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
<[email protected] <mailto:[email protected]>> 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:[email protected]
<mailto:[email protected]>]
Sent: Sunday, March 04, 2018 12:17 PM
To: [email protected]
<mailto:[email protected]>
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 <[email protected]
<mailto:[email protected]><mailto:[email protected]
<mailto:[email protected]>>>
Reply-To: Cortland Richmond <[email protected]
<mailto:[email protected]><mailto:[email protected]
<mailto:[email protected]>>>
Date: Sun, 4 Mar 2018 08:01:31 -0500
To: <[email protected]
<mailto:[email protected]><mailto:[email protected]
<mailto:[email protected]>>>
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:[email protected]
<mailto:[email protected]>]
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