geek out

[golfnut]

Thoughts / questions

1) Throw a mag field at a target. The energy washes through it and decays.

We sense phase and amplitude info from the currents in target.



The target is acting as a tiny transmitter.

Target has no E field screen / shield.


Is it fair to assume then, that there would be E and H available radiated from the target for our Rx.


We listen in H - is there additional signal available from the target in the E.


S

[Gwil]

I think the answer is that there is an E signal, but at metal detector frequencies the wavelength is so great that you'd need an enormous antenna to get a useful signal. Carl M mentions wavelength in his Coil Basics guide:

http://www.geotech1.com/pages/metdet/info/coils.pdf

Gwil

[Farside]

Anyone considered a submersible, semi-autonomous rover with a metal detector an sampling mechanism attached? I figure that there's probably a lot of loot 50ft off shore in about 20ft of water.

Something like this:
http://www.instructables.com/id/Semi...underwater-re/

or

http://blog.makezine.com/2006/03/18/...utonomous-sub/

or

http://www.instructables.com/id/Underwater-ROV/

or generally:
http://www.homebuiltrovs.com/

[golfnut]

wavelength is so great that you'd need an enormous antenna to get a useful signal. Carl M mentions wavelength

Im sort of with you, but I use tiny efeild antennas smuch samller than wavlength in work.

There are lots of E fild VLF Rx


Commercial one here
http://www.bathlabs.com/Files/vlf.gif

S

[simonbaker]

Thoughts / questions

1) Throw a mag field at a target. The energy washes through it and decays.

We sense phase and amplitude info from the currents in target.



The target is acting as a tiny transmitter.

Target has no E field screen / shield.


Is it fair to assume then, that there would be E and H available radiated from the target for our Rx.


We listen in H - is there additional signal available from the target in the E.


S

Good question as to what we are actually picking up.

Our coils give off mainly "near field" patterns because of their size, but targets, being tiny, perhaps give off signal that looks more "far field" like. But probably our loop antennas (coils) are so much more sensitive to the near field components that those components dominate anyway. Do I know what I'm talking about? No, but doesn't mean I'm wrong...

It's possible that the near and far field magnetic components are in phase anyway, so we are perhaps taking advantage of both... or not, if their "poynting" vectors are different directions...I should really figure out what I'm talking about...

The other comments about antenna size, etc, sound credible enough.

-SB

[Vortxrex]

We need a way to make the target slightly radioactive. It is possible to detect (from a substantial distance) that a person or persons on a (cruise ship for example) have been treated by nuclear medicine.

The "Yukon Study" was famous & very detailed report on how gold deposits in recovery systems. They used radioactive gold.

Imagine what kind of "nugget Detector' you could build if gold nuggets were (even slightly) radioactive.

[Overtheedge]

Why the presumption that we detect magnetic fields rather than electrical fields? Is the receiver tuned-input?

Recall that we shield the coils to minimize ground capacitance effects by swamping. The shield is not a shorted coil.

E/M theory, not just for breakfast anymore.

Inquiring minds want to know.

[simonbaker]

Why the presumption that we detect magnetic fields rather than electrical fields? Is the receiver tuned-input?

Recall that we shield the coils to minimize ground capacitance effects by swamping. The shield is not a shorted coil.

E/M theory, not just for breakfast anymore.

Inquiring minds want to know.

In the end we're always detecting some electric field component -- that's what really moves the electrons. Magnetic field is (of course) just a relativistic manifestation of electric fields subject to relativistic motion -- you can't have a magnetic field without an electric field and vice versa, afaik. But that stuff don't get the baby washed, does it?

-SB

[mikebg]

Thoughts / questions

1) Throw a mag field at a target. The energy washes through it and decays.

We sense phase and amplitude info from the currents in target.



The target is acting as a tiny transmitter.

Target has no E field screen / shield.


Is it fair to assume then, that there would be E and H available radiated from the target for our Rx.


We listen in H - is there additional signal available from the target in the E.


S

Steve, please start to think in time domain because we live here. The frequency domain is created about 200 years ago as a tool for easy analisis and design, but it is sometimes misleading.

What happens in time domain?

We make excitation with magnetic field. According Lenz's rule, the conductive target opposes with eddy currents to change of magnetic feld in its volume. Our RX senses the target as change of its secondary field. The target is not transmitter. It operates as a passive retranslator.

Is there an additional signal available from the target in the E?

No, but there is EMV induced from target retranslation in RX coil. The TGT signal from conductive targets is proportional to second derivative of current in TX coil. In frequqnzy domain that means phase lag almost 90 deg. Note that the received AIR signal is proportional to the first derivative of TX current. In frequency domain that means AIR signal has phase lead 90 deg for all frequencies in the spectrum.
A ferromagnetic nonconductive target retranslates something similar to AIR signal (firs derivative of TX current). Because of that, hot rocks induce EMV having phase lead almost 90 deg relative to TX current.

