routing coax through strong DC magnetic fields

[Tracy Hall]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

Thanks,

Tracy Hall
KD7AVV
hthalljr'gmail'com

 

[Tim Wescott]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

Thanks,

Tracy Hall
KD7AVV
hthalljr'gmail'com

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.

 

[Ken Taylor]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

Thanks,

Tracy Hall
KD7AVV
hthalljr'gmail'com

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

 

[ChuckC]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

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)

 

[Hal Rosser]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

Thanks,

Tracy Hall
KD7AVV
hthalljr'gmail'com

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

 

[Rene Tschaggelar]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

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

Rene

 

[W9DMK]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

Thanks,

Tracy Hall
KD7AVV
hthalljr'gmail'com

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
http://www.qsl.net/w9dmk
http://zaffora/f2o.org/W9DMK/W9dmk.html

 

[Dave]

I understand that coaxial cable does not radiate much energy because
symmetric opposing current sheets in the outer skin of the center
conductor and the inner skin of the shield essentially balance each
other.

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

If so, will the coax then radiate and become lossy?

Can anyone point me to an analysis of this problem?

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.

 

[Tim Shoppa]

Suppose I route coax through a strong DC magnetic

field, such as in an MRI, with the magnetic field
perpendicular to the cable axis. Will the
current distribution in the center conductor then
become biased "up" with respect to the
field (right hand rule), and the current distribution
in the shield then become biased "down,"

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.

thereby destroying symmetry?
If so, will the coax then radiate and become lossy?

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.

 

[Phil Hobbs]

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

 

[Jon]

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

 

[Ed]

Suppose I route coax through a strong DC magnetic field, such as in an
MRI, with the magnetic field perpendicular to the cable axis. Will the
current distribution in the center conductor then become biased "up"
with respect to the field (right hand rule), and the current
distribution in the shield then become biased "down," thereby
destroying symmetry?

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 :^)

 

[Tracy Hall]

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

 

[John Smith]

I would read the manufacturers data, attempt to get a 100% braid (silver
plated copper tubing for a shield would be GREAT! <grin>), 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... <grin>

Warmest regards,
John

 

[Ian Stirling]

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.

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.