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While pondering about coil capacitance and cable interfacing in PI detectors, I thought about potential problems of the surface currents in a coaxial cable. As the coil cannot be shielded in a fully enclosed loop, there must be a gap in the shielding which will let the RF current conduct along the outer surface of the screen in coaxial cable.
Coaxial cables in radio use usually have baluns or broadband chokes to cut the outer surface of the screen from the RF path, one broadband method being ferrite rings around the cable used as a choke.
There are a couple issues with the cable being unbalanced in either end, which I imagine might be potential targets for improvement:
-The outer surface of the screen radiates the pulsed power, resulting in potential EMI issues.
-The outer surface of the screen adds to the capacitance of the coil.
-The outer surface capacitance will be variable, which adds a proximity dependent effect.
As the balun would merely be serving to keep the surface currents from the outer surface of the screen, chokes would do. Inserting ferrite toroids or a ferrite tube onto the cable, or wrapping multiple turns of the coax around a toroid ferrite core, would serve this purpose. As radiofrequency ferrites are nonconductive and the toroids could be fixed parallel to the search coil axis, they would not affect the search coil more than magnetic soil which is not an issue with PI detectors? These baluns/chokes would need to be mounted in both ends of the cable to be effective. In the control-box end the RF currents would be partially shunted by the driver transistor's bypass capacitor, but some would be able to flow along the surface of the cable unless the box is a full metal box and the connector is a proper RF coaxial connector making full contact around the rim of the screen.
If someone more familiar with designing PI detectors knows whether these questions are real issues, I'd be pleased to hear about it. Have any manufacturers made PI detectors with baluns or surface-current chokes in the cable? Do any use twisted pairs etc. cabling with an external shield, or double screened (signal/guard/screen) coaxial cable?
Coaxial cables in radio use usually have baluns or broadband chokes to cut the outer surface of the screen from the RF path, one broadband method being ferrite rings around the cable used as a choke.
There are a couple issues with the cable being unbalanced in either end, which I imagine might be potential targets for improvement:
-The outer surface of the screen radiates the pulsed power, resulting in potential EMI issues.
-The outer surface of the screen adds to the capacitance of the coil.
-The outer surface capacitance will be variable, which adds a proximity dependent effect.
As the balun would merely be serving to keep the surface currents from the outer surface of the screen, chokes would do. Inserting ferrite toroids or a ferrite tube onto the cable, or wrapping multiple turns of the coax around a toroid ferrite core, would serve this purpose. As radiofrequency ferrites are nonconductive and the toroids could be fixed parallel to the search coil axis, they would not affect the search coil more than magnetic soil which is not an issue with PI detectors? These baluns/chokes would need to be mounted in both ends of the cable to be effective. In the control-box end the RF currents would be partially shunted by the driver transistor's bypass capacitor, but some would be able to flow along the surface of the cable unless the box is a full metal box and the connector is a proper RF coaxial connector making full contact around the rim of the screen.
If someone more familiar with designing PI detectors knows whether these questions are real issues, I'd be pleased to hear about it. Have any manufacturers made PI detectors with baluns or surface-current chokes in the cable? Do any use twisted pairs etc. cabling with an external shield, or double screened (signal/guard/screen) coaxial cable?