Coil Output power

As I understand it, the strength or power output of a coil is limited by FCC regulations. It is very possible to obtain more depth but the FCC will not allow it at this time. Does anyone know what these regulation are and if they also apply to individuals who like to tweek or modify their units.

Tom in SC

With today's technology....... Sensitivity is much more important over transmit power.

Tom, outside of high quality headphones, any way to goose up sensitivity?

It just seems like sensitivity will still be limited by the depth. Depth by transmit power. Higher frequency and more power should get both. Am I wrong? the Fisher Goldbug 2 is very sensitive but it has limited depth.

Tom in SC

The higher the frequency..... the less penetration depth .... especially in mineralized ground. The higher the transmit power...... the more feedback issues from the dirt and the unit itself.

Right now......... we are incarcerated into 'nearfield' detection boundries. Without making a paradigm shift in platform architect............ a reduction in EMI (from both internal & external sources) will increase depth..... along with mitigation of mineralization effect. I would like to see a leap in technology in coil footprint design. If only to have a much more condensed (focused) electromagnetic footprint..... without infringing upon induction balance principles.

From my understanding , a conciderable amount of power that would be needed to make much of a difference , would only allow a short amount of detecting time due to the extreme drain on the batteries ......AND many more batteries would be needed to make a difference .....Jim

Synth, an external battery pack could be utilized, like for hobby race cars. Custom built.
My $64.00 question is that if there is a limitation as to power levels regarding sens/transmit, how come a Minelab GPX unit which is VLF, can achieve such extreme depth/sens over other VLF's. And it can do it in the worst of soils. It also must stay within FCC regulations. Obviously, at least to me, there appears to be more to achieving depth than sens/transmit power. Superior design seems to be an answer.

Good question Terra.

If you really want to get into it check part 15 of the FCC rules. It happens to be on my Xcal.... says THIS DEVICE COMPLIES WITH PART 15 OF FCC RULES: OPERATION IS SUBJECT TO THE FOLLOWING TWO CONDITIONS (1) THIS DEVICE MAY NOT CAUSE HARMFUL INTERFERENCE, AND (2) THIS DEVICE MUST ACCEPT ANY INTERFERENCE RECEICEDD, INCLUDING INTERFERENCE THAT MAY CAUSE UNDESIRED OPERATION. This harmful interference seems to have various celling based on device ... like cell phones, wireless radios, toys ect. Could be the GPX limits are based on where its intended to be used.... and some people are using it in unintended places.

Dew

GPX is a PI unit.

Tom, hand me a towel. Need to wipe egg off my face

Let me ask a different way then. What is it that some units have (VLF units of course) that make them much deeper than their competitors. Coils/frequencies/etc being somewhat equivalent between units. Is it better quality circuitry components/software?

Now i have to ask how a VLF machine and PI might vary in coil receive and send?

I don't have an electronics background like alot of you folks obviously do.

1) So my donkey mind tells me that it takes power to create deep penetration . . . raw power.
So a TX running at *9V's or *12V's is going to perform better than a TX running on 6V (4 AA's).
*If everything is optimized at that power level and designed for max. depth.

I don't think I've seen any high end MTr's designed to run off of 4 AA's??? Have you???

2) Of course, the RX would need to be fully optimized for high performance to match the TX.

3) And all of the other associated electronics would need to fully handle this increased power
and seamlessly handle EMI ( and other concerns ).

* Some have said that a thicker coil penetrates deeper. Any thoughts?

If so? I wouldn't mind having a double thick ( taller from top to bottom ) 5X8 DD if I could
gain another 4". It would still be lighter than my 8.5X11 DD. Just dreaming . . . .

Terra..... it is in the engineering architect as to 'how well' the unit can 'see & report' VERY minute' signals. If internal "baseline" noise of the unit itself...is too high; it then cannot differentiate between high 'circuit noise' vs 'weak target' response.

Engineering architect, internal component cross-talk, electronic component stability/drift factor, EMI mitigation, resonance....... and a host of other factors......................... of which............... all narrow down to engineering 'attention-to-detail' design.

Dew........... there is a tremendous difference between "HOW" a VLF unit sends/receives........... vs ............ a PI unit.
When a PI unit is 'receiving'......... the transmitter is completely turned off.
When a VLF unit is 'receiving' ...... the transmitter is 'on'. (Induction balance critical).

