Méthodes de mesure applicables aux émetteursradioélectriques – Partie 14: Produits d'intermodulation à l'extérieur du canal provoqués par deux émetteurs ou plus utilisant la même antenne
Introduction
The following procedures assume that measurements are conducted manually using commonly available testing equipment and components However, the precautions to avoid errors are also applicable to automated measurements using specialized equipment designed for the continuous monitoring of critical transmitter facilities.
The methods outlined are applicable for measuring A1-type interferences, down to at least -100 dB relative to the reference carrier level The theoretical considerations, including the nomenclature used in this standard, are detailed in Appendix A.
Ces procédures concernent les mesures sur site des signaux prélevés à partir de coupleurs directionnels dans les feeders entre les émetteurs et les antennes, ainsi que les mesures hors ondes.
Mesures sur les feeders d'émetteurs
The testing equipment is connected to the transmitter system as shown in Figure 1 After disconnecting the reject filters (one for each emission), measure the levels of unmodulated carrier frequencies, which will serve as a reference for intermodulation product measurements If the transmitters do not have the same power, use the emission with the lowest power as the reference Next, connect all reject filters and tune each to achieve maximum attenuation at the corresponding carrier frequency, aiming for an attenuation of approximately 55 dB for all carriers, with the specific attenuation determined based on the spectrum analyzer's dynamic range Finally, using the formulas provided in section A.2, calculate the frequencies at which intermodulation products are likely to occur, and then measure the level of each intermodulation product relative to the reference carrier using a spectrum analyzer.
Précautions à prendre pendant les mesures
To prevent the generation of intermodulation products within the spectrum analyzer, input levels must be kept sufficiently low, ideally by attenuating the analyzer's input level by 3 dB and confirming that all intermodulation products are reduced by the same amount It is essential to verify that the identified intermodulation signals are indeed produced by the transmitter system by sequentially turning off each transmitter Additionally, it is important to adjust the measured values of intermodulation products to account for the differences in performance between carrier frequencies and the frequencies of the intermodulation products.
– des pertes d'insertion du filtre réjecteur (ce point est particulièrement important pour les produits d'intermodulation à proximité d'une fréquence porteuse);
– des pertes dans le feeder d'émission principal.
The measurement outlined in section 4.2 does not reflect the actual radiation level of the antenna To determine the radiated level, it is essential to adjust the results by considering the frequency response and/or the Voltage Standing Wave Ratio (VSWR) of the antenna, as detailed in section 4.3 c).
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The procedures outlined are based on manual measurements with common test equipment, but the necessary precautions to prevent errors are equally important for automatic measurements conducted with specialized equipment for continuous monitoring of critical transmitter installations.
The methods outlined are effective for measuring A1-mode interference levels as low as –100 dB relative to the reference carrier level Detailed theoretical considerations and nomenclature for all formulas in this standard can be found in annex A.
Procedures are given for on-site measurements of signals derived from directional couplers in the feeders between the transmitters and the antennas and also for off-air measurements.
To measure transmitter feeders, connect the test equipment to the transmitter system as illustrated in figure 1 First, with the stop filters disabled, measure the levels of the unmodulated carrier frequencies to establish a reference level for intermodulation products (IPs); use the lowest power transmission as the reference if the transmitter powers differ Next, enable all stop filters and adjust each to achieve maximum attenuation at the corresponding carrier frequency, aiming for approximately 55 dB attenuation, as determined by the spectrum analyzer's dynamic range Finally, calculate the expected frequencies of IPs using the formulas in clause A.2 and measure the levels of these IPs relative to the reference carrier with the spectrum analyzer.
When making measurements, it is crucial to ensure that input levels to the spectrum analyzer are low enough to prevent intermodulation product (IP) generation within the device; this can be verified by attenuating the input by 3 dB and observing a corresponding 3 dB reduction in all IPs Additionally, it is important to confirm that the identified IPs are indeed generated by the transmitter system by sequentially turning off the transmitters Finally, any measured values of the IPs should be corrected to account for performance variations at both the carrier frequencies and the IP frequencies.
– stop filter insertion loss (this is particularly important for any IP close to a carrier frequency);
The measurement outlined in section 4.2 does not reflect the true radiated level from the antenna To determine the actual radiated level, it is essential to adjust the results as specified in section 4.3 c), considering the antenna's frequency response and/or Voltage Standing Wave Ratio (VSWR).
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Mesures hors ondes
Connect the testing equipment to a calibrated wideband receiving antenna positioned at the measurement site Conduct the tests outlined in sections 4.2 b), 4.2 c), and 4.2 d) During the measurements, adhere to the precautions specified in sections 4.3 a) and 4.3 b), and adjust the results as necessary according to section 4.3 c) Additionally, consider the gain difference of the receiving antenna between the carrier frequencies and the intermodulation product frequencies.
