ambient electromagnetic noise environment measurement

Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 1923, 2012 1443

Ambient Electromagnetic Noise Environment Measurement

L. Nowosielski1, B. Bogdan2, M. Wnuk1, R. Przesmycki1, K. Piwowarczyk1, and M. Bugaj1

1Faculty of Electronics, Military University of TechnologyGen. S. Kaliskiego 2 Str., Warsaw 00-908, Poland

2KenBIT Sp.j., Zytnia 15/22 Str., Warsaw 01-014, Poland

Abstract The external Radio Frequency (RF) ambience establishes the minimum usablesignal level for satisfactory radio communication service. Sources of the RF ambience may benaturally occurring or due to manmade sources. In order to evaluate the level of the local RFelectromagnetic environment the level of the electromagnetic field strength for selected frequencieshas to be measured. In the article the automated measurement system for electromagneticfield strength measurement is presented. In the system the precautions were taken to ensurethat the measuring and controlling equipment does not affect the measured electromagneticfields. The measurement system consists of the measurement receiver, antenna and personalcomputer. On the deck of the personal computer the dedicated software for remote control of themeasurement system is running. In the article the software graphical user interface descriptionand the algorithm for the measurement process, data collection and analysis are presented. Thealgorithm is based on the IEEE Std 473-1985 standard IEEE recommended practice for anelectromagnetic site survey. The measurement results of the electromagnetic field strength inHF band in selected rural and urban areas are presented too.

1. INTRODUCTION

In some frequency ranges external noises usually prevail over individual noises of receiving radiocommunication and radio direction finding systems and they are often the main obstacle in ra-dio signal reception. In order to maximize receiving sensitivity of radio communication and radiodirection finding systems it is necessary to choose location characterized by as low level of envi-ronmental noises as possible. Therefore before installation of this kind of devices it is necessary tocharacterize environmental properties of radio engineering disturbances in a chosen location withinfrequency range covering operation of installed receiving systems. The environment of radio en-gineering disturbances consists of natural interferences (atmospheric, solar and cosmic) and thosegenerated by human as a result of intentional and unintentional actions additive noises oftechnical character which are often called as man-made noise.

Quantitative description of fluctuation noises, both external and individual of a receiver, canbe very seriously simplified and reduced to one numerical factor for the needs of analysis of themeasuring system noises. This numerical factor is described below.

Let us mark tuning frequency of the receiver by f and substitute bandwidth of the whole pre-detection path in the receiver by B. A simplified methodology of analysis of the effect of externalnoise consists in assumption that it affects the receiver input only in narrow frequency band B.So we can accept constant value of spectral power density at the receiver input G(f). Meancomputational power of that noise at the receiver input is equal to:

Pn = GB, (1)

On the basis of the value of noise power at the receiver input expressed by Equation (1) anotion of external noise coefficient fa is being introduced, analogically to a well-known notion ofnoise coefficient of element or electronic system. We start with a general equation for fluctuationnoise power:

Pn= kToBfa, (2)

where:

k Boltzmann constant, equal to 1.38 1023 J/K,To absolute temperature in kelvins,B equivalent bandwidth of reference selective system in Hz,fa noise coefficient, with dimension W/W and value 1.

1444 PIERS Proceedings, Moscow, Russia, August 1923, 2012

The Equation (2) binds with each other quantities: Pn, k, To and B which are directly orindirectly measurable and it is used to define noise coefficient fa in non-logarithmic units:

fa =Pn

kToB(3)

In technical practice noise coefficient is expressed in decibels and marked symbolically with thesymbol Fa.

External noise coefficient fa is also defined by properly interpreted formula (3) and it is themeasure of noise of radio channel referred to the receiver input (or to the ambience of receivingantenna). We find non-negative value fa(f) 0 constant in narrow frequency ambience f , butgenerally variable with frequency.

In further part of this study external noise coefficient will be used as the measure of evaluationof a particular location as regards the level of external noises. The values of coefficient Fa for amodel area QRSL (Quiet Rural Site Line), predestined to place receiving radio communication andradio direction finding, have been determined from the dependence [2]:

Fa(f) = 28.6 log10 f + 53.6, (4)whereas the values of coefficient Fa adequately for rural, suburban and urban area have beendetermined according to [2] on the basis of the dependence:

- for rural area Fa(f) = 27.7 log10 f+67.2, (5)- for suburban area Fa(f) = 27.7 log10 f+72.5, (6)- for urban area Fa(f) = 27.7 log10 f+76.8. (7)

In further part of this study a sample implementation of a measuring set for assessment ofthe level of external noises for a particular location and a procedure of measuring external noisecoefficient Fa have been presented. Moreover sample measurement results for rural and urban areashave been shown which goal is to evaluate a given location to be used as a place of installation ofreceiving radio communication systems.

