Low-Frequency E-Field Standard

LOW-FREQUENCY E-FIELD STANDARD

Hubert Trzaska ITA, Technical University of Wroclaw Wyspianskiego 27, 50-370 Wroclaw, Poland

The work discusses methods applied to E-field meters and receiving antenna (Ar) is shown in Fig.1. The E-field standardization till now. Apart from low-frequency E-field generated by the standard antenna of length l at distance d is measurements in far-field with loop antennas, in the near- given by formula 1. In the formula I denotes current fed to the field, whip or dipole antennas are in use. The latter were standard antenna base and k is the propagation constant [1]. standardized with the use of a TEM-cell or, on an open site, with the use of another whip antenna. For this purpose is proposed a similar standard as previously used for loop antennas standardization. It is more effective, less susceptible to interference and surroundings, easier in use and is more universal. A set of standard loop antenna was designed and comparative measurements of a short, symmetrical dipole antenna were performed. Results of measurements in a TEM- cell and with the use of the standard are in good agreement. Fig.1 Standard and receiving antennas above conducting surface. 1.INTRODUCTION Whip antennas standardization methods are troublesome in their use, limited to an open-site and, as a result, dependent upon weather conditions. There are serious problems with The EM Environment Protection Lab of the Technical effective matching of short, transmitting whip antenna to a University of Wroclaw has been involved in the construction source of (usually) standard output impedance. The system is of ElectroMagnetic Field (EMF) meters, dosimeters and very susceptible to the presence of other interfering fields as indicators as well as in susceptibility measurements of wide well as to reflecting objects in its surroundings. This results in range types of electric and electronic equipment. Each degraded accuracy of that standard. measurement must be performed in conditions in which A very convenient and accurate method, based upon a accuracy of the measurement is known with no respect to the TEM-cell, may be applied only on a very limited scale, to number and role of factors limiting the accuracy. It has led us standardization of the whip and dipole antennas of sizes much to involvement in EMF standards as indispensable 'by- smaller than the available cell. products'. The standards must follow (or rather advance) The use of a standard loop antenna for standardization of requirements of measurement; i.e. they must be specific E-field meters has been proposed. It is well known that standards for specific EMF component measurement and electrically small loop antenna has one E-field component as specific types of measuring antennas used and must be for well. However, the component has never been used for E-field different applications of the standardized meters (near- or far- standardization. A standard transmitting loop antenna has been field meters, EMF levels, type of polarisation). designed and comparative measurements of an E-field sensor At low frequencies (say below 30 MHz), for far-field E- (then used as a transfer standard) have been performed in a field measurements, meters with magnetic antennas are in use, TEM-cell and with the use of the method presented. Results of while for near-field measurements whip- or short symmetrical these standardizations are in good agreement, it confirms the dipole antennas (usually active) must be applied. Sometime perspective applicability of the method in low frequency E- the latter was possible to standardize in a TEM cell of field meters standardization. Because of the radiation adequate large sizes; the former needed standardization on an efficiency of the loop antenna, it is convenient when open-site with the use of an another whip antenna fed from a relatively high field intensities are required. source. The arrangement of standard transmitting antenna (As)   2 2 2 2  15 I k 2 4 2 2 2 2  d  4l  2l d  l  l  -1 kl E =  k l  jk4 d  l  3d  d  4l  l  ln tg 1.. sin 2 kl 3 2 2 2 2 2    d  4l  2l d  l  l 2. LOOP ANTENNA AS A STANDARD   E           m 2. e  t The near-field analysis of a loop antenna has been  e presented in many papers. Electric (E) and magnetic (H) field H =     m     3. components of a radiator are given by [2]:  t where:  and  - electric and magnetic Hertz vector 2  jkr 2 e m 2  a  e r  j  respectively. E   I Zo   sin 1   2 2 2 2 In the case of an electrically small loop, constant current    r  a r  a  kr  distribution in the antenna may be accepted; because of the dominating role of the magnetic moment the electric Hertz Standardization of E-field antennas is performed here, vector may be here neglected [3], thus: while standard and standardized antennas are placed in xy- IkZ e-jkR plane. A case of a short, symmetrical E-field probes o standardization is shown in Fig.3 (to uniform formulas used in  m  dS 4.  standardization procedures r is replaced by d): 4 S R where: R = r 2  a2  2rasincos 5. S - area of the loop, a - radius of the loop, r - distance to the loop center.

