Air-Gap

Tom,

Could you expand a little more on the topic of "air-gap"? To the simple minded it seems that if you can raise the search coil several inches above the ground and still detect a target you should be able to bury the target the same distance as the air-gap and still detect it. Unfortunately, that does not appear to be the case based on you testing of the LTD. Air-gap is an interesting topic that I think many others would be interested in learning about.

Thanks,
Leo

Sure,

The LTD accentuates this phenomenon to an extreme. One of the best targets that presents a textbook example..... is the Type-1 U.S. $1.00 gold coin. Using simple numbers............say, the LTD can air-test this tiny coin to a range of 17" in an ideal EMI-free environment.....with 'cl' process invoked. Bury the coin to 8"......and you can detect the coin easily,.,.,.,.....even with the coil sweeping 9" above the ground. This is to say; 9" air-gap PLUS 8" deep in the ground...... for a cumulative total of 17" detectable range.
Now..... bury the coin to 9" deep. You can detect the coin 9" deep PLUS another 8" air-gap above the ground........ for a cumulative total of 9" in-ground PLUS 8" coil height above ground = 17" total detection range.
NOW......... bury the coin to a depth of 10" deep. There becomes a requirement that the coil needs to be scrubbing the ground in order to detect the coin. Any air-gap/coil-height above the ground..... and the coin goes undetected. Scrubbing the ground becomes a requirement.

In summary; For those targets that are at 'fringe' detection depths/ranges.....scrubbing the coil is paramount ... for detection purposes. IF the target(s) are NOT at fringe detection depth ranges.....a large "coil-above-ground" air-gap nicely presents itself; a priceless 'usable' forgiveness factor....especially in tall grass or lumpy soil conditions. The AMOUNT of air-gap forgiveness factor is dictated by % of dirt mineralization content. This is to say; your conditions/experiences will somewhat vary.

Right now......we are studying this phenomenon......to determine "WHY" this attenuation problem is being encountered. Is it operating freq related, dirt attenuation..... or a speed-dependency sampling rate issue. I'd love to be able to 'correct' this problem..............but.......at least........ this is the first detector in history that can air-test these tiny 'relic' targets to.......what used to be; unfathomable air-test ranges. And........at least........ this performance gain is 'user-friendly' usable......above the ground ...... in a air-gap forgiveness fashion (never before achieved).

Tom

Can I take it that Fisher didn't realize this air gap phenomenon when producing the LTD? If so, that will end up being a good thing, because it will most likely open up some new future ideas.

I wonder if the added air-gap benefit was an unintended consequence of the new LTD circuitry or if it was a design target that was achieved. In any case, I know of no other machine that possesses this advantage.

Quite intentional..........and another step in the right direction.

Tom... can you "share" a light on this one..
many years ago members of the detecting society here in Norway attended a tour to
a farm wth history streching back to the Viking Age.
The farm houses were located on top of a rather steep hill with fields streching out to the south and to the east. The type of soil mostly found here comes from
eroded mineral-free chalkish stone.
My detector for this outing was a special one - the GoldBug 2,equipped with the big elliptic coil.
The find i remember well, was a small cupro-nickle coin,not old and not very rare-
but the depth W0W!--- first the air gap 15-25cm.-then soil depth of about 15cm.so that gives a total of aprox.30-40cm.(12"-16") The stubble was high and very tough
to fight.....
Sorry to say I don`t remember the settings I used......
Later,and after further testing I found this machine to be even more powerfull than the ExplorerXs just because of the ability to pick out even the tiniest bit of metal in any type of ground...
throndr

Sure. Air-gap.......can/could eventually lead to in-the-ground dirt performance...... instead of just (currently) an air-gap gain.

Yes, The Gold Bug-II with Iron Disc invoked is one of the most impressive units available today............. and it's a old platform. Once again,,,,,, a Dave Johnson doings....many years ago.