Coil Design and details on nulling (both concentric and DD)

You don't "null" a coil all by itself. The coil is part of a much larger circuit and the entire circuit must be tuned, not nulled.

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The coil does need to be installed on the detector but first it should be designed with the detector itself in mind. Let's say you want to replace a 6 inch coil with a larger one. You first have to know the makeup of that coil. Is it concentric? Double D? Coplanar? Is it a monocoil? Once you know this, you have to measure the parameters of the coil. Let's assume it's 10uh or 10 Microhenry. You must design your coil to the same inductance value. The coil diameter and the end value are plugged into the formula and the wire size and number of turns can then be estimated. When you wind the coil, measure the result before you install it for the best result. In most cases, close is good enough because there are other factors involved in the full circuit functionality.

The coil is part of an oscillator circuit when it is used for transmitting. It doubles as a transmitting antenna. As such, the tuning will be for maximum field generation. This sometimes means replacing a capacitor or adjusting one, if it is variable. That's half of the tuning process. As to what value or voltage to expect? The answer is the maximum, whatever that is. Some technicians like to watch the receiver for the transmitter maximum. Sound confusing? Well, the only way to sense field strength is with a matching receiver and the closest one is the detector's own receiver circuitry.

Which leads me to the second factor in tuning. Your receiver must be tuned on the front end (the coil side) for maximum resonance with the frequency of oscillation. Again, it is sadly not just "measure the voltage here and it should be X volts." You have to adjust for maximum receiver response without swamping the circuitry. You also have to adjust it for a response in the air with no target present and again with a metal target. The multitude of detector types make this impossible to describe in any detail beyond saying it must be adjusted or tuned.

In actual practice, you will be going back and forth between transmitter adjustment and receiver adjustment until you get it right.

Some detector coils are clever beasts. In the connector itself, which may be a multipin device, jumper wires reconfigure the detector for different sized coils which have markedly different parameters. If your machine does that, all bets are off.

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		Quote:			Posted by BitburgAggie_7377
	GD.....darn your good---I actually understood a good deal of that (and you know how technical I ain't)

Run away screaming! This may be the end as you know it! Your innocence is at stake, mate!

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I would like to supplement GoldDiggers description with a bit more information about nulling.

The principle of a VLF detector is that when an alternating current flows in the transmitting coil (Tx) is it will radiate a magnetic field that will energise the target. In turn, the target will then re-radiate its own magnetic field. Depending on the type of metal in the target, the two fields will vary in phase. The receive coil (Rx) is intended to pick up only the target?s field. By comparing its phase with the transmitted one, the target can be identified. That?s the theory anyway. But with the TX and Rx coils in such close proximity, the Tx coil will also induce a signal directly into the Rx coil and it is that induced signal that needs to be nulled out. If not, the combination of the two fields could overload the receiver preamplifier.

So how do we null it out? Well, if both coils were placed one above the other a clockwise current in the TX current would induce a clockwise current in the RX coil because the magnetic lines of force would be travelling  down through the middle of both coils. But if the two coils were placed side by side the RX coil current would be reversed. That is because the lines of force go down through the Tx coil and back up through the RX coil. What we need is a critical combination of those states so the two opposite currents cancel each other out. With a double D this is done by laying the coils side by side then gradually overlapping them until the signal picked up is zero ? or as low as possible.

You can most easily do this when the detector is finished because you will have the Tx coil fired up and the receiver running, but if you want to do it at the construction stage, then apply a frequency of about 5kHz across the ends of the Tx coil and measure the response between the ends of the Rx coil with a scope.


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