Malfunctioning 11" DD Tek/Fisher Coil for Tom D./Others

This being my first DD coil, I want to run this issue past you before I contact the manufacturer. This regards the stock coil on my T2, but input from any users of the Tek/Fisher 11" DD coil is appreciated.
The problem is that the strength of detection does not appear to be equal along the length of the coil. If one were to draw 5 imaginary horizontal lines accross the bottom of the coil, the results are as follow using a copper U.S. penny at 6" from the coil bottom, to circumvent the DD tendency to misread objects if to close... approx 3" from coil..... The area I am measuring is the area covered by the raised spine on back of coil, not the flat areas at the very edge of the heel & toe sections.

(Air Tested)
1. The middle section (2/5th's of area) have a detection strength of lets say, 100%.
2. The heel section (1/5 of area) is 80% strength of the middle section.
3. The toe section (1/5 of area) is 60% strength of the middle section.

Along with the diminished audio strength (toe & heel) is also a diminishment in correct VDI reading.

Correct me if I am wrong, but I was under the impression that coil signal strength was evenly dispersed along the length of a DD coil where the windings are located. The way this coil is behaving, it's almost like a concentric coil, in that the concentric coil center is super strong and the areas going away from the center weaken.
Is the above how the 11" Fisher/Tek coils are supposed to react to targets?

My 5" T2 DD coil does not have these toe/heel extremes.

Also, if I take a iron nail/iron object and sweep it back & forth in a horizontal line from 1/2" outside to 1/2" inside of the outer edges of the coil, it gives the highest tone (coin) even at max disc. That may be normal for a DD coil, but I thought I would ask.

Lastly, the 11" coil seems to do a very poor job on coin VDI's, like a 10 point variance, on a dime at 5" (in the ground). Yes, they are clean target signals with no surrounding trash (this verified by using all metal pinpoint mode). I can understand a couple of numbers variance at this depth, but not 8-10. But again, as I said, this is my first DD.

Could improperly fabricated coil windings be the cause of the contents of this post? Or perhaps software related?

If someone could replicate my penny test on their T2/F75 and see if their coil behaves similar to what mine is, I would be deeply appreciatve. Just want to know before I pull the alarm button

Thanks to Tom D. & all who reply.



Edited 4 time(s). Last edit at 05/27/2011 08:04AM by TerraDigger.

My thoughts are are call the manufacturer and ask to talk to a tech as over the years faulty coils can cause a variety of problems...Perfect scenario if a club member has one do a comparison test.
Years ago bought a XLT new and didn't seem to be working right and gave it to an XLT Guru and he agreed with me and to make a long story short Whites tested and put on a new coil under warranty and worked perfect upon its return..

Heel-to-toe..........the elliptical DD coil DOES INDEED 'footprint' like a concentric coil; contrary to popular belief. The Top Dead Center (TDC) of the coil is the hot spot. As you move further up or down the spine/beam.....it'll get weaker. The 5" DD coil is a completely different EM footprint. Nearly a spherical footprint.....with a bit more energy in the center of the coil......especially heel-to-toe......but not by much. (Enough to make a real-world difference).

HOWEVER; your resultant sounds a bit too excentuated....heel-to-toe; yet, it's the fairly large delta IRT your ID readings......from heel-to-toe. A ID span of 5 or 6 is more normal for these DD coils. But 10 is a bit too much. Can you.............do you have access .... to a different 11" DD coil for testing purposes? I am also concerned about this 'in the ground' condition......as a small (unaudible) fleck of rust in close proximity will cause exactly this......and you would never know it's there. There are also TOO many other variables induced when dirt testing. How does a air-test perform?

Raised spine = you talking about the top of the coil?

Raised spine = you talking about the top of the coil? =Yes

No, I do not have access to another 11" coil

On air testing, the VDI works fantastic with no variance in #'s. It just seems that in-ground with the 11" its giving off short high tones continuously, even if disc is at max and large VDI variances. I have done the reduced GB as you recommended prior. Still exhibits the same behavior. That's why I included the part about the 1/2" edge testing in this post. The 11" coil in the field is almost unusable because of this behavior. If this is the way the T2 operates, it leaves alot to be desired in mineralized soil. It just seems to have a over-fondness for iron/other junk with corresponding high tones. I know, the machine was designed as a relic machine, but I feel a T2 should not high tone this much.
The other thing I noticed is that if I take a coin & a bottlcap (air test), set disc to allow both, switch to 3b mode, the signal sounds the same. Isn't 3b supposed to sound off differently on the bottlecap. And yes, its a old style rusty cap (I dug up), not one of the newer ones that sound like coins. The same bottlecap on my White's sounds like a bottlecap signal.
Then there is the issue, not related to above, where switching to the F1 channel, by pulling the toggle inward only, results in the channel being a continuous high pitch squeal. No frequency can be heard. Have tested at over 10 different sites, always exhibits this behavior on F1.

Thanks for your help.

Hmmmmmm.............the last thing I recommend is...sending the unit in; however, it sounds like it is due. I would state your multiple problems/encounters.......and include the bottlecap with your control panel set-up.....so they can replicate your encountered problem. Still.......sounds like a form of a shielding issue.