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Cal data appears to yield a negative magnetic field  

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jserink
(@jserink)
Active Member
Joined: 7 years ago
Posts: 6
17/03/2014 7:49 am  

Hello:

We were in the field last week at a point 23,500m away from our broadcast station.
Our Trimble DSM 232 was reading 88.5dBuV/m which comes out to 0.0266 V/m.
The problem with this number is that based on an antenna current of 20A, which we got form the ATU, that gives us an effective antenna height of 80m.
An effective height of 80m is impossible since the physical antenna is only 36m. The effective height should be ~25m or so. As such, we know the DSM reads high.

Then we use the NF-5035. The electric field numbers as read from the E-Field sensor come to 87dBuV/m on the averaged trace, which is an improvement on the DSM but still yields and effective height of 67m (still impossible). Plus, there are no calibration values to adjust the Efield sensor to so these numbers are not much good.

So we move to the H Field sensor.

The stored data cannot be displayed in the spectrum analyzer display but I have a question on that in another post, however, from notes in the field the 3D seonsor data was VERY close to the NF 3D Z-Axis so we can use the Z-Axis cal data to adjust this. The average value is ~0.000124 A/m, converting to Electric field is 0.047V/m.

So, our first problem is that this is twice as high as the Efield measurement above.....How can that be? Is the E-field sensor out or the H field?

If we then assume that the 0.000124A/m is the Z-Axis value, that comes out to 155pT.
The calibration sheet I got with my unit says this:
Frequency Z-Cal Z-Observ Z-Offset
122,880.0 7.000E-007 7.003E-007 -3.000E-010
163,840.0 7.000E-007 7.003E-007 -3.000E-010
245,760.0 7.000E-007 7.003E-007 -3.000E-010
327,680.0 7.000E-007 7.004E-007 -4.000E-010
491,520.0 7.000E-007 7.002E-007 -2.000E-010

In short, that cal values tell me that unit reads slightly high.

So my offset at my frequency of 310KHz is ~-3.8x10^-10 T which is -380 x 10^-12.
If I add this offset to my measured value I now have a negative magnetic field of -225 x 10^-12?
I can't have a negative magnetic field in absolute units.
I think I'm making a major error here. Can someone point me in the correct direction as where my foul up is?

Cheers,
John


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Moderator
(@moderator)
Member Admin
Joined: 11 years ago
Posts: 1048
18/03/2014 6:39 am  

For LF fields you can NOT convert electric to magnetic fields or magnetic to electric fields as you can do with RF measurements...

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jserink
(@jserink)
Active Member
Joined: 7 years ago
Posts: 6
18/03/2014 6:54 am  

Hello:

You stated:
"For LF fields you can NOT convert electric to magnetic fields or magnetic to electric fields as you can do with RF measurements..."

This is an RF measurement, at 310KHz.

None the less, this does not explain why I get a negative magnetic field value after applying the calibration offsets. Lets forget about the E filed conversion for the moment.

Is there a problem with the magnetic Field offset values in the cal sheet?

Cheers,
John


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Moderator
(@moderator)
Member Admin
Joined: 11 years ago
Posts: 1048
18/03/2014 4:23 pm  

300kHz is LF and not RF.

No idea what you are mixing up with your offset stuff, sorry i dont get your posting aniway...

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jserink
(@jserink)
Active Member
Joined: 7 years ago
Posts: 6
19/03/2014 4:07 am  

Hi:

Ok, perhaps I'm not being clear, I'll simplify this.

I have a measurement I took outside from a signal that is at 310KHz.
I measured an average of 190pT in the Z direction, 1D.

The calibration sheet says this for the Z axis:
Frequency Z-Cal Z-Observ
122,880.0 7.000E-007 7.003E-007
163,840.0 7.000E-007 7.003E-007
245,760.0 7.000E-007 7.003E-007
327,680.0 7.000E-007 7.004E-007
491,520.0 7.000E-007 7.002E-007

So, at 310KHz we extrapolate and get the following values:
310,000.0 7.000E-007 7.0035E-007

This means that at 310KHz the NF-5035 reads 350pT higher than it should.

So, if I subtract the 350pT from my measurement of 190pT, I'm now at -160pT.

How can I be at -160pT?

Have I applied the offset incorrectly?

Does that make sense?

Cheers,
John


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Moderator
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Member Admin
Joined: 11 years ago
Posts: 1048
19/03/2014 4:44 am  

???? You have to use the error PERCENTAGES and not the total values of the calibration sheet.
So as you now can see in the calibration sheet our NF units are ulta accurate and the error is almost zero. So it makes no sense to add any corrections to the measurements.

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