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Concentric coil options
- Thread starter Ronk
- Start date
vlad
Well-known member
Be nice if somebody was listening-here's an article by Steve Herschbach, and it confirms what I've learned. It seems the FRL 10" concentric is it......for now. 
(of course there are the monster DD's by Nel & Mars, among others)
http://www.detectorprospector.com/forum/topic/597-vlf-concentric-vs-dd-coils/
(of course there are the monster DD's by Nel & Mars, among others)
http://www.detectorprospector.com/forum/topic/597-vlf-concentric-vs-dd-coils/
vlad
Well-known member
and from a design viewpoint easier to layout-with less worrisome problems.
From George Paine (who designed so much found in what we use today.)
There is a difference between the standard round and DD loops. On the coils I designed for Discovery I don’t recall a problem with excessive phase shift between the two coil configurations. When I first started designing coils for Discovery I decided to pick a particular frequency of operation and inductance for the Transmit and Receive coils. In addition the “Q” of the Transmit must be control within a certain range. For those who don’t know......the coil’s Q is the ratio of the coil’s inductance to its resistance at a given frequency. Whenever the coils change in size the turns are modified to return the inductance to the standard value. This tends to maintain winding resistance and more importantly, the Q of both the Transmit and Receive coils. So, its important to have standard coil values to target the design to. Normally this would be the coils, inductance, resistance and effective Q. It these values are maintained the resultant coil phase will be maintained over all coil designs regardless of the coils size and shape.
The DD coils can get you if you are not careful. As you know the Receive is generally the same size as the Transmit on these coils. Coupling that with the tendency to keep the Receive turns constant can result in a serious change in the coils output phase. Therefore, the Receive turns must be reduced considerably to lower the inductance back to the standard value. From a practical standpoint the inductance does not have to be exactly equal to the target inductances. As I said the tendency is to keep the turns the same as you change from one coil design to another. This tends to keep the sensitivity the same across many designs. However, that should not be the consideration. In this case its more important to control the phase across many designs. It’s better to look at it this way. For example, suppose that we build two Receive coils where one coil has twice the diameter of the other. But we keep the turns the same in both coils. For this example the larger coil would have an inductance that was twice the smaller coil. These coils would not have the same output phase. The larger Receive would easily have more sensitivity than the smaller coil because of the greater turns and coil area. However, this would not be a good design. The turns on the larger coil must be cut by .707 times. This would make both coils have the same inductance. Ideally we would also need to change the Receive wire size to keep the Receive resistance constant. Remember the coil Q is the ratio of its inductance to resistance at a given frequency. If we keep the inductance and resistance constant then the Q would also be constant. However, I don’t generally change the wire size on the Receive because if you maintain the inductance constant the resistance tends to not change as well. As I said, math calculations show that the wire size should be changed and to what size. But from a practical standpoint the Receive wire size can be left the same. When we reduce the Receive turns on the larger coil the coils characteristics approach the characteristics of the smaller coil. However, the larger coil will still has more sensitivity than the smaller one because of its greater area. The key here is not to get so concerned about the coil’s sensitivity that you forget about the overall design.
All that being said the DD coils do have the worst phase shift away from the target value. However, it can be control within acceptable limits as outline above. I don’t recall the exact phase tolerance on the Discovery DD coils but I think it’s below 0.5 degrees. We always calibrate the fixed ground phase trimmer to be +0.5 degrees. The phase of most soils do not go below -0.5 degrees. {sic}
Putting some of this together with what Steve and others have said, it begs the question why DD's seem to predominate.
Fad or........
From George Paine (who designed so much found in what we use today.)
There is a difference between the standard round and DD loops. On the coils I designed for Discovery I don’t recall a problem with excessive phase shift between the two coil configurations. When I first started designing coils for Discovery I decided to pick a particular frequency of operation and inductance for the Transmit and Receive coils. In addition the “Q” of the Transmit must be control within a certain range. For those who don’t know......the coil’s Q is the ratio of the coil’s inductance to its resistance at a given frequency. Whenever the coils change in size the turns are modified to return the inductance to the standard value. This tends to maintain winding resistance and more importantly, the Q of both the Transmit and Receive coils. So, its important to have standard coil values to target the design to. Normally this would be the coils, inductance, resistance and effective Q. It these values are maintained the resultant coil phase will be maintained over all coil designs regardless of the coils size and shape.
