Rcpt Ack of F75 Ltd prototype

NASA-Tom Wrote:
-------------------------------------------------------
> Something in dire need of further
> clarification.....IRT 5" DD coil on the F75 LTD.
>
> First, let me start with saying........I hunted a
> few fairly clean (non-trashy) areas/sites with the
> little 5" DD coil......where many targets were not
> masked. I was able to ascertain 10" and slightly
> deeper depths on coin-sized targets. Once
> again.....as long as there were no other targets
> in close proximity.
>
> What appears to be a paradox......is the fact that
> the larger 11" DD coil will unmask targets to a
> greater extent than the 5" DD coil in
> target-rich/trashy sites.....with the targets
> being in the 7" - 10" depth range.
>
> Targets needing to be unmasked (actually...
> extremely enhanced adjacent target separation
> conditions) that are in the 0" - 5" range......the
> 5" DD coil wins.....hands-down. Targets in the 5"
> - 7" depth range......and BOTH coils perform about
> the same (with a few exceptions). Targets beyond
> 7"......and needing to be unmasked...... the large
> 11" DD coil is the winner. You would
> think......because these targets are still within
> range of the 5" DD coil.......that it would still
> be the better of the two choices for unmasking.
> Not so.
>
> There are many things that are happening......but
> a couple 'primary' reasons/justifications. First,
> the signal strength remaining at.....say 8"....for
> the small coil......is a very weak signal......and
> ANY other 'fleck' of a target in close proximity
> to a 8" deep target......and the 5" DD coils
> energy is fatally attenuated/shunted. For the 11"
> coil.........a target at 8" is still hardly a
> challenge.......and signal strength remaining at
> 8"....is still fairly strong. A small fleck of
> rust in close proximity to the 8" deep target will
> distort the energy......probably giving a
> incorrect ID.......but will still acquire the
> desired target.
>
> Chose your coil wisely.......and accordingly to
> the specific site conditions.

tom!.is any of this information relevant to the 4x6" concentric coil?
and if not,is it because of the concentric's coil design?..just wondering!
also,if the 5" dd coil's performance IS as stated,HOW does the 4x6" concentric perform under the same circumstances
in your view?..just wondering,as i have the 4x6",but i do NOT have the 5"dd coil.

thanks!

(h.h.!)
j.t.










(h.h.!)
j.t.



Edited 1 time(s). Last edit at 02/18/2011 05:01PM by jmaryt.

There is a WORLD of difference between the 4x6 elliptical concentric........as compared to the round 5" DD coil. I have yet to fully understand the physics behind "why" the 4x6 does not perform......in somewhat similar fashion......to the 5" DD coil. Not even close. The 4x6 coil does not have the adjacent target separation characteristics/abilities of the 5" DD coil........again............not even close. And the 5" DD coils depth capabilities are ever-so-slightly better. ANOTHER performance characteristic that has me scratching my head. If only to see a O-scope field-intensity footprint of both coils!

NASA-Tom Wrote:
-------------------------------------------------------
> There is a WORLD of difference between the 4x6
> elliptical concentric........as compared to the
> round 5" DD coil. I have yet to fully understand
> the physics behind "why" the 4x6 does not
> perform......in somewhat similar fashion......to
> the 5" DD coil. Not even close. The 4x6 coil does
> not have the adjacent target separation
> characteristics/abilities of the 5" DD
> coil........again............not even close. And
> the 5" DD coils depth capabilities are
> ever-so-slightly better. ANOTHER performance
> characteristic that has me scratching my head. If
> only to see a O-scope field-intensity footprint of
> both coils!

Tom, re: the post j.t. quoted 2 topics up .....are you referring to inert Fl soil.....if so how much of that unmasking info between the 5" and 11" DD will/would transfer over to harsher soils that ground grab, say from 55-80....(depending on the exact area) Also, if we threw some emi in the mix .....say enough to keep you with your sens around 30 in bp disc 4 (w/11" coil) ...... would the 5" start to make more of a "come-back", in your opinion ? or might the 4x6 begin to be a better choice .....heavy iron nails and flat iron....



Edited 1 time(s). Last edit at 02/19/2011 03:24PM by silversurfer.

Yes, IRT Florida inert soil.

The 5" DD coil should handle bad dirt better than the 4x6 coil......AND.......provide much better adjacent target separation over the 4x6 coil.

NASA-Tom Wrote:
-------------------------------------------------------
> There is a WORLD of difference between the 4x6
> elliptical concentric........as compared to the
> round 5" DD coil. I have yet to fully understand
> the physics behind "why" the 4x6 does not
> perform......in somewhat similar fashion......to
> the 5" DD coil. Not even close. The 4x6 coil does
> not have the adjacent target separation
> characteristics/abilities of the 5" DD
> coil........again............not even close. And
> the 5" DD coils depth capabilities are
> ever-so-slightly better. ANOTHER performance
> characteristic that has me scratching my head. If
> only to see a O-scope field-intensity footprint of
> both coils!


thanks!..tom!..as you mentioned,i have used the 4x6" coil,and did not find anything remarkable about it as well!
i have read many posts from users of the 5" dd extolling it's virtues in heavy trash, AND commenting on the
outstanding depth too!.it's somewhat ironic in that fisher marketed the 4x6" coil FIRST, so i believe a lot of guys "jumped"
and acquired one specifically for the trash,however,i have read very few guys commenting positively about it,apparently
a lot of 'em "dumped" it,and got the 5"dd when it was introduced!...just sayin!