The geometry of the system is shown in Fig.2.

Fig.3 E-field probe standardization with a standard loop.

E-field in the plane of the loop, averaged at the standardized antenna, is given by:  a2 Z I E  o 1 k2D2 9. 2 dD now: D  d2  a2  l2 10. If l < d/4 and a < d/4, formula 9. gives E with accuracy better than 1 %. Similarly as in case of the H-field, it is possible to improve accuracy by using more therms in the series expansion, however, it is not essential from the point of view of the total accuracy of the standard. Moreover, formula Fig.2 Co-ordinate system 9. (and 8.) takes on a convenient and clear form. Contrary to the H-field, given by formula 8., the E-field For a < r R may be expanded in power series; the within a frequency range below 30 MHz is proportional to firsts terms of the expansion are [4]: frequency. Similarly as above, the squared therm under kernel becomes significant only at the highest frequencies. a 2 R  r  asincos  (1 sin 2cos 2) For l,d << d formula 9. becomes identical with the 2r modulus of the E-field of the elemental magnetic dipole at the dipole plane. The solution presented may be applied only in 6. the case of one path propagation. It assures good accuracy if The magnetic moment of the loop placed at xy plane has reflected ray may be neglected. The condition may be fulfilled only a z-component, thus: in laboratory conditions. mr  mzcos 7a. 3.EXPERIMENT m  mzsin 7b. Substituting 6. to 4. and then 7a-b to 3., we would have H- field components. The radial component, averaged at distance To prove the concept of a loop antenna use in the d on the surface of a standardized loop antenna of radius b, for generation of standard E-fields, especially at low frequencies, a monochromatic field is given by [5]: the standard loop antenna was designed and a series of 60 a2I measurements was performed. 2 2 The standard antenna is very similar to those applied for H r  3 1 k D 8. ZoD standard H-field generation [6]. The difference here is in the where: plane in which the standardized antenna is placed during standardization. Till now, for standardization H-field meters 2 2 2 D  d  a  b (with loop- or ferrite-rod antennas) a special bench was It is worth noticing here that for a,b << d formula 8. is constructed and applied in order to stabilize the geometry of identical with the modulus of the radial component of the the antennas. The bench makes it possible to place two loop magnetic field of an elemental magnetic dipole. The formula antennas parallel to each other and coaxially as well as to turn assures accuracy of about 1% if d/a > 4 and d/b > 4. The H- the standard loop in 90 degrees for ferrite antennas- and small field is frequency-independent and is a function of frequency E-field antennas (say till l=20 cm) standardization; maximal only at the highest frequencies, where the squared term in distance (d) between antennas at the bench is 1.5m. kernel becomes significant. The standard loop antenna, designed for E-field meters Following similar procedures with the use of 2., we have: standardization, is a single turn circular loop of diameter 9. 2a=20cm. It is symmetrically fed through a balloon (Tr) and in its center an insulated thermocouple for current measurement accuracy of the thermocouple current measurement (the is immersed. Because of electrically small sizes, such Achilles' foot of the method), accuracy of approximate symmetry was not required here, but the same set was formulas and limitations related to the geometry of provided for use at frequencies above 30 MHz, where the propagation and presence of reflecting objects. Detailed symmetry is indispensable. For stronger field's generation a discussion of the H-field standard accuracy has already been multi-turn antenna was developed as well; its use is analogous presented [9]. It is in place to mention here another problem to the single turn one, but in formula 9. current I should be with accuracy that is difficult to estimate explicitly, i.e., the multiplied by n (where: n-number of turns). Multi-turn loops problem of spectral purity of the standard feeding source. The are electrostatically shielded to limit unwanted radiation. The problem was previously discussed in detail [8], but careful antenna's diagram is shown in Fig.4. investigation of the selected power sources, their spectrum control and the use of band-pass filters is always needed. Table 1. Standardization comparison [V/m]. f[MHz] E TEM E loop