2. MEASURING POSITION

A measuring position includes:

- active measuring antenna SAS-550-25 working within the frequency range from 100Hz to60MHz produced by the company A. H. Systems, INC,

- coaxial cable 50,- measuring receiver ESPI working in the frequency range from 9 kHz to 7GHz produced bythe company Rohde & Schwarz,

- personal computer (PC) with software operating a measuring process and the Ethernet inter-face used for remote controlling of a measuring receiver,

- Ethernet cable,- a shielding enclosure with interface panel providing insertion to input of the measuring receivera coaxial cable leading to output of the receiving antenna and power supply AC 230V 50Hz,

- supply filter AC 230V 50Hz of type FN2070-3-06 by Schaffner, placed inside the shieldingenclosure which provides filtration of radio signals generated by the Ethernet cable and activeelements of the measuring receiver inducing on power leads.

Block diagram of the measuring position is presented in Fig. 1. The photo of the measuring setis shown in Fig. 2.

A position for measuring coefficient of environment noises contains components typical for thesystem of measuring electromagnetic field strength. Basic components of such system are antennaand measuring receiver connected to the antenna output by coaxial cable.

The used antenna should have the measured antenna coefficients AF (f) allowing to convertvoltage induced at the antenna output into electric field strength at the site of antenna installation.In order to convert the signal level induced at the antenna output into the input level of themeasuring receiver, after passing of signal through the coaxial cable connecting the antenna withthe receiver input it is necessary to determine its transmittance A(f) for particular measuring


Figure 1: Block diagram of the measuring position. Figure 2: Photo of the measuring set.

frequencies. Both antenna coefficients and transmittance of transmission line are data used bysoftware operating the measuring process for calculations discussed in the further part of thisarticle.

A task of the measuring receiver is detection and measurement of signal levels with radio fre-quencies with the use of RMS and AVG detectors. The used receiver is equipped with the Ethernetinterface used for remote controlling of the receiver with the use of software installed on the PC.

An important element of the measuring position is the shielding enclosure and supply filter AC,which task is to provide separation of the measuring antenna from radiated interference signalsgenerated unintentionally by the measuring system. The above separation is extremely importantduring measurements of environment noise coefficients in rural areas where measured levels ofenvironment noise signals are repeatedly lower than the levels of interferences generated by themeasuring system. The sources of particularly large disturbances generated by the measuringsystem are the Ethernet cable and PC used for remote controlling of the measuring receiver.

3. MEASURING PROCEDURE

An algorithm of proceeding during evaluation of a particular location to be used as the site ofinstallation of the receiving radio communication system working in the frequency range from 1MHzto 30MHz has been described below. The above algorithm includes the below listed activities tobe performed:3.1. Selection of Silent Radio FrequenciesIn order to conduct the above mentioned procedure, first it is necessary to make a spectral analysisof radio frequency band from 1MHz to 30MHz which goal is to select at least m of measuringfrequencies evenly distributed in the whole analyzed band on which emissions coming from devicesgenerating and radiating energy of radio-waves intentionally do not occur (e.g., radio communica-tion and radio diffusion transmitters, radars). In further part of the study the above frequencieswill be called silent frequencies. At the above mentioned frequencies only natural interferences(atmospheric, solar and cosmic) occur as well as those generated by human unintentionally.3.2. Measurement of Electromagnetic Field Strength at Each of Selected Measuring Frequen-ciesAt selected silent frequencies one should measure electromagnetic field strength using the mea-suring set described in the previous subsection. During measurements it is necessary to take intoaccount the value of antenna coefficients AF (f) characterizing a particular measuring antenna andattenuation of coaxial cable A(f) connecting antenna with input of the measuring receiver. Themeasurements should be performed with the detector of RMS value and AVG value. The levelsUAV G(f) measured by the AVG detector will be used in item 5 of this measuring procedure dur-ing calculation of deviation of signal level Vd(f). It is necessary to conduct n of measurementsof electromagnetic field strength for each measuring frequency. With consideration that values ofall components are set in the decibel measure, the value of electromagnetic field strength at themeasuring site should be determined with the use of the dependence:

E(f) = URMS(f) +AF (f)A(f), (8)


where:

AF (f) antenna coefficient [dB/m],URMS(f) value of signal level measured by measuring receiver with the detector of root-mean-square value [dBV],A(f) attenuation of cable connecting measuring antenna with measuring receiver [dB].

3.3. Determining External Noise Coefficient for Each of Measuring FrequencyExternal noise coefficient Fa(f) for each of so called silent frequencies should be determined forevery n of measured values of electromagnetic field strength, from the dependence:

Fa(f)= E(f) 20 log10 (f) + 95.5 10 log10 (B), (9)where:

f frequency of measured signal [MHz],B bandwidth of measuring receiver [Hz].