(h.h.!)
j.t.

Conceptually, I would have never 'halted' Fisher for wanting to invent (and roll-out) this coil.

A paradox, indeed.

NASA-Tom Wrote:
-------------------------------------------------------
> Conceptually, I would have never 'halted' Fisher
> for wanting to invent (and roll-out) this coil.
>
> A paradox, indeed.


as a very wise man one proclaimed "time waits for no man!"
time to get the "little" guy,and "cook" some very trashy sites!

(h.h.!)
j.t.

Push

Rather noteworthy. A few days ago.........in the mid-afternoon, I had a bad EMI day in my test-garden. EMI mitigation not possible. I tried everything. Testing a new-in-box F75 SE. Unit would not detect 10" dime. Immediately, I acquired my old F75 LTD............and..........it too...would not acquire 10" deep dime. Grabbed my 20-year old CZ......and it would acquire the 10" deep dime and 11" deep dime...........but not the 12" deep dime. The CZ was exceptionally stable.........never hinting at any form of EMI. The next morning (6:30am)......all three units would acquire a 12.0" deep dime ((and the 11" and 10" dimes)). No audible EMI encountered. I waited until 2:30pm......mid-afternoon...............and.................sure enough........... both F75 LTD and the new SE would not detect the 10" deep dime. I then switched over to the 5" DD coil for both F75's...........and could detect a dime to 10.2". The 5" coil had better depth, better performance and no 'audible' EMI that I could hear. The next morning (6:30am again)...........both F75's could detect a clad dime to a max depth of 10.4" with the 5" coil. Immediately switching over to the 11" DD coils.............and both F75's could detect a clad dime to 12.0". Immediately switching BACK to the 5" coils..........and both units could still detect a clad dime to 10.4". I did this to verify that no EMI condition had changed......so as to validate the test. ((Some other validation tests were also performed)).
Summational conclusion: It appears the inherent design of the 11" DD coil......is a EMI magnet. With minimal EMI exposure .... this 11" DD coil is a tremendous performer. When EMI becomes moderately heavy...........the small 5" coil will trump the large coil ..... in the depth department.......AND in the stability department. Through years of testing, I knew EMI reduced depth and overall performance......................but.............what I did NOT know (and would have never suspected)...............is that the 5" coil would (paradoxically) trump the performance of the large coil........in every respect.

Not to forget mention of the CZ. This electronic architect suffers with reduced performance (albeit.....not as much); yet, SILENTLY runs at a reduced performance level. Very quiet, stable unit. I would rather KNOW that my performance is running at a reduced level. I would rather be 'armed' with knowledge. When armed with knowledge......you then have a chance of mitigation attempt.

In using the 5" coil, did you increase the sensitivity? Or did all settings stay the same when switching between the coils? Thanks.

Darn glad I have the 5" DD for the T2. Phew!!!!!

Tom, I gotta ask this though. Last time out with the 11" it was high toning (very short duration) on darn near everything. Even items disc'd out. Really was p***ing me off. None of the classic 'EMI' signs (chatter-threshhold instability). Think this could have been a EMI issue due to 11" coil???



Edited 2 time(s). Last edit at 05/08/2011 04:44PM by TerraDigger.

Tom, I would think that before you branded the 11" DD an EMI magnet that at least you would attempt to determine the source of the EMI, the offending frequencies, and whether the EMI is above the ground and/or in it. Do you have access to a spectrum analyzer for just that purpose ??

Tri

With all due respect........what difference would knowing the frequencies/sources do to stop the EFFECT. That's what Tom is talking about. A simple observable effect of fluctuating EMI levels in a urban enviroment and how each coil size reacts.

Terra, I guess I am the type that likes to know the how and why of things.To me, attempting to find what the offending EMI is , seems to be the first step in solving the problem, as Tom said the larger coil is best if EMI is minimal or not present, however if EMI is rearing its ugly head, then my curiosity gets the best of me and I want to solve the problem, especially since both coils are tuned to the same frequency then WHY is the larger coil affected more, is it a certain frequency of external EMI, is it something to do with electromagnetic radiation from the sun, is it microwave radiation related, is it the neighbors wireless router across the street, a kids walkie talkie, the smart phone or your hip..........and even more importantly why does it even happen when we consider ourselves in remote places , apparently far from obvious sources of EMI ? I think we all agree there is something odd going on...........How can you attempt to stop the effect if at least you don't ask WHY....????????

Tri

I will venture a guess......as I am not a engineer. My theory.......the larger surface area of the larger coil....picks up more EMI interference simply due to its larger mass. I like your train of thought though. Could a damping compound be applied to a coil, except for the underside, which transmits/receives the electromagnetic signals. Thus reducing the effects of a sizeable amountof the EMI a coil picks up.



