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On Shielding:
First we are discussing electrostatic shileding. Magnetic shielding
would defeat the whole point of the exercise regardless of how it is
applied (to the sensor that is, magnetic (and electrostatic)
shielding of the amplifier can be useful.
E/s shielding is done by surrounding the device to be protected with
a conductive enclosure. Remember that with a toroid shaped sensor
all of the action, so to speak, is within the toroid. There is
essentially no external field therefore a shield can be used outside
the toriod (including windings of course). If a conductor is used
to construct the toriod form it will act like a shorted single turn
in a transformer, causing high currents and heating with no
effective magnetic field inside the toroid. Because the field of a
solenoid exists outside the solenoid as well as inside shielding is
more difficult because space must be allowed for this external
field. By the way e/s shielding is very common on loop type antenna
and metal detector sensors. These shields generally include an
insulated or air gap to eliminate the shorted term.
So in short be careful with shields and don't use a conductive form
for winding toroids.
While I'm here has anyone built a working magnetometer? I'm working
on my third and believe me I've had everything go wrong. I am using
a solenoid. I think a well wound and dimensioned solenoid is as
good as a bad toroid anyday. I have been playing with J.A.
Koehler's spreadsheet and have noted that it seems easier to get a
little better signal strength with the solenoid. I agree that
signal to noise ratio is important but as is pointed out if you have
a decent signal most likely the SNR is OK. I also note that outside
of device and thermal noise it's generally pretty quiet a few
hundred feet "under the sea" (electrically anyway).
While the sensor liquid is important it represents a means of
possible enhancement or improvement. I know water works and it is
well characterized. I also know that if I can get water to work
then I should be able to get another proton source that is more
stable and improves on water's characteristics.
I think the low noise electronics associated with the front end are
more difficult than the sensor and it is my experience that this is
where all my bench time goes. The rest seems easier. By the way
the front end should be shielded both electrostatically and
magnetically. Noise at these low frequencies is very difficult to
deal with.
Has anyone attempted a cesium unit? I think I may put some effort
into one for next season. This season is here and I'm going
prospecting.
Lee
First we are discussing electrostatic shileding. Magnetic shielding
would defeat the whole point of the exercise regardless of how it is
applied (to the sensor that is, magnetic (and electrostatic)
shielding of the amplifier can be useful.
E/s shielding is done by surrounding the device to be protected with
a conductive enclosure. Remember that with a toroid shaped sensor
all of the action, so to speak, is within the toroid. There is
essentially no external field therefore a shield can be used outside
the toriod (including windings of course). If a conductor is used
to construct the toriod form it will act like a shorted single turn
in a transformer, causing high currents and heating with no
effective magnetic field inside the toroid. Because the field of a
solenoid exists outside the solenoid as well as inside shielding is
more difficult because space must be allowed for this external
field. By the way e/s shielding is very common on loop type antenna
and metal detector sensors. These shields generally include an
insulated or air gap to eliminate the shorted term.
So in short be careful with shields and don't use a conductive form
for winding toroids.
While I'm here has anyone built a working magnetometer? I'm working
on my third and believe me I've had everything go wrong. I am using
a solenoid. I think a well wound and dimensioned solenoid is as
good as a bad toroid anyday. I have been playing with J.A.
Koehler's spreadsheet and have noted that it seems easier to get a
little better signal strength with the solenoid. I agree that
signal to noise ratio is important but as is pointed out if you have
a decent signal most likely the SNR is OK. I also note that outside
of device and thermal noise it's generally pretty quiet a few
hundred feet "under the sea" (electrically anyway).
While the sensor liquid is important it represents a means of
possible enhancement or improvement. I know water works and it is
well characterized. I also know that if I can get water to work
then I should be able to get another proton source that is more
stable and improves on water's characteristics.
I think the low noise electronics associated with the front end are
more difficult than the sensor and it is my experience that this is
where all my bench time goes. The rest seems easier. By the way
the front end should be shielded both electrostatically and
magnetically. Noise at these low frequencies is very difficult to
deal with.
Has anyone attempted a cesium unit? I think I may put some effort
into one for next season. This season is here and I'm going
prospecting.
Lee