RE: Field Strength - Substitution Method
Hi All, Thanks for all your input. I believe I have a better handle on it now. 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: Friday, December 28, 2001 1:31 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 <200112280904_mc3-ec2a-3...@compuserve.com>) about 'Field Strength - Substitution Method', on Fri, 28 Dec 2001: >This isn't the issue. The receiving antenna, as you say, can't tell. The >problem is the substitute antenna. > >We assume the source is a dipole (or, here, a bilog) at every frequency. >Our substitute antenna has a simple pattern (even a bilog - we spend a lot >of money to get it, too). The source is not so simple. Even if it IS a >half-wave dipole at (say) 100 MHz, at harmonics it is a wavelength long or >more, and radiates in sheaves of cones oriented at some angle along the >axis of the wire (a LP does NOT do this!) - most of which may miss the >receiving antenna completely on an OATS. The angles along the wire decrease >as the frequency increases. It will have gain over a dipole; the lobes off >its ends narrower and stronger than those from a dipole fed with the same >power, and at high enough harmonics, close enough to the axis of the >conductor that they are again directed towards the receiving antenna as we >turn the EUT. > >This will bias calculations which assume the source has a simple pattern at >_every_ frequency. It doesn't. I just don't buy that. The receiving antenna is measuring the field strength at its position (actually some sort of average over its volume). How that field strength is produced is irrelevant - whether it comes from the bilog or the EUT. Besides, limits are based on the direct measurement of field strength by a receiving antenna. Only the changes of height and polarization search the actual emission pattern of the EUT, with a VERY broad brush. Emissions in narrow lobes in other directions and emissions at harmonic frequencies are not measured, but that is not an error - they are either measured at another stage (higher frequency measurements or during rotation of the EUT) or are not required to be measured. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
RE: Field Strength - Substitution Method
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
Re: Field Strength - Substitution Method
I read in !emc-pstc that Cortland Richmond <72146@compuserve.com> wrote (in <200112280904_mc3-ec2a-3...@compuserve.com>) about 'Field Strength - Substitution Method', on Fri, 28 Dec 2001: >This isn't the issue. The receiving antenna, as you say, can't tell. The >problem is the substitute antenna. > >We assume the source is a dipole (or, here, a bilog) at every frequency. >Our substitute antenna has a simple pattern (even a bilog - we spend a lot >of money to get it, too). The source is not so simple. Even if it IS a >half-wave dipole at (say) 100 MHz, at harmonics it is a wavelength long or >more, and radiates in sheaves of cones oriented at some angle along the >axis of the wire (a LP does NOT do this!) - most of which may miss the >receiving antenna completely on an OATS. The angles along the wire decrease >as the frequency increases. It will have gain over a dipole; the lobes off >its ends narrower and stronger than those from a dipole fed with the same >power, and at high enough harmonics, close enough to the axis of the >conductor that they are again directed towards the receiving antenna as we >turn the EUT. > >This will bias calculations which assume the source has a simple pattern at >_every_ frequency. It doesn't. I just don't buy that. The receiving antenna is measuring the field strength at its position (actually some sort of average over its volume). How that field strength is produced is irrelevant - whether it comes from the bilog or the EUT. Besides, limits are based on the direct measurement of field strength by a receiving antenna. Only the changes of height and polarization search the actual emission pattern of the EUT, with a VERY broad brush. Emissions in narrow lobes in other directions and emissions at harmonic frequencies are not measured, but that is not an error - they are either measured at another stage (higher frequency measurements or during rotation of the EUT) or are not required to be measured. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Field Strength - Substitution Method
Group, A standard published by ETSI this month is good reading on this subject. TR 102 273-4 V1.2.1 Larry K. Stillings Compliance Worldwide, Inc. 357 Main Street Sandown, NH 03873 (603) 887 3903 Fax 887-6445 www.complianceworldwide.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Field Strength - Substitution Method
I guess what I was really asking was, "What was the technical purpose of the substitution measurement?" From the follow-on messages, I gather maybe the purpose was to determine ERP. Then it makes sense to use an antenna of known gain with a calibrated rf signal level driving it to achieve the same receive signal as the EUT generates with its antenna. If the EUT-connected antenna is also a bilog, then there ought to be good correlation between signal levels driving the antennas unless the frequency of operation was such that there is a lot of VSWR (mainly at the low end) so that a significant portion of the rf source was reflected back into the source. -- >From: "Sam Wismer" >To: "'John Woodgate'" , >Subject: RE: Field Strength - Substitution Method >Date: Fri, Dec 28, 2001, 8:21 AM > > > 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. > > --- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson:pstc_ad...@garretson.org > Dave Healddavehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on t
RE: Field Strength - Substitution Method
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. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
RE: Field Strength - Substitution Method