The harmonics in a standard antenna cause measurements nonrepeatable, especially at different EMF levels and while a power source is changed to another one. Limited spectral purity characterizes modern generators using the frequency synthesis principle and having wide-band output amplifiers. Old types of generators, with selective PAs, needed tuning at Fig.4 Circuit diagram of the standard loop antenna. every frequency, but their signals were 'more sinusoidal'. Previously reference standards and E-field probes were The thermocouple was calibrated on 50 Hz. The antenna standardized with the use of a TEM-cell. Excitation current is set up before any standardization step is taken, and measurement of a cell is well above that of the thermocouples then the thermocouple emf measuring set is disconnected to current measurement. The TEM-cell assures the strongest limit EMF disturbances around the antenna (even though the fields with relatively small excitation power and, even more disturbances caused by wires connecting the thermocouple importantly, has a flat frequency response. However, for with a DC milivoltmeter still have not been noticed ). discussed purposes a relatively large-sized cell would be To illustrate the level of the generated standard field, in necessary. If, at the least, such a cell were available, the EMF Fig.5 is plotted the E-field versus frequency for a single turn uniformity inside it would not be enough [10,11]. loop, at distance d=1m and for I=1A. Apart from frequencies Matching the loop to a standard output impedance source above 10 MHz, the dependence is linear - the scale of the plot characterizes low efficiency; however, the efficiency is much makes it impossible to see faster changes above 10 MHz. better in the case of a loop antenna as compared to a whip one. The method presented has mainly been developed for Use of low output impedance, solid-state power amplifiers or short E-probes, then applied as transfer standards [7], active transmitting antennas would improve the efficiency. standardization. The standardization, in some cases, could be The possibility to generate relatively strong EMF (while low performed in a TEM-cell. However, a large enough cell is power generators are applied), in the case of an open-site necessary; the accuracy of standardization in a cell is measurement, is necessary only to assure an acceptable comparable to the presented one, and the standard may be 'signal/noise' ratio. An antenna is usually a linear applied for other purposes as well. In Table 1. are compared results of the standardization of an active E-field probe in a 4.FINAL COMMENTS TEM-cell and with the use of the method proposed here.

