Hi Sam,
This substitution method has been around a long time, maybe 50 years or so.
FCC has (for licensed transmitters) specified spurious and harmonic
emissions to be X dB below the in - band carrier power, when
measuring conducted emissions out the antenna port, or radiated
emissions from the case.
The specification units are in units of power, not field strength.
Section 2.1053 of FCC rules states:
"Information submitted shall include the relative radiated power of
each spurious emission with reference to the rated power output of
the transmitter, assuming all emissions are radiated from halfwave
dipole antennas."
The procedure generally used is:
1. Make the normal radiated emissions measurement, with the EUT at
far field distance for all frequencies under consideration. Rotate
the EUT, raise and lower the search antenna in H and V polarities,
record maximum levels. For thoroughness the EUT should be oriented
in H and V polarities if it can operate that way (ex: hand-held
portable transmitter)
2. Replace the EUT with an antenna with center of radiation at
approximately the same height as the EUT center of radiation. For FCC
licensed transmitters, the antenna port is generally terminated with
a shielded 50 ohm load so the center can be approximated by the
center of the EUT case.
3. Connect signal generator to the substitution antenna, set to
emissions frequency under consideration, then using the same search
antenna as in step 1, raise and lower search antenna in H and V
polarities to maximize the signal.
4. Once maximized adjust the signal generator to produce the same
levels recorded in step 1.
5. Reported emission = signal gen, dBm - cable loss, dB + {gain of
substitution antenna above dipole (=GdBi - 2.15 dB)}.
Compare this to the dBm level called out by the requirement.
NOTE: Some FCC and ETSI specs will require reference to an isotropic
antenna, in that case the antenna gain term in step 5 is G dBi
In my experience if measurements are made in the far field and on a
good OATS the levels in 5 should correlate pretty well with
radiated emissions readings when using the relationship between E,
P, G, and d in the far field and solving for P.
best regards
Tom Cokenias
T.N. Cokenias Consulting
P.O. Box 1086
El Granada CA 94018
tel 650 726 1263
fax 650 726 1252
cell 650 302 0887
At 9:21 AM -0500 12/28/2001, Sam Wismer wrote:
Hi all,
Thanks for the input.
For Ken's question as to why I am putting myself through this exercise
is because a customer had asked me to, therefore I shall give it my best
shot.
What I have learned from this is that there doesn't seem to be an
industry standard method in performing substitution field strength
measurements and that my deviation of up to 10dB doesn't seem to be
surprising to many.
John's comments below echo my thoughts exactly as to the radiation
pattern of the EUT and the chosen substitution antenna. I don't
understand all the discussion about the radiation pattern of the EUT
versus the transmit bi-log. The transmit antenna and SG are set to
reproduce the field strength of the EUT exactly(in magnitude and
frequency), therefore the receive antenna really doesn't know anything
has changed. I am no physicist so I could be way wrong on this.
By the way, both antennas are matching bi-logs with very close antenna
factors and I suppose SWR's.
Kind Regards,
Sam Wismer
Engineering Manager
ACS, Inc.
Phone: (770) 831-8048
Fax: (770) 831-8598
Web: www.acstestlab.com
-----Original Message-----
From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of John Woodgate
Sent: Thursday, December 27, 2001 5:14 PM
To: emc-p...@majordomo.ieee.org
Subject: Re: Field Strength - Substitution Method
I read in !emc-pstc that Cortland Richmond <72146....@compuserve.com>
wrote (in <200112271228_mc3-ec1b-a...@compuserve.com>) about 'Field
Strength - Substitution Method', on Thu, 27 Dec 2001:
>What you don't have -- and what, I think, is most difficult -- is a
model
>that reliably correlates a substitution antenna as a source with the
>equipment under test. A rack seven feet tall and two feet on a side --
with
>wires overhead and off to its sides -- will NOT radiate the same as a
>dipole. And it will differ more from an antenna as its dimensions
become
>larger than the antenna. I would expect differences to become more
>pronounced, in other words, at higher frequencies. This is what you
saw.
I don't think that's relevant. The EUT, whatever it is, produces X
dB(uV/m) at the receiving antenna. The sig gen and bilog is then set up
to produce the same field strength at the receiving antenna. The problem
seems to be *independent of the EUT*. The calculated field strength of
the sig gen and bilog doesn't agree with the *measured* value.
Having said that, an EUT may have an entirely different radiation
pattern from that of an antenna, but the normal OATS procedure is
'blind' to this, even if the EUT is large enough to subtend quite a
large angle at the receiving antenna, so that the measured field
strength is actually an average over a considerable volume of space.
--
Regards, John Woodgate, OOO - Own Opinions Only.
http://www.jmwa.demon.co.uk
After swimming across the Hellespont, I felt like a Hero.
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