The probe manufacturer says something like “keep the probe box away or out of
the field” or “best results are obtained with the probe placed on a slant.” But
then they show us isotropicity data that promises +/- ¼ 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 “potato masher” 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 ¾” diameter by 4” 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.
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 -
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
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?)
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
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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