not shielding the TX coil

[punagold]

Is there any reason to shield the transmit coil? Thanks

[WM6]

For working MD not so much, but for comply radio EMI regulation yes.

[punagold]

Thank you WM6

[Silver Dollar]

I thought that you want the TX and RX to react the same. So shielding both allows
them to see the same (attenuated / distorted) signal so they track together allowing
a better indication of the target...

[Koala]

I have wondered why the TX is shielded. Asked the question years ago on the IDX thread but there was no answer. What I have found is that if you make a DD if the crossing places are not insulated then the coil falses.

Don't know why but I can not get individually wrapping the windings in space blanket materiel to work well at all. In some cases worse than having no shield at all.


Moved away from individually wrapping the coils to using conductive card above and below the coils. Works so much better for me. Not sure why ??

IDX are a bit funny with -V and ground

[punagold]

Silver Dollar,

I would assume that the TX is not "seeing" at all. It is just transmitting. All of the "seeing" is done by the RX coil, therefore the need for shielding.

[Davor]

I made some simulations of coils with and without shield, and criss-crossed, in a way they react with aluminium foil shield. For some reason there is a difference in alu-foil response content in received signal due to varying coupling if shield is not applied to both coils, as in a case of bobbing coil(s) against a normal soil. The foil response was equal if shield was applied to both coils, so this kind of response is lost in motion filter circuitry. At that point I did not have the viscous soil model to check it against difficult soils as well. But I concluded that if I ever wish to apply shield, it has to be both coils. The other conclusion was that shielding reduces sensitivity of VLF detector only a little. PIs are a different story for completely different reasons.

This should be checked in real life.

[Silver Dollar]

Silver Dollar,

I would assume that the TX is not "seeing" at all. It is just transmitting. All of the "seeing" is done by the RX coil, therefore the need for shielding.

Um I meant that the TX and RX have to be subjected to the same conditions so they "track" together.
A shield is a fixed capacitance so if RX only is shielded the TX will change with ground capacitance
and RX will not...

[dfbowers]

I assumed you always needed to shield the TX coil unless it's driven. Wet grass would be a killer otherwise.

[Davor]

Not so, it is simply a function of eliminating the common mode signal that gets rid of the infamous wet grass. You achieve that with a balanced input, and no shielding is necessary. There is a snag in form of some 3dB noise compared to the single-ended input.

[golfnut]

With vlf IB, no sheild - I guess that there must be a signal detected to play the tone.

The coupling of the Tx to Rx with grass/ground present - with no shield the mechanism may be the grass/ground viewed as a block of dielectric material.


So the e field from the Tx coil gets into the Rx coil - the phase shift of this signal must be in the correct quadrant as you here it

[Koala]

Don't fully understand shielding and find it frustrating.

IDX DD coil

both RX and TX have shield and insulated from each other. RX edge and center of coil are sensitive to a electrical charge moving towards them. Less so at the cross over point and the TX side.

Coil will not ground balance over wet grass will over wet stone.

Coil unusable. Same windings work with the shielding removed and connected to a piece of card covered in graphite.

IDX does use -V for the TX and GND for the RX


Not tried a CC

[golfnut]

I think Don in US did a test to detect E field using a fet.

When he offered up his E field sensor to a Tx coil it got loads of E. or voltage field.

When he put graphited card in the way with a drain wire? The sensor got no E.


So the Tx with out a screen is bad I think.


I think the coil stuff is a lot harder than it should be but I dont have the answer!!!

Just use plenty of decent fine carbon/graphite - whether on the shell or on paper or card or maybe painted onto something like bandage and laid into the shell

Drain wires too, No complete loops. all to same common point. And then onto the Rx ground I think.

If the carbon is really low ohms like low tens then its conductive enough reduce sens. i.e. it can support circulating currents and dissipate Tx H-field as heat

In this case a break in the path would likely be necassary.

Ive used graohited paper and it did improve on grass.

Ive used ally foil on IGSL and it was ok, not blinding though. didnt like wet grass