DD coil issue

Two comments:

1)   A VLF/IB detector doesn't have to radiate energy out into space like a radio transmitter, it just has to create an oscillating magnetic field.  This will still happen if energy bounces to and fro between the coil magnetic field and the capacitor electric field.

2)   Many (possibly most) commercially produced VLF detectors use a transmit coil plus parallel capacitance which is operated at resonance (and which determine the operating frequency).  They work perfectly well. 



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		Quote:			Posted by GoldDigger1950
	What you need is at least 3db down from resonance at a minimum and more if you want a large field

Do you have to be 3db down or can you go 3db up?

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Xavier

Ha yes OK got it. Now how can one measure this? what would the formulae be to calculate it and where would you do the adjustment?   

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Xavier

		Quote:			Posted by Gwil
	Thank you for replying to my comments.  I'm not sure if I understand you correctly - you said that " a resonant circuit will mean that no (zero) magnetic field is developed in the transmitter coil ".  This would mean that at resonance there is zero current in the coil, since a current must be accompanied by a magnetic field.  Is this what you meant?

Almost. The resonant circuit, when in perfect resonance, consists of three components. Resistance(R), capacitance(C) and inductance(L). When in perfect resonance, all of the energy is kept inside of the RCL circuit with no loss to the outside. In order to develop a field around the coil, there must be some de-tuning. The resistance of the circuit might be enough to develop a small field but to develop a large one, you must de-tune by at least 3db from the center frequency. The coil is the one component that must match the frequency in order for the circuit to work. The capacitor and the internal resistance are the components to fiddle with to get a field built. Remember, we're not talking about DC in a coil making a field. We're talking about current swinging back and forth in a tank or resonant circuit.

In a perfectly resonant circuit, a single burst of energy would be circulated back and forth in the tank with no loss at all. In reality, the internal resistance develops a very small field around the coil and the loss from that causes the pulse to die very quickly. If you keep feeding it with a series of pulses, the same tiny field will build up constantly and then die off. If you want a bigger field, you have to throw in a component of resistance to set it off balance and keep the field longer and make it larger. This is accomplished by changing the wire size using smaller diameter wire to increase the resistance. If you increase it so the circuit is 3db down from the center frequency, you get the optimum field buildup and a reasonable collapse time so the receiver can sense any residual magnetic fields in our target.

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GD , Being the Obstinate bone head I am, I dis agree on the the No Current being emitted at the Natural Residence of the the coil.

Any time there is a current flowing in the coil it will produce a Magnetic Field.

There is or was a Tank Circuit pinging in the Smithsonian for more the 70 years.

The only reasoning it was still ringing was it was in a Vacuum and no drain being put on the power.

If there was a Ground, Target or  any other Item to Influence it it would have dissipated and made a mark of were it was.

In this one They use Gold to Tilkle it to keep it running.   ( Give it a kick OUT of phase to it Running ).
 

Ok they Knock it out of phase.

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