Note that 1.5² is just over a tenth wavelength at 1 GHz.  12.7%, to be
exact, and of course Ed was going from memory here.  We expect no resonances
with this dipole loaded at a high, capacitive impedance, or if we measure
its current output into a low impedance (allowing the short dipole to look
like a current source).

No doubt there are non-idealities, as Ed describes, but they are going to be
very much site-dependent: the answer you get depending on how the probe is
used, which is not something that can be corrected for in a lookup table.

Ken Javor
Phone: (256) 650-5261




From: Edward Price <e...@jwjelp.com>
Reply-To: Edward Price <e...@jwjelp.com>
Date: Tue, 6 Mar 2018 21:57:28 +0000
To: <EMC-PSTC@LISTSERV.IEEE.ORG>
Conversation: [PSES] Field probe calibration
Subject: Re: [PSES] Field probe calibration

Patrick:
 
The probe manufacturer says something like ³keep the probe box away or out
of the field² or ³best results are obtained with the probe placed on a
slant.² But then they show us isotropicity data that promises +/- 1Ž4 dB. It
seems like those are contradictory statements.
Back around 2002, I decided to dissect a dead Narda 8762(?) probe which a
customer helped me drop. The Narda was a white Fiberglas, 300 MHz to 1 GHz
probe that looked like a very elegant German ³potato masher² hand grenade
equipped with a cable that plugged into an IFI EFS field sensor. I wish I
had taken some good pictures of the project, as few people venture that deep
into such expensive territory.
The head contained the orthogonal three-dipole array, with the conductive
arms looking like gold foil on a thin Fiberglass substrate. The length of
each dipole was about 1.5 inches. The sensing elements might have been
thermocouples, thermistors or diodes, and they were mounted in the dipoles.
Each sensor was also connected to a pair of very high resistance plastic
wires (possibly doped with carbon like automotive spark plug wires) that ran
to the far end of the stalk where an analog signal conditioning amplifier
summed the three channels and provided a DC output proportional to field
strength. I was struck by the delicacy of the sensor head, looking at what
must have been a very labor intensive assembly.
BTW, the signal conditioning amplifier was enclosed in a 3Ž4² diameter by 4²
long section of tubular steel, so it was obvious that this conductive mass
(not to mention the shielded, multi-conductor power & signal cable) would
distort the measured field and degrade the isotropicity.
 
Ed Price
WB6WSN
Chula Vista, CA USA
 
From: Patrick [mailto:conwa...@gmail.com]
Sent: Tuesday, March 06, 2018 7:37 AM
To: Edward Price
Subject: Re: [PSES] Field probe calibration
 

Hello Ed - 

  Good morning!

 

  You are correct - the factors stored in the probe correct for the
non-linearities of the diode detector.

      (  as a side note- hearing a presentation live, and asking/answering
questions, is always more educational than a sterile slide deck.

      i wish we all could sit through this presentation, ask questions, and
have dialog. )

 

Did you notice the warnings about errors possible in the calibration
process?

For instance, the "probe on a stick" is calibrated at an angle???

And for that probe they say the electronics box should be kept out of the
field???

   I understand the reasons, but keeping the box out of the field is nearly
impossible for most of our semi-anechoic chamber applications!!

   And I rarely see them used at the same angle as calibrated.

  How does one quantify those effects?

  (...a topic for another thread ?)

 

But, getting back to the frequency response question...

Here is what I recall-

... As shown in the slides, the detector is connected across a small (tiny?)
dipole.  

     the size of the dipole has some real-world limits.

     large enough to capture enough power to make a measurement possible.

     small enough to minimize disturbance in the field.

     the three orthogonal dipoles have to be close enough to represent the
same physical space.

     So the size, placement and response is a compromise away from "ideal"
(i.e. not flat ).

 

... IMHO, there is nothing revolutionary in that information.

    But, maybe I missed something?

    Let me know your thoughts.

 

I would encourage local chapters to contact ETS, and ask them if they could
present at the local chapter meeting.

(DISCLAIMER - I have no affiliation with ETS, I just like the idea of
learning something new about something that was thought to be settled...)

 
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