Dual D Coils

[Anonymous]

This one is for Eric and / or the Guru's. Does it make much difference if you Faraday shield only the receive coil on a Dual D for a PI or is it wise to shield both coils? Dave. * * *

 

[Anonymous]

Hi Dave,
Glad to hear that you had an enjoyable visit to the UK and a safe journey home to the US. It was good to meet and talk about PI, plus an enjoyable pint or two of Speckled Hen!
Back to your project and question about DD coils. I would definitely shield both coils, particularly for running at short delay times. An unshielded TX winding would be affected by ground capacitance. This would alter its self resonant frequency and hence the damping. You could over damp the coil to counter this effect, but then you would have to set the delay time back. What happens in the TX with regard to ringing and damping is reflected into the RX coil. Shielding both coils independently keeps things tidy. As well as eliminating ground to coil capacitances, it also prevents direct capacitive coupling between the coils. The only coupling is inductive which makes balancing simpler. I would use a cable with two independently screened conductors and only common them together back at the electronics.
Another benefit of shielding both coils is that spurious radiation is eliminated from the TX and prevented from getting into the RX. An important point in regard to EU emissions and immunity regulations.
Eric.