Rational Comparsion of field strength between Semi-anechoic chamber and fully anechoic chamber1 by xuyuzhu


									    Rational comparison of field strength between semi-anechoic
                        chamber and fully anechoic chamber
                                     Ruoting Chang
                    The State Radio Monitoring Center(SRMC) ,China
                     No.80 Beilishi Road Xicheng District ,Beijing P.C 100037

                       e-mail :changrt@srrc.org.cn
Abstract:                                                2. The fundaments and problems of SAC
     In article the Validation of fully anechoic              Both NSA test and field strength test in SAC
chamber adopted to perform EMI compliance test           are based on the below geometrical optics model
is explored by conducting test and theoretical           (Figure 1)
analysis, an preferred limit which is correlated to      Transmit                     Receive
the present limit based on OATS is provided.             Antenna                      Antenna
Key words                                                                   direct ray
      open-area test sites (OATS), semi-anechoic
chamber (SAC), fully anechoic Chamber (FAC),                                   reflected ray
normalized sit attenuation (NSA).                        Ht                                        Hr
         1. Introduction:                                                  ground plane
      The primary intention of emissions
specifications is to prevent unwanted interference
to radio services, and it is also the key measure for
making sure spectra effectiveness. OATS is                                 Figure 1
adopted as standard test site for radiated emission           E=EO(1+Γe ) (equation 1)
test would provide a controlled environment that         Where:     E=the resultant field
not only simulated read ground reflection but also                  strength
greatly enhanced the repeatability of measurements.                 Eo=the direct ray field
With more and more numbers wireless devices and                     strength
other apparatus, the presence of strong and                         Γ=reflect factor
complicated        background       noise   resultant           φ=phase difference result from direct ray
un-corrected errors of the test                          and reflected ray
results in OATS. As an alternative, semi-anechoic             The difference phase relation of these two
chambers have been installed world-wide in recent        rays make an interference pattern which
years. But SAC is very expensive. The relative           corresponds to the function of the height of the
large deviation of comparison results among              receive antenna above the ground. The resulting
different EMC labs indicates the inherent problems       effect rays from obliteration to almost doubling of
of SAC, and as a result, FAC are getting more            the direct ray. So in the relevant standards, the
attention. the correlation between FAC and SAC           receive antenna should scan the height among 1 to
have to be determined for validating he use of FAC       4 meters. So as to locate the maximum strength.
for certification to existing SAC limits. This article   There are following known problems related to
verifies this correlation values with comparison         this methodology.
tests. FAC should play more important role for                1). According to equation 1, the interference
field strength test as well as radiated power test.      pattern depends on the distance between transmit
and receive antenna, height of transmit antenna,              amplitude accuracy of receiver or analyzer is very
polarization, frequency and type of antenna. In               high and precise calibrated, antenna, the error
horizontal polarization, because of Γ= -1, and                results from test equipment can below 0.5dB. For
the distance difference between direct and                    most 10m SACS, deviation of NSA is around
reflected ray can not reach half wavelength or                3.5dB below 60MHz, so the error budget of site
more below 100MHz, so constructive interference               imperfect in relevant CISPR standard is
of direct and ground reflected signal can’t be found          unpractical the large deviation of comparison tests
in the 1 to 4 m height scan range. Therefore the              among different labs is nature.
same magnitude of radiated emission will give a                    5).The huge test time because of the scan
different reading in the received field strength for          receive antenna height for different polarization,
horizontal and vertical polarization on OATS                  scan and sweep, so test flexibility is impossible.
(15dB at 10m distance, 0.8m source height). So the            Normally the above problems are not present or
horizontal polarization radiation is underestimated           can be resolved in SAC.
strongly, at same time, in real world, the reflect                 The cost of 5m or 3m FAC is great lower than
factor of ground is never -1, so there is big risk to         that of 10m SAC. In FAC, because of near-field
evaluate the potential interference of horizontal             effect, differences in theoretical NSA related to a
radiated emissions to radio reception according to            frequency of 30MHz to far field condition of 3m
present standards.                                            and 5m are 1.1dB and 0.4dB respectively at
     The height of maximum field strength of                  30MHz. it is acceptable.
horizontal polarization and illustrated in below              3. Test Comparison between FAC and SAC
table. (10disiance, scan height 1 to 4m, transmit                  A simple relation can be established for
height 0.8m).                                                 perfect, ideal cases (which means free space, far
                               maximum (location              field conditions) between E (v/m), distance
        frequency (MHz)
                                       (m))                   between transmitting an antenna and the point of
             30-230                     4                     measurement (m), e.i.r.p (w).
              240                     3.931                                 30 e.i.r. p 
                                                                    E                           (Equation 2)
              250                     3.777                                     D
              300                     3.150                        A maximum value of E can be calculated,
              350                     2.699                   representing the maximum reading obtainable on
                                                              an OATS by adjustment of the measurement
              400                     2.361
                                                              antenna height. It is
              450                     2.097                        Emax≌1.6E, This means a site gain of 4dB.
              500                   1.88689                        For the free space condition without
           950-1000                   1.00                    consideration of near-field effect, test field strength
                                                              of 3m and 5m will 6dB and 1dB larger compared
     2).The variation in antenna factor with height
                                                              with the situation of 10m SAC respectively. The
of receive antenna, for horizontal polarization in
                                                              comparison tests have been conducted in SRMC
low frequency band up to 2dB.
                                                              10 SAC, 5m FAC and 3m FAC to verify those
     3).The variation in distance to EUT caused by
                                                              value. Height of transmit antenna is 0.8m, height
varying phase center with frequency of
                                                              range of receive antenna is 1 to 4m, in additional,
log-periodic array antenna, the error result from
                                                              below 100MHz of horizontal polarization, receive
this factor is unavoidable and un-corrected in
                                                              antenna height was adjusted among 1 to 6m for
                                                              locating the first peak point.
     4).The error budget of site imperfect of SAC
                                                                   The deviations of NSA of 10m SAC, 3m FAC,
or OATS is 1dB, actually, because of relative
                                                              5m FAC are as below figures:

