Multimedia products need to comply with requirements to limit exposure to electromagnetic fields (EMF). Multimedia (MM) products have to comply with requirements to limit the human exposure to electromagnetic fields (EMF).
The quantities in which the basic restrictions are specified are often impractical to measure which makes the proving of compliance complicated. Besides using the basic restrictions and the reference levels to prove compliance, it may be also possible to derive compliance criteria based on realistic assumptions such that a simple measurement can be used to prove compliance.
This paper describes the measurement method developed to measure the maximum total radiated power using the reverberation chamber (RvC). The reverberation chamber is a well-known tool for rapid measurement of total radiated power of intentional transmitters [5]. It should be noted that the method of testing described in this paper determines EMF compliance of an EUT in a specific frequency range only, namely 80 MHz – 6 GHz since this is the practical usable frequency range of the used RvC and its associated measuring equipment. Before TRP measurements can be carried out on actual EUTs, an RvC calibration has to be performed to determine the losses which are inherently associated with the chamber. All these factors are determined over nine locations inside the chamber that represent the boundaries of the working volume. Only the ACF is determined since the measured average power is required for the 20 mW rule.
Power measurements are carried out in the frequency range 80 MHz – 6 GHz, this being the workable frequency range of the RvC in combination with the associated equipment. Continuous power measurements are carried out over 6 minutes per frequency point; 6 minutes being the averaging time for SAR in the frequency range 10 MHz – 10 GHz (see [2]).
The aim of the calibration is to determine the transfer of energy between the transmit antenna and the receive antenna.
For correct TRP measurements it is necessary to determine the average power so that this can be compared to the low power criterion of 20 mW.
Figure 4 and Figure 5 show the ACF of the RvC in the frequency ranges 80 – 1000 MHz and 1 – 6 GHz. In order to get more confidence in the results obtained from the TRP measurement method, it was decided to verify the method by measuring a number of artificial EUTs with well-known and tuneable radiation properties.
Verification measurements have been carried out with independent intentional radiators which were realized with the help of a set of two horn antennas and signal generators. The difference between the estimated TRP and the actual TRP gives an indication of the accuracy with which TRP measurements can be performed in the RvC (3).
Several verification measurements have been performed in which various combinations of factors such as frequency, modulation, number of sources and positioning of source have been varied to get an optimal set of data (Table 1).
In case of a single source, the estimated TRP can be found by simply adding the ACF at that particular frequency to the measured received power. In the case of multiple sources at the same frequency, the estimated TRP can be determined again by simply adding the ACF at the appropriate frequency to the measured received power.
When the sources are modulated, one should be careful in determining the correct chamber calibration factor (ACF) depending upon the frequency span of the spectral components of the modulated signals.
In terms of accuracy it can be said that for a single source, the TRP can be determined with an accuracy of approximately 2 dB (?).
Overall, it can be concluded that a good correlation exists between the measured received power and the calculated received power which is obtained by simply adding the individual power components. In general, a product may contain spurious sources of emission (unintentional radiators) as well as one or more wireless technologies (intentional radiators).
Primarily, TRP measurements were carried out on a range of MM products classified as unintentional radiators, i.e. Additionally, TRP measurements were carried out on MM products having one or more intentional radiators (e.g.
The low power criterion of 20 mW can be used as a limit to demonstrate EMF compliance of MM products. A recent study reveals radiation from solar events is unlikely to affect earth & the dose of radiation emitted during the events isn’t enough to harm people. Substations are buildings where the voltage is changed from extra high voltage to high voltage or medium voltage to low voltage by transformers. Concerns have been expressed about the possible health effects of living in the vicinity of cell phone towers. Public health organizations such as the WHO have concluded that the overwhelming weight of scientific evidence shows that cell towers cause neither cancer, nor EHS. The level of exposure to the general public, even for those living in close proximity to a cell tower, is at least 50 times lower (usually thousands of times lower) than established safety limits. One of the concerns raised by alarmist groups is the amount of EMF power emitted by cell phone towers. But as noted in the SCENIHR statement quoted above, many radio and TV stations which have been operating for more than 60 years are far more powerful sources of EMF. However, what is more important than the absolute power from the transmitter is the actual exposure level to the public. Compliance can be demonstrated by using the low-power criterion by proving that the total radiated power is lower than 20 mW. Since no product-specific EMF standard exists for MM products, the generic EMF standard IEC 62311 [1] may be applied for compliance demonstration.


