DD-coil sweep direction ?

[F117]

This will probably sound like a stupid question and my apologies if it has already been covered somewhere else on the forum.
When capturing oscillograms of the Deus2 transmit coil with different settings, I noticed that the transmit coil was the left half of the DD coil and the RX coil the right half.
Determined empirically with a pickup coil attached to the scope probe. See https://www.geotech1.com/forums/foru...813#post417813
This triggered my following question:
When you sweep the coil from left to right, a target passes first under the RX coil and next the TX coil.
When you sweep from right to left, a target will first pass under the TX coil and next under the RX coil.
Is there a theoretical difference between both directions ? From right to left, the target will first be energized by the TX magnetic field and next pass under the RX coil.



I would expect that the target will only produce eddy currents after having passed under the TX coil field ?

Experience from the field could more or less back this up: at the end of many hours of treasure hunting, your walking pace gets slower.
That is when the small targets start to pop up. Probably because you pass over the same target in both directions.
And with a lightweight detector like the Deus(2), the swing speed will probably be something like 1m/s at the start of the day, and gradually slow down, resulting in multiple passes over the same target in both directions.

Can somebody confirm / debunk this theory ?

[pito]

This affects the polarization of the phase, say in the case of a coin you will have + 10 degrees then - 10 degrees or vice versa, to eliminate this the absolute value is used for calculations.

[F117]

A central TX coil with an RX coil on both sides should eliminate the sweep direction ?
Like this ML GPZ19:




My main question is: will you miss 50% of the targets if you only sweep in one direction horizontally and then take a step forward that is bigger than your coil diameter ?

[pito]

This coil was design for ground cancelation. You will not lose 50% of target, however people do swing both directions = double check .

[Gwil]

"From right to left, the target will first be energized by the TX magnetic field and next pass under the RX coil."
"I would expect that the target will only produce eddy currents after having passed under the TX coil field ?"

This isn't really what happens. A signal is produced when a target is in a position to change the coupling between the Tx and Rx coils. No significant 'energising' takes place. The signal in the Rx coil is the same for a given target position, with or without movement in either direction.

Of course movement can make a lot of difference to the signal in the headphones, depending on the filtering in the receiver circuit.

At least that's how I see it.

[Carl-NC]

When the target is centered under the TX coil it will have maximum coupling with the TX coil so the induced eddy current and its "reflected" field will also be maximum. However, it is on the outside of the RX coil so the RX voltage it induces is negative and weaker.

When the target is centered under the RX coil it will have maximum coupling with the RX coil. However, it is on the outside of the TX coil so the induced eddies and the reflected field are negative, and also weaker. So, again, the induced RX voltage is negative and weaker.

Only when the target is in the overlap region will you get a positive induced eddy current and a positive induced RX voltage. And even though neither one is at their maximum potential, the combination of the two is at a maximum.

[F117]

Thanks, Carl!

[ivconic]

The whole process that Carl explained happens very quickly.
If only there was a way to so precisely measure the differences between swings to the left and swings to the right;
these would be very small and instantaneous differences in the domain of a few mS.
If there is a "loss" on one side; so it will last in the range of mS...
Completely irrelevant, on the limits of statistical error.