routing coax through strong DC magnetic fields

I think the signal in the coax and the DC magnetic field will obey superposition and that the coax will stay shielded as well as it would otherwise.

no, the coax fields still cancel at a distance (small)

But most coax is between 90% and 98% covered, not 100%, so there can be leakage into or out of but not at DC, there is no effect on the coax (which is high frequency)

If you the coax real hard in the dc magnetic field, you probably still won't affect the coax on hf or vhf.

I think you're distorting the Hall Effect here. One side of each conductor will have a positive charge and the other side a negative charge, and there will be a potential across the shield and across the inner conductor. But at the ends of the cable these effects are gone.

No. Superposition of fields. Even for an AC magnetic field.

If you have an alternating AC magnetic field you can induce a current in both the shield and inner conductor, but these will match. If you've got ground loops you may misinterpret this as a signal. Don't have ground loops.

Tim.

A constant (DC) magnetic field has no effect on either the current distribution or the voltage in a coaxial (or any other type) of cable. Regards, Jon

Contrary to the common perception, an average coax with a wire screen is loyy anyway.

Rene

Dear Tracy,

No, I don't believe that the "biasing" up and down that you refer to will cause any destruction of symmetry, because everything is linear. So long a linearity prevails, there will be no effect whatsoever due to the DC field, in my opinion.

Bob, W9DMK, Dahlgren, VA Replace "nobody" with my callsign for e-mail

analyze this way... if the current in the coax is AC then each half cycle the electron flow reverses so the forces would reverse, and since the actual electron velocity is relatively low there would not be enough time for the electrons to really move before the current reversed and they had to go the other direction. so i would say that it would be unlikely to have any effect on coax carrying AC. now, if you go to very low frequencies where the electrons have a chance to move you might be able to measure something, but since the wavelength increases with the lower frequency the relative size of any imbalance becomes smaller. and essentially if it did completely separate you still end up with balanced currents but in a twin lead arrangement and you still have currents that cancel at any distance away from the line. what would be more interesting is to calculate the torque on a piece of twinlead carrying dc in a strong field.

To leading order, the superposition argument is right, but it assumes that the current distribution in the coax is unchanged by the applied DC field, which is not true in detail.

The Hall effect operates in metals, so an AC current in the shield will indeed cause an AC voltage across the diameter of the coax shield, outside as well as inside, and this will radiate as an electric dipole.

However, the Hall effect in metals is so small that it's difficult even to measure it, and the electric dipole radiation from a source very small compared to a wavelength is weak. The resulting leakage is therefore (miniscule)**2, far smaller than that caused by the poor shield coverage and poor shield continuity.

The shield conductors are just laid on top of each other, and since their contacts are neither gas-tight nor self-wiping, they are somewhat noisy and unreliable--we don't rely on that sort of contact elsewhere in electronics.

Cheers,

Phil Hobbs

Can't point you to any studies or the like, but coaxes in our klystron amp's pass by the klystrons with no ill effects noted.

Ken

Thanks, all, for your helpful responses.

The general consensus appears to be that a tiny electric dipole will be induced that oscillates transverse to the cable at the same frequency as the carrier. It might possibly leak a tiny amount of energy to the area immediately surrounding the coax, but probably not a significant amount compared to that which is already lost due to imperfections in the cable.

I'm not planning any installation in an MRI, but I do have an application where the cable is routed through some fairly strong permanent magnetic fields, and I wondered if I needed to magnetically shield the cable to avoid substantial losses. Sounds like I don't need to worry.

I suppose I could actually run a test, but it's a lot easier just to freeload off all your experience!

Tracy Hall hthalljr'gmail'com

I would read the manufacturers data, attempt to get a 100% braid (silver plated copper tubing for a shield would be GREAT! ), and limit exposure to the fields in as much as it is possible.. But hey, I am a natural paranoid--but that trait kept me alive when I was younger and carrying on with women of questionable character in cheap bars...

Warmest regards, John

I assume you only ask, in theory? First thought coming to my mind is what is going to happen with that coax near the MRI when it is turned on!

Ed :^)

Hmm. I suppose that for largish low frequency components in the signal, you might also get the cable bits physically moving, both together, and in relation to each other, which might screw up the impedance a little.