coil transmit booster question

[kasparov747]

has anyone here ever made a transmit booster for their coil? one that would boost the voltage just a little but not enough to disrupt the phase. i wondered if it would help add a little depth if one were to boost the transmit voltage to the coil? any help would be appreciated. thanks and happy hunting.

[Qiaozhi]

has anyone here ever made a transmit booster for their coil? one that would boost the voltage just a little but not enough to disrupt the phase. i wondered if it would help add a little depth if one were to boost the transmit voltage to the coil? any help would be appreciated. thanks and happy hunting.

In practice, boosting the TX signal slightly makes very little difference. It may even make the detector unstable and result in lower performance. In order to double the depth of detection you would need to increase the TX power by 64 times. This is because the received signal falls off with distance from the coil by the 6th power. So a little extra transmit power achieves very little in terms of increased depth.

[Bill512]

In practice, boosting the TX signal slightly makes very little difference. It may even make the detector unstable and result in lower performance. In order to double the depth of detection you would need to increase the TX power by 64 times. This is because the received signal falls off with distance from the coil by the 6th power. So a little extra transmit power achieves very little in terms of increased depth.

this approximation is true for distances much bigger than the coil diameter and also for targets much smaller than the coil diameter.
But even if the factor is at the 4th or 5th power for some cases, this is not the only reason for someone, to not try this type of boost.
Many other issues are rising ,such are the handling of the (limited) dynamic range, the extra sensitivity to "ground signals" and signals from tiny targets at shallow depths etc...
Even somebody, finally achieve to increase the power and at the same time to maintain stability, i think that the "overall gain" -in the case of a mono sensor and "bad" ground- is extra confusion.

[wam]

What about connecting dummy LC resonating load on connector, make little transformer to dump TX signal into 20W audio amplifier with original coil instead speaker.
Just the idea .. may be someone already did it.
I have tried to improve Sterlings output by 6V by changing gain (resistors for ops).

[Melbeta]

Some information from others about this...

I once asked Charles Garrett about how to soup up a detector. He told me that the manufacturers themselves would do it, but they were restricted by the Federal Communication Commission (FCC), who regulates the metal detector's radio emissions, as a radio device. They are restricted, as I recall, so that one cannot detect the radio emissions from as far away as 100' distance.

He said, other than that, they tried to use esthetic things, making them look better, making the dials look better, better balance, pretty colors, and so on. This was before computer microprocessor chips.

Now I know of some guys, engineers with local electronic plants, that went into the transistor areas, of both the Tx and Rx coils, and changed the bias resistors, lowered the values, which permitted more of the battery power to enter the coils. We tested their detectors against my brand new stock, as I was a Garrett dealer, and they definitely were more sensitive to a air target, and also a target buried in the ground, over the standard non-modified unit.

Also Keith Wills once told me the very same thing about amplification....I have what Keith once told me stored somewhere in my computer.

And others have gone the other route, enhanced the audio sound, that comes out of the speaker wires, to the speaker, and others have even built an audio enhancement circuit, that amplifies the faint sounds, and brings the loud sounds down, so you do not hurt your ear drums, and they definitely permitted one to hear a deeper target than normal...

I use one of those latter devices all the time now on my detector, as I am older and harder of hearing, and with it, I can hear as good or better than the other detector user. I used to find older deeper targets, using a Compass Judge 2, and another guy, had a Compass Judge 1, we searched the same park, and I had a bunch of older coins, and he had none. So he was concerned, and then, I would find a faint target, and he put his detector on it, and could not hear anything, then I would dig it up, and a deep Wheat penny. He was very frustated, and wanted one of my audio units, but was not able to find one for sale.

[douglas]

I find that a higher tx output contributes to more stable operation on the RX alwing the user to crankup the RX signal sensitivity. On a VLF detector.

[giuhalftrack]

this audio circuit you tell looks like an audio compressor

[golfnut]

more tx grunt may be ok on low trash, low or uniform mineralisation ground.

You may need some way of catering for the additional dynamic range required - eg gain control in real time of more than one of the stages.

[golfnut]

Getting more volts into a Tx may not give you what you want - which is more current in the Tx coil. For more H field.

You have to keep it linear so you dont upset the Rx with a forest of spurs.



I have a clamp for my scope and my deus Tx signal has a little kink in it which I was surprised to see

[Skippy]

It doesn't surprise me. I don't know if you followed Henrik's "Deus small coil" thread, but the TX circuitry is surprisingly simple, and the Q factor varies a lot, depending on the frequency selected. Coil L is constant, there is just capacitor switching to give some kind of matching/tuning. I think it's likely these high frequency blips will be removed by the time the synchronous detector has done its job.

The Deus is a good example of why TX power has no significant effect. It has 3 different TX power settings (and corresponding RX gain settings to keep everything the same) to get long battery life. Users say there is no detectable difference between the 3 modes, though I would think that in an environment wth electrical interference, a higher TX level would help a bit.

[golfnut]

I think full power has no advantage in say a busy iron rich roman site.

[Silver Dollar]

A good question is "What is low power and what is high power?"
Can we put 20ma into the coil and get good depth or do we need
100ma or more? I've been playing with oscillators recently and generally
rate them by current draw but I do not know how much current is in
the coil. They don't always agree with the sims and my current probe
gives different results on different days so I do not know how to test this...

[Carl-NC]

The coil current is all that matters (ampere-turns). The supply current is just a measure of efficiency. Since an ideal tank can oscillate with very high currents while "burning" very little current, the concept of "transmit power" in a metal detector has little meaning. A well-designed transmitter will have a lower transmit power than a poorly designed transmitter.

You can calculate coil current with good accuracy if you know the coil voltage and coil impedance (XL+RL).

[Silver Dollar]

So if I measure the voltage across my coil and have the inductance from a
inductance meter can I calculate the ampere-turns or "power"?

I understand some oscillators are more efficient and am looking for a good one.
The Minelab with Davor's AB/C is the best I've seen so far. My current project
is using an exisiting coil with 12.6mh center tapped TX so looses some efficiency
due to the fact the resistance is 28 ohms across both coils. Unfortunatly the
minelab oscillator doesn't like high resistance...

[Qiaozhi]

So if I measure the voltage across my coil and have the inductance from a
inductance meter can I calculate the ampere-turns or "power"?

I understand some oscillators are more efficient and am looking for a good one.
The Minelab with Davor's AB/C is the best I've seen so far. My current project
is using an exisiting coil with 12.6mh center tapped TX so looses some efficiency
due to the fact the resistance is 28 ohms across both coils. Unfortunatly the
minelab oscillator doesn't like high resistance...

If you know the coil inductance, you can calculate the inductive reactance from:

The dc resistance of the coil is

Therefore, the impedance of the coil can be calculated from

As an example, let's assume the coil has an inductance of 6mH and dc resistance of 4 ohms, and the TX frequency is 10kHz:



If the TX peak voltage (sine-wave) is 12V, then the RMS voltage is:

Now you can calculate the rms current in the coil using Ohm's Law:

If the number of turns (N) on the coil is 100, then the ampere-turns are:

From this you can calculate the amount of power wasted in the coil as heat, and the amount used to generate the magnetic field:





It's actually easier to do this in LTSpice, which gives the same answer.