The basic restrictions are specified using different quantities in different frequency ranges. In order to simplify the compliance measurements, alternative limits called ‘reference levels’ have been derived.
This approach is considered in [1] where it is said that if the equipment under test (EUT) can be demonstrated as compliant for both intentional and unintentional emissions with the generic low-power standard IEC 62479 (i.e. A description of the calibration of the chamber as well as the verification of the measurement method is given. In practice the upper frequency range for TRP measurements in an RvC may be much higher and depends on the frequency restrictions of the required measuring equipment.
In IEC 61000-4-21 [6] detailed descriptions are given on the RvC calibration and testing methods for both emission and immunity. The RvC has two stirrers (horizontal and vertical) which are set to a continuous stirred mode, both having different speeds such that modes in the chamber are not repeated. The aim of this calibration is hence to determine the transfer of average power between the transmit antenna and the receive antenna.
The application of synthesised and multiple intentional radiators offers the possibility to study and verify the issue of summation of power of multiple and wideband type of intentional signals using the RvC. The actual injected power at the input of the horn antenna was measured beforehand and is called the actual TRP (TRPactual).
The noise level of the RvC was determined by performing a TRP measurement on an empty chamber.
Each measurement was carried out three times to get an idea of the repeatability of the measurement method.
In case of multiple independent sources the TRP is obtained by adding the TRPs of the individual sources [7].
In the case of multiple sources at different frequencies, the TRP can be determined with an accuracy of 3 dB (?).
Picture of verification measurement in RvC with multiple sources at a single frequency pointing in same direction. This implies that all power sources inside the RvC can be treated as uncorrelated; which is known to be an intrinsic property of the RvC. Power measurements are done over a complete frequency range as opposed to a fixed frequency (as was the case in the verification measurements) meaning that a broadband measurement is performed. Depienne et al, The reverberation chamber: a useful tool to characterize the radiated power of small RF devices, IEEE conference publications 2003, page(s) 461-464.
Converter stations are used to convert alternating current to direct current and vice versa (Converter station).
The SCENIHR scientific advisory body to the European Commission has released a comprehensive new report (Jan.
The American Cancer Society has published an information bulletin on their web site on Cellular Telephone Towers. A study entitled Mobile base stations and early childhood cancers: case-control study by Elliott et al. A Double Blind Study on EHS individuals exposed to mobile base station radiation showed that they were unable to detect real radiation from sham radiation. A Two Part Study of EHS like health effects on people of living near cell towers in Germany has recently been published (Feb. Pictured above is a collection of cell phone base station antennas and microwave relays on the roof of a building at Brock University in Ontario Canada. A typical FM radio or TV transmitter might produce 50,000 watts, which is 25 times greater than the antenna cluster at Brock University. This depends on many other variables such as the directionality pattern of the antenna, distance, absorption and reflection from other objects, etc.
This paper describes the measurement method developed to measure the maximum total radiated power using a reverberation chamber.
These are based on the various adverse health effects that can be caused by EMF exposure in that specific frequency range. Reference levels correspond to the basic restrictions under worst case exposure conditions and are defined in more easily measurable quantities such as electric field strength (E), magnetic field strength (H), magnetic flux density (B), power density (S), etc. Basically, the TRP of an EUT is determined by placing an EUT in a RvC and measuring the output power of the RvC using an rms power sensor (see Figure 2). The TRP is measured over a time period of 6 minutes, this being the averaging time in the case of time varying signals for Specific Absorption Rate (SAR) values (10 MHz – 10 GHz). Two types of antennas were used depending upon their effectiveness in certain frequency ranges, namely, a log-periodic antenna for frequency range 80 – 1000 MHz and a horn antenna for the range 1 – 6 GHz.
This transfer factor is called the antenna calibration factor (ACF) and can be seen as a loss factor of the chamber. Therefore, for measuring the TRP of multiple sources in the RvC at different frequencies, the estimated TRP is determined by adding the worst case ACF in the frequency range spanning over all sources to the measured received power.