Hi Jim, How is that additional loss you mention not accounted for in the antenna factor? How can the loss of antenna change depending on what you are measuring? Isn't an antenna factor an antenna factor? How does one know that they should consider that there may be additional loss of the antenna over an above the antenna factor, by the way that they are using it? Also, what is the basis for rotating the antenna as you mention? Isn't that more or less creating the desired result by changing the set up? If that is acceptable, I suppose I could raise the receive antenna to 4 meters or to whatever arbitrary height that allows me to pump as much power as I need into the transmit antenna that I need a correction for. Not meaning to jab, because whose to say that I am doing it right by setting both antennas to 1 meter in height in the horizontal polarity. 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 Jim Conrad Sent: Thursday, December 27, 2001 3:38 PM To: Cortland Richmond; Sam Wismer; ieee pstc list Subject: RE: Field Strength - Substitution Method Sam, You should also consider the radiation may be from multiple sources on the EUT. Polarization of the source may not be the same at the substitution antenna. Try 0, 45, and 90 degree polarization of the bi-con. A tuned dipole would also give you better results as a bi-con has a very high SWR. This gives you addition loose which you may not have accounted for. Jim -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Cortland Richmond Sent: Thursday, December 27, 2001 12:26 PM To: Sam Wismer; ieee pstc list Subject: Re: Field Strength - Substitution Method Sam, I think you did it right with one AF and one gain. There's a problem with that method. You need more information needed to make the _results_ right. Given a certain power at the antenna terminal, and a known gain and efficiency, you can calculate the free-space field strength at some distance due to RF applied to that antenna. Then you add loss or gain due to reflection from the ground plane. This gets you down to some fairly reliable way to estimate what field strength will be created over a ground plane at some distance from an antenna. 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. Cortland (my own opinion and not that of my employers) --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, s
Re: Field Strength - Substitution Method
>> Can you please explain why? The receiving antenna just responds to the field strength at its position; it doesn't 'know' anything about the source - it cold be an EUT at 10 m or a distant TV transmitter or even a cosmic source. << This isn't the issue. The receiving antenna, as you say, can't tell. The problem is the substitute antenna. We assume the source is a dipole (or, here, a bilog) at every frequency. Our substitute antenna has a simple pattern (even a bilog - we spend a lot of money to get it, too). The source is not so simple. Even if it IS a half-wave dipole at (say) 100 MHz, at harmonics it is a wavelength long or more, and radiates in sheaves of cones oriented at some angle along the axis of the wire (a LP does NOT do this!) - most of which may miss the receiving antenna completely on an OATS. The angles along the wire decrease as the frequency increases. It will have gain over a dipole; the lobes off its ends narrower and stronger than those from a dipole fed with the same power, and at high enough harmonics, close enough to the axis of the conductor that they are again directed towards the receiving antenna as we turn the EUT. This will bias calculations which assume the source has a simple pattern at _every_ frequency. It doesn't. Cheers! Cortland --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Field Strength - Substitution Method
Either I or everyone else has misunderstood something important in this message. The message states that the EUT is an intentional radiator. That implies an antenna-connected rf source. Assuming that the fundamental frequency is what is of interest and that it is high enough and the antenna low enough gain such that the OATS measurement is a true far-field measurement, correlation should be easy to achieve between an EMI instrumentation antenna and the EUT-connected antenna. That is, if we are in the log portion of the bilog antenna and the gain is 10 dBi, whereas the EUT-connected antenna has 3 dBi gain, then the rf generator driving the bilog would need to be set 7 dB lower than the rf source in the EUT. This assumes that VSWR driving both antennas are comparable. What I don't understand and why I think I may be missing something is I have no idea what is the value of this exercise. The only kind of substitution measurements I have heard of are for antenna gain determination or field intensity determination on older EMI receivers. The antenna gain determination consists of driving the antenna-under-test (AUT) with a known signal level and measuring the pickup with a remote antenna in the far field. Then you disconnect the AUT and substitute a test antenna of known gain. The difference in dB measured at the receiver connected to the receive antenna is a direct measurement of the difference in gain between the AUT and the standard. EMI receivers cannot be calibrated at a single frequency as spectrum analyzers are because the preselector and other factors change the path losses and gains from rf input to baseband output as a function of frequency. Older EMI receivers coped with this by using a substitution technique. When a transducer had delivered a signal of interest to the receiver the meter deflection was noted and a coax switch was toggled which disconnected the transducer input and connected a calibrated rf signal source. The rf signal source was adjusted until the meter deflection matched the actual measurement. The rf signal source level, adjusted for any losses such as those in an antenna balun, was compared to the limit which in those days for radiated limits was called an antenna-induced limit, meaning the limit was not in terms of field intensity but rather in terms of the signal delivered to a properly matched load by an antenna of specified construction and gain. Ken Javor -- From: "Sam Wismer" To: "EMC Forum" Subject: Field Strength - Substitution Method List-Post: emc-pstc@listserv.ieee.org Date: Thu, Dec 27, 2001, 8:57 AM Hi Group, I have been asked to perform field strength measurements using standard radiated methods as well as substitution methods on the same sample(an intentional radiator). While I have plenty of experience making radiated measurements, I have never made a substitution measurement. I thought I have read enough about it to at least give it a go, but I am getting some results that I didnt expect. I assumed that both measurement methods should produce similar results within reason(+-5dB or so or perhaps even closer). The results I am getting in some cases have deltas of up to 10dB. The procedure I am employing basically consists of: 1) Making the radiated field strength measurement on an OATS and recording the raw result at the receiver 2) Substitute the EUT with a signal generator and an antenna(I am using a Bi-log). 