Edited 2 time(s). Last edit at 05/09/2011 02:12AM by TerraDigger.

Seems plausible, Hey Tom, could it really be a case of "Occam's Razor" ??

I did try every possible set-up configuration......with both coils.......to find out (if/what set-up configuration) would present best possible performance. I started by using a 'standard'...so as to have a base-reference ... for initiation.

Yes, I would like to know the exact cause(s) of EMI...........as this would be the first step (and a major one) in resolution (or mitigation) acquisition. Kinda hard to 'fix' something.......if you do not know the source/cause of the problem.

It would be nice to have a spectrum analyzer that you could run (across a very wide bandwidth) for several days and verify a correlation between certain EMI frequencies and time of day depth attenuation validities that you yourself have absolutely verified. Like you say there are many variables. I remember the old ma bell days of C-band sat-tv when microwave repeater towers facing into your dish caused me many a headache. Back then there were two solutions, inline IF filters or better to relocate the dish using a structure to block the terrestrial interference or situating said dish so the TI was facing into the rear of the dish.................thereby mitigating all.....problem solved.

Tri

I'm sure Fisher has a standard test where they subject a new design to a controlled, pure sinewave radiation then measure the detector circuit output. They know their Achilles heel but that's going to be proprietary information.

It's just not so easy to design a countermeasure across an infinite range of frequencies.

But I agree. It is the first step to measure the offending environment.

I ran into a similar situation a couple of days ago. I was in the middle of BFE at high noon with no electrical lines etc. around for miles. The 11" DD coil was too noisy to be effective. I ran through all of the frequencies and all of the settings, no joy. I put the 5" DD coil on and the difference was night and day. In fact, the difference was so drastic that if not for Tom's recent post here I would have been convinced the 11" coil was damaged.
I understand that the hotter the circuit the more noise you should expect but there has to be some sort of compromise for ground coverage, sensitivity etc. I wonder if the 6.5" elliptical or the 10" elliptical coil would yield better results than the 11" DD in this type of situation?

So, can someone confirm/deny my larger coil mass theory. I am not a engineer for cryin out loud

Well Terra, I do not have an engineering degree but I perform the duties of a mechanical engineer all day every day so I don't know if this will be credible or not but here goes.
The efficiency of an antenna/coil is determined by many things. The magnet design engineers I work with have explained it to me but I'm a mechanical guy and have not had the time to research it in depth.
The gauge of the wire, diameter of the coil, number of turns etc. all play a part in this. I copied the following from [www.vlf.it]

Some rules of thumb for loop sensitivity are:

* For a fixed number of turns:

* Sensitivity goes up as loop diameter squared, and up as wire length squared. For a fixed wire length:

* Sensitivity goes up as the loop diameter, and down inversely as the number of turns. For a fixed loop diameter:

Sensitivity goes up as number of turns, and up as wire length.

Which shows that "Turns are good, but size is better!" and "Use as much wire as you can!"

Does this make sense?

Sure does make sense. Thanks.

Good capture.

Resonance also plays a important role. And..............For some reason......the elliptical DD coil is more susceptible to the vertically polarized plane of EMI.......even whilst horizontal///parallel to Earth.

Tom, this may sound a little odd, but when you used the term, " more susceptible to the verically polarized plane of EMI" , that got me wondering if
doing your normal tests perpendicular to your normal earthly parallel plane would make any difference, (ie mitigate) the EMI .....? The reason I say this is because it may narrow down the source of the beast.

Tri

Tom, do you have the 10" elliptical to repeat the test? This may be the optimum, deep "low EMI" coil for the LTD.

I wonder if the new 15" DD presents a worse configuration.



Edited 1 time(s). Last edit at 05/11/2011 01:27AM by go-rebels.

The non-DD 10" elliptical coil is a fair drop in depth/performance......and EMI would have to be quite bad....so as to mitigate EMI to the point of where the 10" DD coil starts to become an advantage......BUT.....this 'level' of EMI DOES occur.

The horizontal plane of EMI is quite minimal.....as most of planet Earth EMI is on a vertical axis. So far.............in all of my testing........... the attempt to mitigate 'vertical plane' EMI has the most pronounced effect in real-world performance.

But then who'd thought that the 5" DD would have trumped the 11" DD?

Tom, is your bad afternoon EMI condition repeatable day in and day out?

What are your techniques to mitigate 'vertical plane' EMI?

I can confirm 'day time'......yet, the specific time 'shifts'. I seek to find commonality........or......at least a 'window of time' that is fairly repeatable. So far; no joy.

Vertical plane EMI. Simply hold the coil vertical.......and rotate your body until you find a specific direction with the most EMI. Then......F1 - F7. (((Sometimes you can find leaking powerlines by using this procedure))).

I know coils have electrostatic shielding...........but do you think a damping material exists that would shield the top & sides of coil from EMI...and would not compromise the proper operation of the coil. And I guess if the material exists it could be applied to the control housing also. My one, still existing, 'theoretical' cell in my brain keeps telling me its technically feasible.