the coil are not nearly as deep as the center anyway and this removed 33% of the heaviest part of the coil which is down the center. It created more space behind the coil to walk. It improved greatly the ability to work around the trash as the coil is only 12 inches long now.
Equally as important it also reduced 33% of the hottest part of the coil e.g. its gulping less mineralized ground, more akin to a 12 inch coil than an 18.
I took this coil out to a field I had detected to death for a couple years with coils up to 15 inches in size. Targets at this site are very deep. I had spent (wasted) many hours at this site since only to leave empty handed yet the 18x12 inch coil picked off half a dozen deep silvers and wheats in the two test hunts.
So I say think Wide DD, you get most if not all the depth without all the issues.
 
Hi Charles,
Interesting post. Can you post a pic of the final coil you ended up using? Maybe you could post a pic of the previous "wide" coil also.
Thanks,
Reg
 
Reg the coil is long gone, I typically cut the coil cable off for use on the next prototype but I had saved some pics.
Here are the original wood molds for the 18x12 WDD coil.
<img src="http://www.coilbuilders.com/pics/coilmolds1.jpg" alt="" />
Here are the 18x12 coil finds, note IH's at this site are 8-12 inches down.
<img src="http://www.coilbuilders.com/pics/18x12ih1875.jpg" alt="" />
Here's the silver, some of these were just deep, most of the mercs were on edge, but all were good signals I would not have missed prior.
<img src="http://www.coilbuilders.com/pics/18x12silver.jpg" alt="" />
While we are strolling down memory lane heres a monster coil, the vacuum table is 28 inches across if that gives it any scale. Did I have depth feaver or what! <img src="/metal/html/biggrin.gif" border=0 width=15 height=15 alt=":D">
<img src="http://www.coilbuilders.com/pics/form1.jpg" alt="" />
 
Hi Charles,
The coils I have tried are long, front to back, and narrow in the other dimension. The picture shows an 18 x 6in mono I made for the SD2200. It would have looked better if the ends were semi-circular, and it would have had a true 18 search swathe. This makes it almost impossible to miss anything, unlike a circular coil, where at depth you are virtually searching with the point of a cone. The largest I made was a 30in x 10in which was pulled over the ground on a sled or trolley.
These coil shells were made by machining a block of high density polyethylene on a CNC mill. The poly. is very wear resistant, so the coil channel is on the top side and filled with lightweight resin. Even so, the shaft mounted coil was a bit heavy.
Eric.
 
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