Figure 2   10m SAC                       Figure 5       5m FAC
                                 The comparison test results are as below:
                            ee  d]





                             3M       0M
                                      20   0M
                                           30   0M 50
                                                40  0M 60
                                                        0M      0M
                                                                70   0M
                                                                     80      0M
                                                                             90   1G
                                                 rqec  H]

                                MS    A5-SC0VR
                                MS    A5-SC0Hm
                                MS    A5-SC0H

                           Figure 5 Comparison between 10m SAC and 5m
                            ee  d]



Figure 3   3m FAC


                             3M       0M
                                      20   0M
                                           30   0M 50
                                                40  0M 60
                                                        0M      0M
                                                                70   0M
                                                                     80      0M
                                                                             90   1G
                                                 rqec  H]

                                MS    A3-SC0VR
                                MS    A3-SC0Hm
                                MS    A3-A1mP

                           Figure 6 Comparison between 10m SAC and 3m

                                                                            Class B           Class A
The above figures show:                                                  Level1   Level1   Level2   Level2
      1. The deviation between theoretical and test                      (dBμv    (dBμv    (dBμv    (dBμv
results of vertical polarization of f≥60MHz is                distance
                                                                           /m)      /m)      /m)      /m)
within ±2dB.                                                    (m)
                                                                         30-230   230-10   30-230   230-10
      2. For horizontal polarization the theoretical                      MHz     00MHz     MHz     00MHz
test values of 5m FAC and 3m FAC are 10 dB and
15 dB larger compared with in 10m SAC at                         3        36       43       46       53
30MHz respectively. The actual test results are
14dB and 18dB (receive antenna height from 1 to                  5        31       38       41       48
6m). Above 150MHz, the deviation from
theoretical value is within ±2dB.                             5. References
      When transmit antenna are 1.0m and 1.2m,                  1) Technical Report EN 50147—3:2001
the same tests were also carried out in SRMC, final             2) Dr.Gam, ”proposal for a new radiated
results are almost same.                                           emission test method using a completely
      Comments on larger deviation below                           absorber lined room with a ground
60MHz:                                                             plane”,th9 Zurich symposium on EMC
      The expand uncertainty of SAC and FAC                     3) The Expression of uncertainty in EMC
facilitates of SRMC are as follows:                                testing
      For 5mFAC, the expand uncertainty is 3.5dB                4) CISPR 22 1998
(f≤60MHz)                                                       5) CISPR11 2001
      For 3mFAC, the expand uncertainty is 3.2dB
      For 10mFAC, the expand uncertainty are
3.6dB (f≤60MHz,horizontal polarization)
and 3.4dB (f≤60MHz,vertical polarization).
      So the larger deviations of comparison tests
compared with theoretical values below 60MHz
are reasonable.
4. Conclusion:
     The advantages of FAC compared with SAC
are obvious , both theoretical analysis and
comparison tests shows the strong correlation of
field strength test between FAC and SAC,
especially, adopting FAC provide the objective
estimation of horizontal polarization field strength.
The preferred limits in FAC given in following
table can achieve the same compatibility levels for
electromagnetic interference as class B (level) and
class A (level 2) in CISPR 11 and CISPR 22.


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