3.4. Determining Median of External Noise Coefficient for Each of Measuring FrequencyValues of external noise coefficient Fa(f) determined in the above mentioned way should be subjectto statistical processing. For set n of external noise coefficients for each measuring frequency it isnecessary to determine median and confidence interval in which 90% of determined value Fa(f) isincluded. The determined values of external noise coefficient Fa(f) along with marked confidenceintervals should be presented in a graphical form.3.5. Determining Deviation Value of Signal Level for Each Measuring FrequencyExcept the value of coefficient Fa(f) for each of so called silent frequencies it is necessary todetermine so called value of the level deviation Vd(f) understood as the difference between thevalue of signal level measured by the measuring receiver using the detector of RMS (root-mean-square) value and the detector of AVG (average) value, according to the dependence:

Vd(f) = URMS(f) UAV G(f). (10)Also in this case every set of deviation values of signal level for a particular silent frequency

should be subject to statistical processing in order to determine confidence interval in which 90%of calculated deviation values is included. The determined deviation values of signal level alongwith marked confidence intervals should be presented in a graphical form.3.6. Evaluation of Particular LocationAfter calculating the values Fa(f) and Vd(f) one can decide about quality of particular location asregards the level of external noises. A good location should be characterized by the value Fa(f) notlarger from 3dB to 4dB than the value of coefficient Fa(f) for model QRSL area. A good locationshould also be characterized by deviation value of the level Vd(f) for signal levels measured duringthe day which is not larger than 3. The measurements which the algorithm has been presented aboveshould be performed for each of four seasons. Time intervals for carrying out the measurementsshould cover hours from 800 to 1200 (the lowest levels of interferences of technical origin) and from2000 to 2400 (the highest levels of interferences of technical origin). Number of frequencies at whichmeasurements for radio frequency band from 3MHz to 30MHz should be conducted is at leastm = 12. Each series of measurements should be performed every 15 seconds for 3 minutes within10 to 14 days.

4. RESEARCH RESULTS

In order to check operation correctness of the designed measuring position and measuring proce-dure, evaluation of the fallowing three locations have been conducted: interior of anechoic shieldedchamber, urban and rural area. The measurement results for these locations have been shownadequately in Figs. 3, 4 & 5. Diagrams in colours: green, yellow, orange and red mean values ofexternal noise coefficient Fa(f) adequately for: QRSL, rural area, suburban area and urban area.The diagram marked blue means values Fa(f) and Vd(f) for selected location. Error columns showconfidence interval for significance level equal to 0.1.

Comparing levels Fa(f) measured inside anechoic shielded chamber (Fig. 3) and in rural area onecan state that the levels of external noises are at comparable level. The level Fa(f) at frequencies


Figure 3: Values of ambient noise coefficient Fa(f) and value of the level deviation Vd(f) for shielded chamber.

Figure 4: Values of ambient noise coefficient Fa(f) and value of the level deviation Vd(f) for urban area.

Figure 5: Values of ambient noise coefficient Fa(f) and value of the level deviation Vd(f) for rural area.

below 2.5MHz seems to be disputable. In case of interior of anechoic shielded chamber a componentof such frequency should not have such a significant level. The source of the above signal is emissionof radiated interferences of PC operating the measuring process. The above mentioned interferencesinduce on power leads AC 230V 50Hz and find their way outside a shielding enclosure of themeasuring system through a path conducted by supply filter. The above can be prevented by usingsupply filter AC 230V with greater attenuation in stopband.

5. CONCLUSIONS

On the basis of the presented in Figs. 3, 4 & 5 measurement results of ambient noise coefficientFa(f) and the value of level deviation Vd(f) for selected location we can state that the designedprocedure and measuring set work correctly. The obtained measurement results confirm a characterof electromagnetic environment chosen for the test at: interior of anechoic shielded chamber, urbanand rural area.

Exceptions are signals at frequencies below 2.5MHz for which the level of external noise coef-ficient exceeds assumed admissible levels for a particular location. The above phenomenon resultsfrom insufficient filtering off by the filter AC 230V conducted interferences generated by the mea-


suring system. For prevention it is necessary to use power line filters with a greater attenuationvalue in stopband.

The designed procedure and measuring set can be successfully used for choosing location forradio communication and radio direction finding systems which provide the lowest level of externalnoises.

ACKNOWLEDGMENT

The research work financed from the financial funds assigned to the science in the years 2010/2012as the development work. The research work is realized in Poland. The number of work is0005/R/T00/2010/11, No. OR00000511.

REFERENCES

1. IEEE Std. 473-1985, IEEE recommended practice for an electromagnetic site survey (10 kHzto 10GHz), 1985.

2. AECTP-250, Electrical and Electromagnetic Environmental Conditions, 2nd Edition, 2006.

ambient electromagnetic noise environment measurement

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