frequency of a magnetic field?

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kdrifter440

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Does any one know how to measure the frequency of a magnetic field? They sell handheld meters but they only measure Mhz and higher
 
your question begs another: If not MHZ, why not tell what freq you are interested in?

Guess, lower than mhz and I hit on 20-20,000hz and can say use a microphone and free spectrum analyzer software....

Another guess, lower than mhz and I hit on less than 20 hz and can say use a piece of magnetic metal, hold it in the field, and count the bumps....

But alas, sorry, no way to actually guess what freq you want less than mhz........ maybe a tad more info?
 
I'm sure that you mean a "electromagnetic field". As a magnetic field has a DC field.

For low frequencies, you could use a pick-up loop and a DMM. For radio frequencies, you could use a wide-band radio receiver like many HAM's use.
 
I still think my suggestion of an inductive loop, a few coils of wire, connected to an oscilloscope is the most practical suggestion for determining fundamental frequency.
Which what the OP wanted, I think.
 
Well, the field's frequency is going to be the same as the AC component, isn't it? US, 60 Hz, Europe, 50 Hz, aircraft 400 Hz, etc.
 
121211-1927 EST

kdrifter440:

Can you try to explain what your goal is? What do you want to find out?

If you have a moderate strength magnetic field, then I would use a Hall sensing element. With constant current excitation its out voltage will be proportional to magnetic field intensity (Gauss) independent of frequency up to some upper limit.

A simple pickup coil without mathematically integrating the output will have an output voltage proportional to frequency. Thus, a waveform of a base sine wave with added harmonics will accentuate the harmonics in the waveform. The resulting coil output voltage waveform will have a shape different than the waveform of the magnetic field signal. The distorted signal results from e = N*dPhi/dt. Integrating this gets you back to e = N*Phi.

To view or measure the intensity of the magnetic field you can use an oscilloscope, voltmeter, or some other means responsive to the sensor's output voltage.

Note a Hall device works down to a DC frequency, 0 Hz.

If you use a coil method and only want to measure the fundamental frequency, then a filter or other processing is necessary.

Your original question asked how to measure the frequency. Suppose the signal is a clean sine wave, and the coil produces a sufficiently large signal, then connect the coil to a general purpose frequency meter and adjust to readout the frequency. If you use an old oscilloscope, then measure the time between zero-crossings and calculate frequency. Or use an oscillator into one axis of the scope and the unknown frequency into the other and adjust the oscillator to get a straight line, ellipse, or circle.

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121211-2204 EST

Additional comments.

The other day someone was concerned with radiation from a smart meter in another thread. The probe I am going to describe would not address the problem of the other thread, but several comments in that thread took us on a tangent to which the probe measurements were pertinent.

I have a simple probe from many years ago to measure magnetic field density. This is simply a calibrated coil. The output is 20 mV/Gauss at 60 Hz. The output will be 20*f/60 mV/Gauss for other frequencies up to some cutoff point that relates to the self resonant frequency of the coil. Lower frequency limits will depend upon the meter noise level.


When I did the tests the other day I had no filtering, and close to an electronic ballast fluorescent tube I got higher readings than close to a magnetic ballast type. This was because of the high frequency from the electronic ballast system.

A simple filter, I didn't use it the other day, I use with this sensor to concentrate on 60 Hz is simply a shunt capacitor with the coil, 0.47 ufd in this case, that approximately resonates the coil-capacitor combination near 60 Hz.

For frequency measurements over a wide range this is not going to work. That is why you need to tell us what you want to do.

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gadfly56 said:
Well, the field's frequency is going to be the same as the AC component, isn't it? US, 60 Hz, Europe, 50 Hz, aircraft 400 Hz, etc.
Click to expand...
I believe the field frequency will be determined by the current. If there is a rectified output, the RIPPLE frequency, which I SUSPECT will be represented in the input current, depends on the circuit phases and full wave vs half wave rectification, from 1 to 6 times fundamental.

My only experience here is that the 360 Hz ripple from a 3 phase bridge is lots easier to filter that the 120 Hz from a single phase bridge. Indeed, ripple % from a 3 phase bridge is under 5% directly off the bridge, vs. about 50% in the single phase bridge.
 
Besoeker said:
Won't it depend on what's producing the magnetic field. It could be, for example, a SMPS.
Click to expand...

I'll admit I'm making a huge assumption regarding the intended use, but the OP said in his initial post he couldn't find any meters for under 1MHz, and SMPS's can operate up there. NMR's, PET's, etc are likely also to be in the MHz range. We really do need to know the intended application.
 
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