The DD coils can get you if you are not careful. As you know the Receive is generally the same size as the Transmit on these coils. Coupling that with the tendency to keep the Receive turns constant can result in a serious change in the coils output phase. Therefore, the Receive turns must be reduced considerably to lower the inductance back to the standard value. From a practical standpoint the inductance does not have to be exactly equal to the target inductances. As I said the tendency is to keep the turns the same as you change from one coil design to another. This tends to keep the sensitivity the same across many designs. However, that should not be the consideration. In this case its more important to control the phase across many designs. It’s better to look at it this way. For example, suppose that we build two Receive coils where one coil has twice the diameter of the other. But we keep the turns the same in both coils. For this example the larger coil would have an inductance that was twice the smaller coil. These coils would not have the same output phase. The larger Receive would easily have more sensitivity than the smaller coil because of the greater turns and coil area. However, this would not be a good design. The turns on the larger coil must be cut by .707 times. This would make both coils have the same inductance. Ideally we would also need to change the Receive wire size to keep the Receive resistance constant. Remember the coil Q is the ratio of its inductance to resistance at a given frequency. If we keep the inductance and resistance constant then the Q would also be constant. However, I don’t generally change the wire size on the Receive because if you maintain the inductance constant the resistance tends to not change as well. As I said, math calculations show that the wire size should be changed and to what size. But from a practical standpoint the Receive wire size can be left the same. When we reduce the Receive turns on the larger coil the coils characteristics approach the characteristics of the smaller coil. However, the larger coil will still has more sensitivity than the smaller one because of its greater area. The key here is not to get so concerned about the coil’s sensitivity that you forget about the overall design.
All that being said the DD coils do have the worst phase shift away from the target value. However, it can be control within acceptable limits as outline above. I don’t recall the exact phase tolerance on the Discovery DD coils but I think it’s below 0.5 degrees. We always calibrate the fixed ground phase trimmer to be +0.5 degrees. The phase of most soils do not go below -0.5 degrees. {sic}
Putting some of this together with what Steve and others have said, it begs the question why DD's seem to predominate.
Fad or........
I've never had a concentric coil on any of my F 75's, but I am about to get the Fisher 10" concentric. The 10" is a wonderful coil on my Omega 8000 so seemingly should be on the F 75. Our ground right here is fairly mild and concentric's seem to get a bit more depth and work well in mild ground. HH jim tn
REVIER
Well-known member
jim tn said:
A 10" elliptical concentric came on my F70, for three months I swung it, listened to that thing and learned to love it before I ever mounted a DD.
That coil paid for my rig in those first few months in the clad, silver and gold it found and it went on to find me much more treasure in both great soil out west, (10"+ on targets out there), and here in my mineralized SE. dirt in Alabama...which is not mild.
It is still an important part of my arsenal and I will never get rid of it...as a matter of fact I just switched to it yesterday and it will stay on there for awhile while experiment with some new settings I am trying that worked really well on the Nel Sharpshooter.
The tone is different than when using a DD, pure, sweet and responsive...especially over silver and gold and particularly sweet in multi tones.
Enjoy it, I do every time I listen to it sing.
http://www.findmall.com/read.php?37,2296414,2296729
Found this bucket lister using that coil here.
http://www.findmall.com/read.php?91,2240767,2241161#msg-2241161
It was in some relatively rarer black dirt but it it is still mineralized which GB'd in the mid 60's with two bars on the dirt meter.
The numbers said 85-86 and never wavered from two directions which shocked me, depth read 7-8" and when I dug down this was actually sitting there that deep.
It behaved and sounded too good, I had to go after it and I am thrilled that I did.
To this day it is still the deepest and most stable target I have dug so far in my strange devil dirt and I credit that great coil.
Mike Hillis
Well-known member
You can use the 7.8 kHz Triangulated coils on the F75 but they cut off weak signals beyond a certain signal strength. I use the 11" Triangulated concentric coil on my F75 fairly often. It ground balances and is good to about 8" there abouts. Its on or off. No modulation at all.
HH
Mike
HH
Mike
vlad
Well-known member
I.D. is more stable, its deeper, and works better on round iron.
Note some Fisher units are offered with a concentric (notice that the CZ is not offered with a DD, but I have seen two...so evidently it was tried, but not produced)
D. Johnson on concentrics vs DD's:
http://www.fisherlab.com/hobby/davejohnson/SearchcoilfieldshapeApril2012.pdf
Note some Fisher units are offered with a concentric (notice that the CZ is not offered with a DD, but I have seen two...so evidently it was tried, but not produced)
D. Johnson on concentrics vs DD's:
http://www.fisherlab.com/hobby/davejohnson/SearchcoilfieldshapeApril2012.pdf
I've always used mostly concentric coils and never had too much trouble around iron except one time in Arizona I searched where an old gas station use to be and it was carpeted with the old iron crown bottle caps and it was even too much for the DD coil. It was the worst mass of iron I have hunted. I have the Fisher 6.5 football concentric and I'm very happy with it's performance. I was just wishing there was a 7 or 8 inch concentric for my F75.