Coil shield polarity question

[simonbaker]

With double-D coils, such as for TGSL, we have shields on both coils.

(I'm not sure why on TX coil, but that's another question...)

Shields have a gap, so we can attach ground wire to either side of gap.

Does it matter which side of gap we choose?

For example, if we choose left side on TX, should we choose left side on RX? Or opposite? Or doesn't matter?

Regards,

-SB

[Fred]

With double-D coils, such as for TGSL, we have shields on both coils.

(I'm not sure why on TX coil, but that's another question...)

Shields have a gap, so we can attach ground wire to either side of gap.
Does it matter which side of gap we choose?

For example, if we choose left side on TX, should we choose left side on RX? Or opposite? Or doesn't matter?

Regards,
-SB

While we are here, and since the shields cannot touch each other but must be connected to the same ground, i wonder how important is the lenght of the ground wire connecting both shields, in the case of a single cable (one braid).

[WM6]

For example, if we choose left side on TX, should we choose left side on RX? Or opposite? Or doesn't matter?

-SB

If your shielding are made in compact form (as usual) I think doesn't matter.

But if you made shielding in spiral form from (thin) enameled stainless steel wire (about 1 Ohm / 1 meter resistance) selected coupling point may affect sensitivity.

[ivconic]

TX shield is desireable to eliminate stray capacitances between TX coil and soil surface or coil and enclosure material (closest to coil).
In coils where graphite spray is used also both coils are "protected" with shield.
Gap... gap is just a gap. Doesn't matter which side of gap will be connected to common. I never pay attention on that. By some inertion i always wound naked wire over Al foil at spot close to end leg of coil, so later i could easilly join that naked wire and end of coil, cose both ends (TX end and RX end) are going togather with both Al shields on common. Doesn't make any difference.

[simonbaker]

TX shield is desireable to eliminate stray capacitances between TX coil and soil surface or coil and enclosure material (closest to coil).
In coils where graphite spray is used also both coils are "protected" with shield.
Gap... gap is just a gap. Doesn't matter which side of gap will be connected to common. I never pay attention on that. By some inertion i always wound naked wire over Al foil at spot close to end leg of coil, so later i could easilly join that naked wire and end of coil, cose both ends (TX end and RX end) are going togather with both Al shields on common. Doesn't make any difference.

Ok, I didn't think of that -- stray capacitance to ground could cause small frequency/phase shift in oscillator which would look like signal (I guess).

It's too bad only material in universe with medium resistance is graphite.

I won't worry about which side of gap then, thanks.

-SB

[Altra]

Hi Simon,

You right about ground minerals and capacitance effecting the transmit signal. This is why many detector designs have the zero crossing detectors connected to the TX coil. As the TX coil's frequency and phase change, so does the zero crossing point. Therefore the demodulators self adjust to maintain the original phase relation to the RX signal. The Tesoro circuits are a good example. The LM393 comparators are the zero crossing detectors. The design acts as automatic ground tracking circuit although not perfect.

Regards
Mark

[simonbaker]

Hi Simon,

You right about ground minerals and capacitance effecting the transmit signal. This is why many detector designs have the zero crossing detectors connected to the TX coil. As the TX coil's frequency and phase change, so does the zero crossing point. Therefore the demodulators self adjust to maintain the original phase relation to the RX signal. The Tesoro circuits are a good example. The LM393 comparators are the zero crossing detectors. The design acts as automatic ground tracking circuit although not perfect.

Regards
Mark

Another good point -- most fluctuations in TX signal are not seen by synchronous detector because driven by TX signal. Maybe very fast ones could still cause problems???? -- but person moving coil would not make very fast fluctuations probably.

I am tempted to try unshielded TX coils to see how well they work. I never learn....

Regards,

-SB