Had a question for Tom about the air gap T2/F75 LTD

Hi Tom,

In one of your posts you wrote that:

In 'bp' mode...... I am not ascertaining additional 'dirt'
depth on targets. (Many different sweep-speeds tested). What I AM finding.....
is additional air-gap depth. What I mean is: In my test-garden, I have a clad
dime at 12". In any of the old modes (except 'bc'), the Ltd and also the older
F75 will ascertain this dime JUST BARELY.....and SCRUBBING the ground is a
requirement. It is a 'fringe depth' target. Invoking the 'bp' mode.... and the
Ltd will acquire the dime with a much longer duration audio response.... and up
to 3" additional air-gap above the ground. This leads one to believe that the
unit can detect the target to deeper depths. 12" + 3" = 15". Not true. If the
dime is buried to 12.4"........ it is not detectable in ANY mode.....including
the newer modes.

Can you explain why the 3" in the air will not translate to at least 1" in the ground? Are we at the limit of how deep a VLF detector can penetrate the ground or is it because the sensitivity of the receiver can't be increase due to EMI, and any additional gains in sensitivity will be due to better signal filtering and only improve depth in the air and signal strengths of fringe targets, but only on targets around 12" in the ground? Is it because the transmitter in the F75 LTD is the same power as the F75, but the processors and programs allow for better filtering and that's harder to do in the ground? I am referring to the dime as the standard here.


Dave Johnson answered this question somewhat, he wrote:

"Getting extra depth out of a VLF, multifrequency, or PI machine is very difficult, because these machines follow an inverse 6th power law relationship between signal voltage and depth. If everything else is maintained equal, doubling the depth requires 64 times as much signal. If this is done by increasing transmitter power, doubling depth requires 4,096 times as much battery drain. That’s the basic reason why depth increases come so slowly in this industry.

The biggest impediment to getting usable depth in the ground, is interference from magnetic and electrically conductive minerals in the ground, which can produce signals hundreds of times as strong as that of the metal target you’re trying to detect and hopefully identify. There are several approaches to extracting the metal signal from the ground mineral signal, but they all have their limitations. That’s why you see several different technologies coexisting in the market."

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So let's say you want to improve depth by 50% on the F75, so that 12'' dime would now be detected at 18" with a more powerful transmitter, that means the battery requires 2050 times the battery drain. I don't know what the battery drain on the F75 is but I am sure it's not very much since it uses 4 AA batteries. Batteries have come a long way, some of the Lithium Polymer batteries used in RC planes and race cars can sustain very high drain, something that would quickly kill the 4 AA, those are not really designed for high drain devices. I have no idea but I think a Lithium Polymer battery pack the size of a car battery that can be carried in a small backpack can putt out 2000 times more drain then the 4AA batteries. Lipo's are very light, so that battery would probably only weigh about 5 pounds. And if such a battery can put out 2000 times more, that detector would be able to hit a dime at 18", according to what Dave is saying. I don't think you need to go more then 18" really! Now if this depth gain is from an improved transmitter, EMI should not be a limiting factor, although still a problem.

I think a lot of time has been spent in recent years improving the sensitivity of the receiver and using the increased processing power to filter the signals, that's fine if you can reduce the effect of EMI, and I am sure they are trying that. But the transmitting power aspect might have been overlooked because off battery limitations and impracticality of carrying a large heavy battery. But I think it might be time to consider this direction, I think everyone here would not mind carrying a 5 or even 10 pound rechargeable backpack to power their F75 if it can detect dimes at 18", and I think a 10 pound Lithium Polymer battery would be able to provide 2000 times more drain then 4AA , I am sure there are infinite variables but if it all boils down to having a super battery I think they should consider this. What do you think?

Thanks!