What is misleading in frequency domain for this case?

It seems that the response appears before excitation, however for ingineers this is not important.

[golfnut]

more fuel

For the discussion..

http://www.w8ji.com/magnetic_receiving_loops.htm

Another guys view.


Good stuff here from same article..

"
Loop Shielding and Balance

Loop shields do not sort noise out, nor do they prevent electric fields from affecting the antenna. They do not change the field impedance of the antenna. For a description of how shields work, look at the Concentric and Coaxial Transmission lines page and also skin depth. You can also read a few pages of "Transmission Lines, Antennas, and Waveguides" (no longer protected by copyright) that deals with loop shielding and balance.
From those pages you will see the shield actually becomes the antenna in a "shielded" loop.
There are many construction articles about small loop antenna available. It is VERY important that all conductors exit the loop at the ground point of the shield, and that the loop is grounded exactly at the electrical center of the shield. The loop must also be symmetrical, each side must be excited equally, and you must mount the loop so the feedline and any metallic supports leave the center area of the loop with maximum symmetry. If you DON'T do this, the loop can actually use the feedline as the antenna. This can greatly increase sensitivity of the loop to conducted noises! Improper design or construction can also distort the pattern.
Remember the following guidelines:

• The shield is the actual antenna
• The shield must be perfectly symmetrical moving away from the inner conductor exit point
• The gap in the shield must be exactly opposite the grounded point
• The ground must be at the inner wire exit point
• The shield will not make an unshielded loop that is properly balanced any quieter
• The shield only is a tool to help you balance the system. The shield helps only when the shield is properly implemented

S

[simonbaker]

For the discussion..

http://www.w8ji.com/magnetic_receiving_loops.htm

Another guys view.


Good stuff here from same article..

"
Loop Shielding and Balance

Loop shields do not sort noise out, nor do they prevent electric fields from affecting the antenna. They do not change the field impedance of the antenna. For a description of how shields work, look at the Concentric and Coaxial Transmission lines page and also skin depth. You can also read a few pages of "Transmission Lines, Antennas, and Waveguides" (no longer protected by copyright) that deals with loop shielding and balance.
From those pages you will see the shield actually becomes the antenna in a "shielded" loop.
There are many construction articles about small loop antenna available. It is VERY important that all conductors exit the loop at the ground point of the shield, and that the loop is grounded exactly at the electrical center of the shield. The loop must also be symmetrical, each side must be excited equally, and you must mount the loop so the feedline and any metallic supports leave the center area of the loop with maximum symmetry. If you DON'T do this, the loop can actually use the feedline as the antenna. This can greatly increase sensitivity of the loop to conducted noises! Improper design or construction can also distort the pattern.

Remember the following guidelines:

• The shield is the actual antenna
• The shield must be perfectly symmetrical moving away from the inner conductor exit point
• The gap in the shield must be exactly opposite the grounded point
• The ground must be at the inner wire exit point
• The shield will not make an unshielded loop that is properly balanced any quieter
• The shield only is a tool to help you balance the system. The shield helps only when the shield is properly implemented

S

This is good stuff to ponder. Would you say though that for our VLF coils, we are low enough in frequency to ignore transmission line effects?

-SB

[golfnut]

Unsure really. what effects are we talking about..

S

[simonbaker]

Unsure really. what effects are we talking about..

S

It seemed those references might involve higher frequency effects than we deal with?

However, I'm very interested in learning more about the properties of our shield configurations which are like a discontinuous torus surrounding our coil loops.

-SB

[Overtheedge]

The 15th edition of the ARRL Antenna Handbook confirms that shields are effective against electrical fields. See page 14-2 figure 2. Therefore the electrical field is kept out, the magnetic field gets in and induces AC in the antenna wire (loop within the shield).

Same cautionary statements about symmetry, feed point exiting at center of shield and the shield break opposite the feedpoint. see page 5-5 The issue of symmetry had to do with the wire feeding the loop becoming part of the antenna and distorting the null points that each antenna has.

In effect, it appears that we have Lenz's Law at work within the shield inducing an opposite phase current flow in the wire from the magnetic field.

Thanks for setting me straight. For some strange reason I spaced out the magnetic field to electrical current flow conversion within the shielded loop.

Interesting blurb about shielding loopstick antennas on page 14-3 of the same book.

Newer editions of the ARRL Antenna Handbook usually only have different projects than relatively older editions. Theory doesn't change (much). I consider it a great resource.
eric