The standard may be used for standardization of any types and sizes of E-field antennas using 'the standard antenna method'. However, one needs to take into account the presence of the ground and the multipath propagation (formula 9 will have additional correction therms). The procedure is evident; it needs smaller space as compared to the 'whip standard', is more effective, less sensitive to interference and the presence of conducting bodies (reflected rays and EMF disturbances) in its proximity, etc. However, because of practical reasons, another way was applied - it is based on 'the standard field method'. A standardized probe (or transfer standard) is applied for field measurements as shown in Fig.6. An arbitrary transmitting loop antenna (At) serves as the EMF source. From its one side, at the plane of the loop, a standard probe Fig.5 E-field versus frequency. (As) is placed whereas a standardized antenna (Ar) is placed at the same distance, from the other. After measurements the standard and the standardized antenna are replaced and the The most important parameter of the standard is its measurements repeated. This indirect procedure reduces accuracy. Accuracy was estimated at a level similar to the standardization errors dependent upon propagation geometry magnetic field standard working within the frequency range, and allows automatization of the measurements. Contrary to i.e. 5% [8]. The result of the estimation is evident, as the the procedures applied up to now, it does not need factors limiting standards accuracy are the same. They are: troublesome replacement of the standardized antenna and the transfer probe during each measurement. The proposed 3. G.Gonzalez, M.A.Huerta - Fresnel region fields produced procedure is especially advisable for large sized objects by circular loops and annular slot antennas. J.Appl. calibration as well as for E/H antennas (with a cardioidal Phys.vol.44, Nr 8/1973, pp.3500-3504. radiation pattern) measurements. The approach fulfils the 4. I.N.Bronstein, K.A.Semendiaev - Manual of mathematical basic requirement of the standardization: the technique applied functions (in Russian). Moscow 1954. must reflect, as far as possible, the conditions existing during 5. F.M.Green - NBS field-strength standards and the normal work of a standardized device [5]. measurements (30Hz-1000MHz). Proc.of the IEEE Nr 6/1967, pp.970-981. 6. M.Kanda - Standard probes for electromagnetic field measurements. IEEE Trans.vol.AP-41, Nr 10/1993, pp.1349-1364. 7.W.Bitterlich, N.Nessler - Uberlagungen zur Eichung einer Ferritstabenantenne fuer Messzwecke (in German). Intl.Elektr.Rundschau Nr 11/1971, pp.277-282. 8. E.Grudzinski, H.Trzaska - Selected problems of EMF standards. Proc.1992 EMC Symp.Wroclaw, pp.265-269. Fig.6 Open-site calibration. 9. H.Trzaska - Magnetic field standards at frequencies above 30 MHz. HEW Publ.(FDA Nr 77-8010), pp.68-82. The main disadvantages of the proposed low-frequency 10. E.Grudzinski, H.Trzaska - The TEM-cell in EMC E-field standard are: frequency dependence of the generated investigations. Proc.1983 Intl. EMC Symp. Zurich, EMF and the accuracy of the thermocouple excitation pp.407-410. measurement, however, there are chances to change the 11. L.Carbonini - A transmission line device for EMI method to more accurate one. susceptibility measurements with enhanced field The advantages of the standard in relation to the 'whip' uniformity. Proc.1992 Intl.EMC Symp.Wroclaw, one may be sumarized as follows: pp.219-223. 1. Accuracy. The accuracy of the standard is better in any its version and it is less susceptible to external EMF and other BIOGRAPHICAL NOTE type interference, 2. Open area requirements. Small-sized transfer standards and E-field probes may be standardized in laboratory conditions; Dr.Hubert Trzaska was born in 1939 in Wilno, now in requirements for an open-site for larger antennas are less Lithuania. After W.W.II he moved to Wroclaw and here rigorous. completed his education. He received his M.Sc., Ph.Dr. and 3. Efficiency. The loop antenna, as the transmitting one, can D.Sc. degrees at the Technical University of Wroclaw in 1962, be better matched to an exciting source, it assures better 1970 and 1989 respectively. Since 1962 he has been with the efficiency and higher EMF strengths, the latter improves Inst. of Telecomm. & Acoustics. At present he is head of the 'signal/noise' ratio. EM Environment Protection Lab. He's been involved in EMF 4. Susceptibility. Because of the above mentioned item, standards and measurements, in selected EMC problems and susceptibility to interfering signals is less. Open site tests are bioelectromagnetics. Author of many publications. less sensitive to the presence of reflecting objects- it results in reduced area requirements (see p.2). 5. Universality. The same set of standard antennas and auxiliary equipment, makes it possible to standardize E-, H- and E/H-field meters. 6. Practical aspects. The method is faster, more effective and less absorbing. The procedure needs a smaller measuring team - it is important when field measurements are being performed.

ACKNOWLEDGEMENT

The work was partly supported by the National Institute of Standards and Technology grant Nr NIST/MEN-94-177, whose support is gratefully acknowledged.

REFERENCES

1. D.A.Tschernomordik - Standardization method for whip antennas in frequency range 0.15-30 MHz (in Russian). Trudy NIIR Nr 3/1968 pp.13-27. 2. D.J.Bem - Antennas and radio-wave propagation (in Polish). Warsaw WNT 1973.