Hi David -
I had this same problem.
I'm actually glad to see another engineer looking into this!
The answer is not test setup,
not ground plane,
not distance to tabletop, etc.
Each of those can be significant, and should be controlled, but...
The problem you describe sounds exactly like one I had two years ago.
If it is the same problem, the root cause is the *probe calibration*.
The "normal" calibration data that every cal lab provides is too course.
It hides (skips over) resonances in the probes.
Only a finer calibration step size will resolve this problem.
Here is my abbreviated story...
I started with data just like yours.
I had 6 probes: they were two model FP4000, and four of the HI6053.
... <skip the description of months long investigation of setup,
antennas, amplifiers, chambers, height above table, orientation, etc....>
The root cause is the "normal" cal.
It uses a course step size in frequency.
When it is too course, it misses resonances in the probe.
Those resonances are significant, as much as +/- 20% in my equipment.
To prove this, I sent a probe out for "enhanced" cal.
I requested both a "normal" cal and a higher resolution cal.
I asked for 5% steps below 1 GHz and 100 MHz steps above 1 GHz.
When the data came back I plotted the cal factors on top of each other.
It was obvious.
The course cal points of a "normal" calibration will hide resonances
that are +/- 20% deviations.
(above sentence should be BOLD, UNDERLINE, asterisks)
My conclusion: Any probe used for accredited test must have
calibration data showing the resonances.
If it doesn't, then the lab is guaranteed to be over-testing and
under-testing.
DM me and I'll be glad to discuss.
Patrick
On Sun, Mar 4, 2018 at 8:33 PM, Schaefer, David <dschae...@tuvam.com
<mailto:dschae...@tuvam.com>> wrote:
Ken,
This data was not taken with 61000-4-3 primarily in mind. We do
-4-3, but also MIL, RTCA, and ISO testing. I should have had the
probe at least 15 cm for ISO or 30 cm for MIL like you said, but
10cm is how I took the data.
Uniform field calibrations will be a concern eventually, but the
variance is my problem. This was not four probes set up on a bench
next to each other. This was data with one probe on the bench,
centered in front of the antenna, then removed and replaced as
precisely as possible with the next probe.
So if I do a single point cal for ISO 11452-2, one probe might
tell me 100 V/m and another 140 V/m. I'll get questioned by
customers if they fail one day and pass another. This also runs
into another issue - purchasing amplifiers. If I specify an amp to
reach a desired field strength but when it shows up we can't hit
levels due to using a different field probe, there will be hell to
pay.
Standards are silent on probe orientation as well. Do you
position the probe to maximize field strength? If I can get an
extra half a dB of power by having it an angle instead of straight
on, why not do that? I can save that amplifier cost - at least
until I get a new probe. The calibrations don't seem to mean that
much based on my data, so with a composite reading whichever probe
orientation gives me the highest field should be ok.
Also, any replies I make may be delayed. It seems like I usually
see a 4+ hour delay between when I email the listserve, and when
it is delivered.
Thanks,
David Schaefer
From: Ken Javor [mailto:ken.ja...@emccompliance.com
<mailto:ken.ja...@emccompliance.com>]
Sent: Sunday, March 04, 2018 12:17 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG <mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: Re: [PSES] Field probe calibration
In turn:
It is not surprising at all that it takes less power to generate
the vertical field than the horizontal field. That's the effect of
the conducting ground plane. The OP doesn't say what spec they are
working to, but that is why MIL-STD-461 below 1 GHz has the probe
30 cm above the ground plane, to limit that effect.
Comments, such as Gert Gremmen's, that measurements in the
presence of a ground plane (or any conducting structure) are
useless, are themselves useless. The comment reflects a
difference in standards of value. If one is starving, food is the
most important priority. If one is asphyxiating, oxygen is the
primary need. It is logically incorrect for two people suffering
these two conditions to point at each other and say the other one
is wrong about his priorities: they are both correct within the
scope of their individual circumstances. The only logical
observation that can be made is that oxygen needs to be supplied
sooner than food, if the standard of value is immediate survival.
