Exchange field noise voltage at various locations

[Prospector Paul]

Hi, I'm new to prospecting & metal detecting, but I've designed a metal detector & built a few versions. I'm interested in exchanging field noise voltage density per m^2 (nV/√Hz/m^2) measurements at various locations? I don't have much so far, just a few locations out here in the noisy city, but hopefully within 2 to 3 months I'll take measurements at various Southern California wilderness locations. The method I'm using to measuring the noise is a low noise op-amp, LT1028A, typically set to a measured confirmed gain anywhere from about 100 to 1000, connected to a coil. For now I'm using a single turn 1 meter OD loop. The op-amp output goes to an oscilloscope that displays a voltage spectrum. If for example the scope is set to 400us/div and each FFT sample for 10 divisions is 4ms, then to get the nV/√Hz I divide the spectrum noise voltage for a given frequency by 1/sqrt(4e-3 seconds). And of course I divide that answer by the single turn coil area, m^2.


I appreciate it! You can send a private message if you prefer.

[Skippy]

I'm curious as to why you chose a 1 metre diameter loop ? I can see the logic of using 113 cm diameter, as that would give you 1 square metre area, making the maths easier. I assume if you used a more physically practical size, like 30cm diameter ( and more representative of a detector search-coil ) do you get insufficient signal to make decent measurements? Would multiple turns of wire cause problems, eg. due to inter-winding capacitance ? I would think that 3 turns would bind up tightly into a neat stable triangle if you were careful in your construction.

[Prospector Paul]

It depends on the frequencies you're interested in measuring noise and the location. I haven't taken measurements in rural areas, yet. Perhaps rural noise would be too low for my op-amp with a 1 meter OD 1 turn loop. Here in a big city a 1 turn 1 meter diameter loop is fine with an LT1028A op-amp, but I probably wouldn't make the coil much smaller otherwise the noise will be too close to the op-amp noise. Before I travel for hours to a rural location I'll add more turns, perhaps 10 to 15.

[dbanner]

Based upon my rudimentary knowledge, the band(wavelength) would determine the size of coil one would implement. A square coil(or triangle)would make computations a bit easier? Maybe just a dipole type arrangement would suffice. Multiple turns coil would necessitate a more sophisticated calculation of parameters. Multiple Filters could be used for some spectrum quantification.

[dbanner]

Or maybe you could just put a large rabbit ears antenna on your back and walk around the Forrest. Be careful nobody sees you.