Food-for-thought: A PI unit transmits substantially more power over a VLF unit. In fact........... when a PI's transmitter is 'on'............. it may be transmitting 1000-times more power than a VLF.
1) How can we get away with this? (((Hint: Duty Cycle)))
2) Then why does a PI not detect objects 1000's of times deeper than a VLF?

Does the PI unit use a alternating signal or a single pulse simular to flipping a switch on a DC circuit?

Tom in SC

Usually a one-shot pulse.

thanks, that explains a lot to me.

Tom in SC

Tom, your answer fits. Especially since you are always reminding us that lack of consistent manufacturing precision is what is holding some of the current machines back.



Edited 1 time(s). Last edit at 02/03/2012 05:14AM by TerraDigger.

Even more food for thought... how much power does a typical VLF detector transmit?

As an example, let's say we have a tuned (resonant) coil running at 10kHz on 10 volts with 100mA of TX coil current... so how much power gets transmitted?

CZ transmit power is 27mW.

Is this a standard for all VLF units or just the CZ?

NASA-Tom Wrote:
-------------------------------------------------------
> CZ transmit power is 27mW.


Are you sure?

I suspect there is some confusion over what constitutes 'Coil Power' here. Several possibilities exist: A: How much power is consumed from the batteries as a result of driving the TX coil, B: How much total power the coil itself dissipates (heat and RF transmission) and C:How much power is actually transmitted as RF/magnetic field. Clearly case A is largest, B next, then the smallest by far will be C. I don't have any decent figures for any detector, but ballpark ideas would be 30mW from the battery, 15mW heat loss in the coil, a couple of mW transmitted.

This is a fairly common standard......... not just the CZ.

The largest power drain on most detectors........ is driving the external speaker. Especially in pinpoint mode (or batt test).

One very experienced designer made a nice comparison: putting more TX power is like turning on big lights in the fog.... you will not get much of improvement.

Geotech Wrote:
-------------------------------------------------------
> Even more food for thought... how much power does
> a typical VLF detector transmit?
>
> As an example, let's say we have a tuned
> (resonant) coil running at 10kHz on 10 volts with
> 100mA of TX coil current... so how much power gets
> transmitted?

The correct answer: Zero.

Detectors only develop a local magnetic field and, unlike radios, don’t propagate electromagnetic energy. Magnetic field strength is determined by the size of the coil and the “ampere-turns” — that is, the number of turns in the coil, and the peak amperes flowing through those turns. In electronics, an ideal inductor does not dissipate power, so “output power” for a detector has no real meaning.

You might think that a certain amount of “power” from the transmit circuitry is needed to achieve a desired magnetic field strength, but that will depend on the design of the circuitry. A transmit coil in a resonant circuit will effectively recycle much of the coil current, making it far more fuel efficient than a brute-force non-resonant “driven” coil (as found in a multi-frequency design). In fact, an ideal LC tank circuit is purely reactive so even the power consumption will approach zero.

The practical limit on detector field strength has more to do with practical battery life and signal-to-ground ratio than FCC regulations. As you crank up the transmitted field strength ground and target signals go up proportionally. Because deeper targets have more ground above them, this is a losing situation for achieving greater depths. Ergo, the FCC doesn’t really need to limit detector “power” (and we are no where close to any FCC limit anyway) — physics sets the limit for us.

- Carl

The power put into the transmit-coil drive on a modern machine is still a modest proportion of the total consumed (amplifiers ,comparators, A to D converters, microcontrollers, LCD drivers, etc all take power) so TX power could be increased and still give presentable battery life. Eg. if an F75 ran for 10 hours instead of 40 hours, that would be OK with most folk. But as Carl says, there's little need for it.
One other issue is the fact that such a detector would interfere with other machines more. Would you be pleased if your friends' super new detector stopped yours working from 400 metres away?

> Would you be pleased if your friends' super new detector
> stopped yours working from 400 metres away?

Put another way, would you be pleased if your detector ran everyone else off?

When this was discussed recently on Geotech's forum, I'm sure someone (Qiaoshi?) reckoned that this WAS a good thing. A win-win situation?

Quote
DEWCON4414
Now i have to ask how a VLF machine and PI might vary in coil receive and send?

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