Dave J. & I racked-our-brains-out on this one. These results on the clad dime took place in my test-garden ....which has low/nearly-no mineralization. And since Dave's brain (and my brain) never sleep..........resolution acquisition is still a quest. This is to say.......we don't have an answer (yet). I surmise it is a geophysics problem more than electronics. Yes, signal strength remaining at 12" is near-to-nothing. Signal strength departing a coil/antenna is inversely asymptotic......rapidly approaches zero.....IRT remaining 'usable/measurable' energy. EM attenuation at 12" is a DE's nightmare. Increasing X-mitter power may not be the answer......but may help a bit. Rcvr sensitivity is more important (and comes with a plethora of inherent 'challenges' ). ---Rcvr timing/sampling still in question.--- From a technological standpoint.....a paradigm shift in current thought-process may be due/overdue. (Easier said than done).

Just for the record; I also tried a smaller coil on the LTD.........just to see (and measure) linearity differential. Nearly the same problem..........in linear fashion......possibly leaning towards mineralization attenuation (geophysics problem); yet, microprocessor clock timing/sample rate still unresolved. There lies a multiplicity of other problems/possibilities unto which would create this phenomenon/anomaly.

I know this is comparing apples-to-oranges......IB VLF technology vs PI........but makes a wonderful point. A CZ w/10.5" coil will just barely detect a nickel at 12" in my test-garden. This particular CZ utilizes 27mW for Xmitter power.
Utilizing an AquaStar... and the same nickel at 15".... is barely/fringe detectable. The AquaStar is around 60W of Xmitter power (when Xmitter is switched on). The difference between 27mW vs 60W (said differently: 60,000mw) is tremendous...................yet..................the gain in real-time performance is negligible.

So far, Dave & I can only speculate as to why the F75 LTD gains air-gap ONLY additional performance over comparable/other units. Utilizing a low conductor United States, Type-1, 1-Dollar gold coin.......and the air-gap differential is much more accentuated.

Dave cringes when more depth is requested. It hurts badly......is asking for something nearly impossible...............yet, poses one of the greatest opportunities.

I am continuously exposed/reminded of depth criticallity when.......at a construction site.........say a nominal 7"-8" of topsoil is removed. Then.......only 9" or 10" deep........here comes early wheats, Indian Head's and Barber silver. This is to say that 8" + 10" = 18" depth is where 'success' is located.

Yes, a barrier/plateau NEEDS to be broken through. With the advent of the 1266-X......we have been at the 12"-on-a-dime barrier...for the past 20-plus years. I'm certain folks would be willing to dig just a few more inches for EXCEPTIONALLY rare/older targets. If only folks could hunt with a (16" capable on a dime) unit......even for only just a few days.....would certainly startle AND EDUCATE all folks as to: HOW DO YOU KNOW WHAT YOU ARE MISSING.......IF YOU DON'T KNOW IT EVEN EXISTS. And ................until then............................YOU DON'T KNOW....WHAT YOU DON'T KNOW!

Tom

Thanks Tom,

I had to read over this a few times since I know nothing about electronics, but I think I got the idea. Believe me, I know what I am missing, I can just feel those old and deep coins in the ground that are out of reach. Obviously it's my imagination, but I tend to believe that on most old sites there are a vast amount of old coins in that 15 to 20" layer. The construction site example is perfect.

Regarding my idea about the huge battery to power the transmitter, after doing more research, it seems they actually had options on Teknetics ST where you could increase the amount of power going to the transmitter, or something similar. It seems however that if you increase the power of the transmitter drastically, a whole set of other problems comes up that need to be solved on the PI, but I am not sure about VLF.

Lets say we do have a powerful battery that can power a much more powerful transmitter, what problems would arise from that for VLF?


Finally, if we do someday get a detector that can pick up a dime at 18" then we would have to get rid of control boxes and hang them around out necks!

Thanks Tom and good luck hunting this weekend!

Feedback overload ..... along with coil gauge windings...... heavy duty components enhancement...... heavy duty batteries/heavy unit.....,,,,,, are just a few problems with hi-powered Xmitter.