In the world of goods slated for use in home, office and factory,
the coin of the realm is accuracy and minimum uncertainty, so that
qualifications everywhere result in a level economic playing
field. Required field intensities (1/3/10 V/m) are very low
compared to the world of vehicle EMI testing (as high as 200 V/m,
sometimes beyond), so that (again relative) low power amplifiers
may be used with antennas separated from the test area by three
meters instead of one, facilitating the calibration of a quiet
zone in the complete and utter absence of any conducting surfaces,
because the end-item use does not include installation on or near
a conducting ground plane.
This is all in sharp contrast to the qualification of equipment
slated for use on a metal or partially metallic vehicle. The
ground plane is of material use in reducing the intensity of the
horizontal field near it, as noted in the OP and taken advantage
of by the very standards that deal with such qualifications: the
ground plane is our friend. Let us count the ways:
As mentioned above, the ground plane reduces the intensity of a
horizontally polarized field in its immediate vicinity.
The presence of a ground plane causes cables in its vicinity to
react to fields not as an antenna as in the 61000-4-3 and
61000-4-6 paradigm, but as a transmission line. Transmission
lines radiate less per unit of current they carry, and couple less
power from an incident field, than do antennas. Given the very
stringent RE and RS requirements in vehicle standards, we need all
the help we can get.
And finally, metallic equipment enclosures bonded to a ground
plane allows filters to efficiently shunt incoming noise to the
ground plane and away from internal circuitry, and perform that
same function for noise currents coupled to shields that terminate
in a low impedance manner to the exterior of such metallic
equipment enclosures.
Now having dealt with Gert - his recurring comment about the
futility of vehicle EMI testing re ground planes consistently
eliciting the above response from me, ad infinitum and ad nauseum,
lets look at the OP provided test data, especially in light of the
61000-4-3 required UFA (uniform field area) uniformity requirement
of +6 dB, -0 dB for 75% of the required sixteen measurement points.
What I see is that for a given polarization and frequency every
single position measured is within 6 dB of the others. In the
immediate presence of a ground plane, no less! The cup is not
half full - the cornucopia is overflowing. This performance
greatly exceeds the MIL-STD-461 requirement: there is no
requirement for multiple measurement points, and if such are used,
the only requirement is to use the average of the measured points
as the leveling field intensity. In other such standards, such as
RTCA/DO-160, there is a requirement to precalibrate the field in
the absence of the test sample; I would say that the OP test data
is evidence of an excellent chamber.
If I wanted minimal variation between various positions down the
length of a ground plane, I would not use an aperture antenna such
as a DRG horn, but rather one with constant beamwidth vs.
frequency, such as a log periodic. Assuming I could get the
required field intensity with the amplifiers at hand.
Finally, while all the standards of which I am aware allow
leveling on the composite output of field probes looking in three
orthogonal direction at once, it is unsurprising that this results
in significant variations in required power level. Better testing
results when the test equipment allows leveling on the
polarization of interest. Although, as I said above, I consider
the cited test data below to be exemplary.
I would have been bragging to my colleagues, not complaining!
Ken Javor
Phone: (256) 650-5261 <tel:%28256%29%20650-5261>
________________________________
From: Cortland Richmond <k...@earthlink.net
<mailto:k...@earthlink.net><mailto:k...@earthlink.net
<mailto:k...@earthlink.net>>>
Reply-To: Cortland Richmond <k...@earthlink.net
<mailto:k...@earthlink.net><mailto:k...@earthlink.net
<mailto:k...@earthlink.net>>>
Date: Sun, 4 Mar 2018 08:01:31 -0500
To: <EMC-PSTC@LISTSERV.IEEE.ORG
<mailto:EMC-PSTC@LISTSERV.IEEE.ORG><mailto:EMC-PSTC@LISTSERV.IEEE.ORG
<mailto:EMC-PSTC@LISTSERV.IEEE.ORG>>>
Subject: Re: [PSES] Field probe calibration
I'm with Gert.
Anything "antennas" is checked in the far-field -- especially
if testing for accuracy.
I'm a BIG fan of near-field probing for relative measurements and
localizing emissions, but we use probes appropriate to what we are
looking for; if I wanted to "calibrate" one there, I'd use a known
current on a wire/trace or a known voltage on a small plate --
and not trust *that* much.
Cortland Richmond
On 3/4/18 5:35 AM, Gert Gremmen; ce-test wrote:
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
<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
<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
<mailto:dschae...@tuvam.com>]
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