# Factors to be considered measuring Magnetic Field of 130KHz

1. Feb 13, 2014

### Mag_field

I need to measure a magnetic field of Frequency of around 130KHz in all 3 X,Y,Z axis.
I have a plan to make 3 coils perpendicular to each other and measure voltages from each coil in Oscilloscope channel.

As per Faraday's law of induction. I could measure (dB/dt) as follows.
V = - N A (dB/dt)
But I am sure it's not that easy with AC 130KHz signal

So followings are my confusions:
1. How to select measuring coil wire thickness(Thick/Thin)?
2. How to avoid skin effect, capacitance?
3. How to measure/calculate effective impedance, Inductance of the coil and oscilloscope probe?
4. Which type of connecting wire Coaxial or twisted wire need to use from coil to oscilloscope probe?
5. What should be the setting in Oscilloscope input channel 50Ω Dc, Ac or 1MΩ ?

Please suggest how to proceed with it.
I would be really grateful for your valuable suggestions.

2. Feb 13, 2014

### jim hardy

I used to do that at 60hz.
I used a home-made flat coil of ten turns encircling 0.1 m^2 which gave a size handy to carry around. I could tilt it in any direction. Next time i'd include a factor of pi because ω=2∏f and it'd simplify converting the reading of volts into webers. I was chasing sinewaves, but found the leakage from some transformers far from sinusoidal.

To your questions: one experiment will be worth a thousand opinions.

How to select measuring coil wire thickness(Thick/Thin)?
I'd pick wire small enough that it's easy to form but stout enough to make a robust coil of whatever size you pick. I used #20 solid taped to a piece of thick cardboard. I dont know what size coil you anticipate. For your frequency I'd start experimenting with probably #24 wrapped on a cardboard tube perhaps from center of a toilet paper roll , or the voice coil from a junk loudspeaker.
How to avoid skin effect, capacitance? Small wire and not a lot of it

How to measure/calculate effective impedance, Inductance of the coil and oscilloscope probe? Z of scope probe should be far larger than the coil, probably you can neglect both. The coil you can either calculate or measure with an inductance meter, the scope probe should be marked or specified in the manual.
See http://www.arrl.org/files/file/Technology/tis/info/pdf/9708033.pdf for a practical "how-to" on the coil...

Which type of connecting wire Coaxial or twisted wire need to use from coil to oscilloscope probe? For short distance I'd use twisted pair . How far must you reach?

What should be the setting in Oscilloscope input channel 50Ω Dc, Ac or 1MΩ ? Any current flowing in the coil produces its own flux which cancels some of what you're trying to measure. So you want a high impedance connected to your coil. I'd use a 10X probe to minimize loading of your coil, that'll give you 10 megohm and a few pf.

You might think about building an electronic integrator to give you a display of flux in addition to its derivative. For non-sinewaves they'll look different to the eye.

So - grab some parts and start climbing up the learning curve.
I hope you post some photos of your apparatus, and results .