One should keep in mind that in this case a conservative value of TRP is determined since the worst case ACF is used as correction. For this reason, the worst case ACF per frequency range has to be taken as correction factor (Figure 4, Figure 5 and Table 3).
After performing a relatively quick calibration, the RvC can be used to measure TRP over a time period of 6 minutes.


Substations and converter stations are surrounded by metal fences which restrict the access to trained personal only for maintenance purposes.
Certain short wave transmitters such as Radio Vatican and Voice of America have operated at 500,000 watts, which is 250 times greater than the same cluster. Since cell phone antennas focus their energy in narrow beams aimed at the horizon, exposure levels near such towers are much lower than ICNIRP limits. The transmit antenna is placed in the middle of the working volume of the chamber at a height of 1 m above the floor. During TRP measurements on an actual MM product, the received average power (Pavg) needs to be corrected with the ACF value at the measured frequency to attain the actual TRP radiated by the EUT (1). The average received power (Pavg) is measured with the help of a receive horn antenna and a broadband power meter.
For multiple sources at one frequency, the TRP can be determined with an accuracy of less than 0.5 dB (?). An additional pre-amplifier is applied to get sufficient dynamic range since power levels emitted from MM products are low.
The maximum measured TRP level of intentional radiators in which a Bluetooth link was set-up was in the order of 100 – 150 ?W. In the alarmist Radio-Canada Découverte program, the measured level of EMF was 10,000 times lower than the internationally recognized ICNIRP limit. This has given rise to the development of a product-specific standard for MM products, which is published as an ECMA technical report [2]. This position of the transmit antenna is chosen such that it emulates the position of an EUT during actual TRP measurements.
After correcting the measured average received power with the appropriate ACF factor, the estimated TRP (TRPestimated) can be calculated (2).
By applying a pre-amplifier, this noise level could be further reduced to 1 ?W which was needed in the case of measuring the TRP of unintentional radiators (Figure 5, Table 4). It was found that a typical level of TRP for these unintentional radiating MM products is in the order of tens of ?W and hence is much smaller than the low-power limit of 20 mW [8].
This is much lower than the maximum permitted power of 2.5 mW (Bluetooth 10 m, class 2) as well as the TRP-limit of 20 mW (Table 4). Also a good correlation is found between the actual and estimated TRP for single as well as multiple sources. These issues have been the subject of a number of scientific studies and have been examined by a number of public health organizations.
Note that the existing exposure limits already have a safety factor of 50 for the general public. According to the Découverte program on Radio-Canada the total power from this group of antennas is 2000 watts. The highest level reported by the alarmist group SEMO, after extensive measurements in the Montreal metropolitan area, were 250 times lower that the ICNIRP limit.
This technical report provides detailed guidance for the assessment of EMF exposure to the general public from MM products in accordance with [1]. A known level of power is injected into the RvC with the use of the signal generator and transmit antenna. Measurements carried out on actual MM products show that TRP values lie far below the 20 mW limit for both unintentional as well as intentional radiators. Although new exposure sources such as mobile phone base stations, cordless phone base stations or wireless networks are relatively recent, exposures from these sources are generally lower than the ones investigated in these studies on broadcast transmitters. The results of this study are similar to the results of at least 31 other double blind studies on EHS individuals. In phase 2 of the study, actual EMF measurements were made inside the homes of 3526 subjects. While we're on the subject of EMF power, the power output of a typical WiFi device is under 100 mW or 0.1 W, which is 20,000 times lower.
The directional coupler and the power meter are used to determine the forward and reflected powers so that losses due to mismatch are accounted for. For both Bluetooth and RF4CE it is a known fact that the transmitted power is scaled down after the wireless link is established.
The EMF levels from nearby cell towers as well as cordless phones, WiFi, and radio and TV stations were measured. Alarmists claim that transmitters such as the ones at Brock University, and even WiFi devices, can cause health effects ranging from fatigue and headaches, all the way to brain cancer.
Per frequency point the average power is measured by the receive antenna and spectrum analyser over a period of six minutes. Residents who considered themselves electrosensitive, possibly electrosensitive, and not electrosensitive were included in the study.



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Comments

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