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Author Topic: hmc6352 electronic compass and RF fields  (Read 5679 times)
ON4CKM
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Posts: 40




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« on: August 02, 2012, 12:18:57 AM »

Hi,

I'm building a rotator control unit for an old antenna rotator, using an arduino and an hmc6352 electronic compass. I want to attach the compass module to the rotator in the mast and read out the heading of my antenna. I have the basic setup working.

During my tests, I brought a magnet close to the electronic compass and as expected, it got confused by it and gave me wrong readings. But when I removed the compass, it was still giving me wrong readings. I thought for a moment that it got permanently damaged, but the next day it worked fine again. Seems like it needed some time to recover?

But this makes me wonder about how it will operate when it is up in the mast. It will be close to the antenna, both HF and VHF. Can it be damaged by the RF field, or temporarily be deranged? Does anybody have experience with this? Putting it in a shielded box is no option I assume as it can no longer read the earth's magnetic field.

Cedric
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G3RZP
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« Reply #1 on: August 02, 2012, 12:34:12 AM »

Is the compass a flux gate type? That could explain why it took time to recover. As far as the RF susceptability is concerned, the only way is to try it. But you could shield it in an aluminium box - it should still sense the earth's magnetic field.

How much accuracy do you need?
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KA4POL
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« Reply #2 on: August 02, 2012, 01:19:12 AM »

RF is electromagnetic. So you always have a magnetic component which influences the measurement. Use a good old potentiometer which will be much easier to handle. I'd contact the manufacturer of your sensor for detailed information, they usually have excellent support.
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ON4CKM
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« Reply #3 on: August 02, 2012, 01:28:59 AM »

Don't know if it is of the flux gate type. I looked at the datasheet (http://www.sparkfun.com/datasheets/Components/HMC6352.pdf) but can't find that info in there.

The datasheet does state that it is best to keep it away at some distance from soft metals, as they "bend the earth’s magnetic field". So the aluminium box doesn't sound like a good idea.

I don't mind about an inaccuracy of 2 or 3 degrees.

Cedric
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KA4POL
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« Reply #4 on: August 02, 2012, 01:39:34 AM »

If you look at the data sheet you also can find the value of 20 gauss for start of degrading and as long as you remain below 10000 gauss no damage seems to occur.
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ON4CKM
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« Reply #5 on: August 02, 2012, 02:57:27 AM »

Hi KA4POL,

I'm not familiar with Gauss. Is there a way to calculate how much Gauss my signal will generate if I know my output power, my frequency, the gain of my antenna and the distance between the sensor and the antenna?

Cedric
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KA4POL
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« Reply #6 on: August 02, 2012, 03:56:23 AM »

You'll see some ham radio measurements in this paper: http://www.rsl.ku.edu/~eecs501/energy_harvesting/Mantiply1997BioEpp563-577.pdf
It shows you are always below 10 A/m. 1 Gauss is 79.58 A/m. So you certainly will be safe from destroying your sensor. Also most likely the 20 Gauss limit should not be reached. The lower the frequency the stronger the magnetic field as per the paper.
Calculation from x W to y Gauss is far from reality. You would need to measure.
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G3RZP
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« Reply #7 on: August 02, 2012, 04:36:08 AM »

Will it react to a radio frequency magnetic field? I suspect a bigger worry is the rest of the electronics going mad with RF, and without shielding, you will have problems except at very low power (milliwatts in all likelihood). It isn't a fluxgate, but appears more probably a Hall effect device. What sort of antenna are you using that you need 2 or 3 degree accuracy? Given that, how are you going to account for the changing magnetic deviation?

I think 4POL's suggestion of a pot has merit. Except for the fact that you need 5 wires, a synchro is another good approach.
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ON4CKM
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« Reply #8 on: August 02, 2012, 04:51:46 AM »

The rest of the electronics is in the shack, only the sensor is in the mast. And I can shield it off there.

The couple of degrees is acceptable for me for a VHF yagi, but when it is 10 or more degrees it becomes annoying. For my rotary dipole on 20 meter, it is far less critical of course.

I think I will just go ahead and test it out. If it doesn't work, I can still go for a pot or a even a rotary encoder.

Cedric
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G3RZP
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« Reply #9 on: August 02, 2012, 06:18:31 AM »

You could check by adding an aluminium shield with it all on the bench and see if it makes a difference.
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G8HQP
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« Reply #10 on: August 02, 2012, 10:42:18 AM »

I don't think aluminium will bend the earth's magnetic field by any noticeable amount, but it will keep out RF. The reference to 'soft metals' is almost meaningless; could it really mean soft iron?
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KE3WD
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« Reply #11 on: August 02, 2012, 05:23:27 PM »

While it is still on the bench, hold a 2-meter or VHF business band HT with the rubber duckie directly overtop of the sensor, key the radio on its highest power setting and note what happens.  

My "Quick and Dirty" RF sensitivity test.  

No, it doesn't cover all bases, no it is not quantitative, but often such is not needed, for if the RF from the HT swamps the device, well....

If you do find that RF swamps the compass, don't just assume it is the sensor itself and nothing can be done. 

Tacking a small value cap across any negative feedback resistor in an opamp circuit will serve to Integrate.  This is sometimes all that is needed to RF proof sensor circuits such as this one.  Other times it *may* be necessary to install RF chokes in series with signal path.  Surface Mount RF chokes are great for adding to pre-existing circuits.  Cut a trace on the board, clean the resist off of each side, tin the traces and solder in the SM choke. 

73
« Last Edit: August 02, 2012, 05:27:07 PM by KE3WD » Logged
KB4QAA
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« Reply #12 on: August 04, 2012, 04:18:29 PM »

Aluminum will not affect magnetic compasses.  It is fine for a cover.
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WB6DGN
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« Reply #13 on: August 19, 2012, 08:40:31 PM »

Quote
The reference to 'soft metals' is almost meaningless; could it really mean soft iron?

Suspect they're referring to metals such as copper, brass and similar.  They do funny things to magnetic fields.
Tom
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