3) I adjust the signal generator level to produce the same level at the receiver 4) I take the SG level and subtract the losses and add the gains. The calculated result I figure should be close to the measured value obtained in step 1, but its not. Ive tried applying the correction factors in several ways to Back into the proper equation, but the results are all over the place. One question I have is should I use the AFs of both antennas or just one? And if just one, should I consider the gain of the other antenna? My logic is as follows: I presume the AF takes into account the 3 meter Path Loss in some fashion. Therefore, following my logic, if I use both AFs, the 3 meter Path Loss is applied twice. Using both AFs produces a result nowhere close to the measured value. I then used the AF of only one antenna and the gain of the other antenna. Logic is that the 3 meter path loss and one antenna is considered by the AF of one antenna, and the other antenna is considered by applying its gain. Using this method, I can get with 4dB of the measured result at the fundamental, but I am still out by 10dB at the 3rd harmonic. Also, should I be using tuned dipoles? If so, why would that make a difference. Any publications I can reference? Any help is appreciated. Kind Regards, Sam Wismer Engineering Manager ACS, Inc. Phone: (770) 831-8048 Fax: (770) 831-8598 Web: www.acstestlab.com
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. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
RE: Field Strength - Substitution Method
Sam, You should also consider the radiation may be from multiple sources on the EUT. Polarization of the source may not be the same at the substitution antenna. Try 0, 45, and 90 degree polarization of the bi-con. A tuned dipole would also give you better results as a bi-con has a very high SWR. This gives you addition loose which you may not have accounted for. Jim -Original Message- From: owner-emc-p...@majordomo.ieee.org [mailto:owner-emc-p...@majordomo.ieee.org]On Behalf Of Cortland Richmond Sent: Thursday, December 27, 2001 12:26 PM To: Sam Wismer; ieee pstc list Subject: Re: Field Strength - Substitution Method Sam, I think you did it right with one AF and one gain. There's a problem with that method. You need more information needed to make the _results_ right. Given a certain power at the antenna terminal, and a known gain and efficiency, you can calculate the free-space field strength at some distance due to RF applied to that antenna. Then you add loss or gain due to reflection from the ground plane. This gets you down to some fairly reliable way to estimate what field strength will be created over a ground plane at some distance from an antenna. 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. Cortland (my own opinion and not that of my employers) --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server. --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Field Strength - Substitution Method
Hear, hear. It may have been "hand waving" but I always assumed that test sites never correlated very well (although their calibration curves do) because the actual radiators one is measuring are "strange" and not well controlled impedances like an antenna. The sources can be "low" or "high" impedance and each act completely differently than one expects. With that in mind, I don't see how it is possible to substitute a controlled antenna, measure the power going to it, and predict anything to the original - within plus or minus 12 dB. Just opinion here and experience fighting battles at test sites. - Robert - Robert A. Macy, PEm...@california.com 408 286 3985 fx 408 297 9121 AJM International Electronics Consultants 619 North First St, San Jose, CA 95112 -Original Message- From: Cortland Richmond <72146@compuserve.com> To: Sam Wismer ; ieee pstc list List-Post: emc-pstc@listserv.ieee.org Date: Thursday, December 27, 2001 10:12 AM Subject: Re: Field Strength - Substitution Method > >Sam, > >I think you did it right with one AF and one gain. There's a problem with >that method. You need more information needed to make the _results_ >right. > >Given a certain power at the antenna terminal, and a known gain and >efficiency, you can calculate the free-space field strength at some >distance due to RF applied to that antenna. Then you add loss or gain due >to reflection from the ground plane. > >This gets you down to some fairly reliable way to estimate what field >strength will be created over a ground plane at some distance from an >antenna. > >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. > >Cortland >(my own opinion and not that of my employers) > --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
Re: Field Strength - Substitution Method
Sam, I think you did it right with one AF and one gain. There's a problem with that method. You need more information needed to make the _results_ right. Given a certain power at the antenna terminal, and a known gain and efficiency, you can calculate the free-space field strength at some distance due to RF applied to that antenna. Then you add loss or gain due to reflection from the ground plane. This gets you down to some fairly reliable way to estimate what field strength will be created over a ground plane at some distance from an antenna. 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. Cortland (my own opinion and not that of my employers) --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson:pstc_ad...@garretson.org Dave Healddavehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
