IMHO all probes are calibrated under far field conditions.
In general: Using probes in the proximity (< lambda) of anything
conductive (including ground planes at 10 cm and including EUT) makes
the measurement data useless.
As James correctly states, the construction of the probe makes this
effect different per type of probe, be it the construction, the size of
battery or electronics on board or the lead (fiber or copper) , as long
a other conductors are in proximity the read out has no relation to
calibration data anymore.
Using a probe near a ground plane, such as usual in automotive test set
ups, indeed says not much about the test level of the EUT.
Repeating this test under far field conditions, preferable on an antenna
calibration facility, might give you much better results. (not that you
are allowed to generate this much of power on air ;<)
Gert Gremmen
On 4-3-2018 11:06, James Pawson (U3C) wrote:
Hi David,
An interesting set of results! I’m going to ask some questions that
I’m sure you’ve already considered so please bear with me being
Captain B. Obvious.
Do your field probes use frequency correction? I’m not familiar with a
wide range of probes but my Narda PMM field probe has an internal
calibration table; you tell it what the field frequency you are
applying is and it makes the appropriate correction. However, looking
at the typical correction data from the manual (see PDF page 12 of
this doc:
https://www.emctest.it/public/pages/strumentazione/elenco/Narda/EP%20600/Manuali/EP600-EP601EN-90302-2.02.pdf)
it doesn’t look like a large difference.
Is there a difference in the probe construction between the probes
used? Some probes like the Narda one above have two antenna per axis
whereas ones like this Amplifier Research probe -
https://www.arworld.us/html/18200.asp?id=636 only have one antenna per
axis. Perhaps the proximity of copper plate makes a difference.
On the subject of copper plate, what are the differences without this
present? What are the dimensions of it and are they significant at the
frequencies selected?
Have you acquired just spot readings or a full frequency sweep? There
may be some patterns in the frequency sweep data that give you more of
a clue as to what’s happening.
An interesting puzzle and I look forward to hearing about your results
further!
All the best
James
*From:*Schaefer, David [mailto:dschae...@tuvam.com]
*Sent:* 04 March 2018 05:22
*To:* EMC-PSTC@LISTSERV.IEEE.ORG
*Subject:* [PSES] Field probe calibration
I took data with 4 field probes, 3 different models. All calibrated.
Two calibrations by the manufacturer, two by a reputable cal house.
200-1000 MHz data, 10 MHz step size, 60 V/m level. I recorded the
forward power, and all equipment and software in the setup was the
same, barring only the measuring field probe and associated probe
factors. Composite values only. No 3-axis data as I don’t have 3-axis
calibration data for all probes. Probes were 10 cm above a copper
bench, DRG antenna 90 from the bench.
The results are not encouraging. The tables below show the results in
watts of forward power for select frequencies.
Antenna Horizontal – values in Watts
Probe 1
Probe 2
Probe 3
Probe 4
Max-Min(Watts)
200 MHz
85.17
144.4
135.9
97.75
59.23
220 MHz
92.81
171.6
157.4
113.5
78.79
500 MHz
21.7
34.93
28.58
26.94
13.23
900 MHz
25.57
37.25
25.6
32.42
11.68
Antenna Vertical – values in Watts
Probe 1
Probe 2
Probe 3
Probe 4
Max-Min(Watts)
200 MHz
18.94
25.12
22.55
18.82
6.3
330 MHz
34.1
40.69
46.29
39.41
12.19
780 MHz
35.52
53.03
29.87
32.83
23.16
930 MHz
56.63
47.01
64.26
107.7
60.69
There are trends in the data. Probe 1 was usually the lowest. Probe 2
was usually the highest, rarely the lowest.
If you want to talk field strength effects this will mean, depending
on the probe, you could have an E-field 40% higher between two
‘identical’ calibrations. The large variance between which probe was
highest or lowest based on freq. is troubling, as is the clear
difference between horizontal and vertical. I took additional data
with two probes of the same model rotated around a center axis. I
don’t have that all compiled, but just comparing one probe against
itself, laying on the left, right, and bottom sides, results in up 20%
difference in required power.
I have not read IEEE 519, but plan to soon. So my question to this
group - do you think field probe calibrations are accurate? How can we
have confidence in our results with such widely varying results?
Thanks,
David Schaefer
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