Précautions spéciales à prendre pour effectuer des mesures hors ondes
The quality of received signals is critically influenced by the location of the receiving antenna It is essential to select the measurement site at a sufficient distance and at an appropriate angle, ensuring that the receiving antenna is positioned vertically and horizontally within the main radiation lobes of the transmitting antenna.
When directive arrays are involved, the antenna radiation patterns can vary significantly if there is a substantial difference between the carrier frequencies and the intermodulation product frequencies.
In these cases, off-air measurements typically do not provide useful information unless there is a specific interest in the level of intermodulation products in a particular direction or location, such as when there is a risk of interference with aviation services.
Cet effet est particulièrement significatif pour les centres VHF utilisant des émetteurs couplés,
In a system with two completely independent channels per program, each transmitter in a coupled set feeds a separate half-antenna When intermodulation products arise due to insufficient isolation between the antennas, the interference signal's phase becomes erratic Consequently, the phase of the intermodulation products generated in the two half-antennas will not have a direct relationship with the carrier phase.
Dans ce cas, le diagramme de rayonnement de l'antenne sera probablement différent entre la fréquence porteuse et chaque produit d'intermodulation.
It is crucial to ensure that off-air measurements have not been distorted by local reflections or multipath propagation effects To achieve this, it is advisable to repeat measurements by relocating the antenna to several nearby positions to verify the consistency of the results However, locations exhibiting significant changes in the relative level of the carrier and intermodulation product components should be avoided.
5 Méthodes de mesure pour une configuration ou un service d'émetteurs particuliers
Mesures pour émetteurs de radiodiffusion en VHF
Pour effectuer des mesures sur les émetteurs de Radiodiffusion en ondes métriques conformément à l'UIT-R Recommandation 329-6, les équipement suivants (voir figures 1 et 2) sont nécessaires:
– un analyseur de spectre: bande passante de 3 kHz à 1 MHz, dynamique d'au moins
– un coupleur directif: directivité de 20 dB à 30 dB, par exemple;
– des filtres réjecteurs (un par émission): affaiblissement des fréquences indésirables d'environ 55 dB, perte d'insertion d'environ 2 dB, par exemple;
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To conduct off-air measurements, connect the calibrated broad-band receiving antenna to the test equipment at the designated measurement location Perform the tests outlined in sections 4.2 b), 4.2 c), and 4.2 d) While measuring, adhere to the precautions specified in sections 4.3 a) and 4.3 b), and apply corrections as detailed in section 4.3 c) when necessary Additionally, adjust the results to account for variations in receiving antenna gain at both the carrier and IP frequencies.
4.5 Special precautions to be observed when making off-air measurements
The effectiveness of signal reception is significantly influenced by the positioning of the receiving antenna It is essential to select a measuring site that is adequately distanced and oriented correctly, ensuring that the receiving antenna is situated within the primary horizontal and vertical radiation lobes of the transmitting antenna.
Where directional arrays are involved, the radiation patterns may differ considerably if the frequency spacing between carrier and the intermodulation product frequencies is significant.
Off-air measurements are typically not significant, except when intermodulation product levels are crucial in specific directions or locations, particularly where there is a potential for interference with aeronautical services.
The impact of intermodulation products is crucial in VHF installations that utilize parallel transmitters, where two independent transmission chains serve each service, with each transmitter connected to a separate half of the antenna In this setup, the imperfect isolation of the antennas leads to random phases of the interfering signals, resulting in intermodulation products that do not maintain a consistent relationship with the carrier phase.
As a result, the radiation pattern of the antenna is likely to be different for the carrier frequency and each intermodulation product.
To ensure the accuracy of off-air measurements, it is crucial to verify that they have not been affected by local reflections or multipath effects This can be achieved by repositioning the receiving antenna at several nearby locations and repeating the measurements to confirm the consistency of the results.
Locations where significant variations in the relative levels of the carrier and IP components are observed shall be avoided.
5 Methods of measurement for specific transmitter service or configuration
5.1 Measurements for VHF radio transmitters
In order to make measurements on VHF broadcasting transmitters operating in accordance with ITU-R Recommendation 329-6, the following equipment requirements (see figures 1 and 2) apply:
– spectrum analyzer: bandwidth 3 kHz to 1 MHz, dynamic range at least 70 dB, for example;
– directional coupler: 20 dB to 30 dB directivity, for example;
– stop filters (one for each transmission): attenuation at unwanted frequency approximately
55 dB, insertion loss approximately 2 dB, for example;
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– un filtre passe-bande accordable: bande passante de ±100 kHz à 3 dB (pour les mesures hors ondes), par exemple;
– un amplificateur large bande à faible bruit: gain de 20 dB (pour les mesures hors ondes), par exemple.
Mesures en cas d'antennes adjacentes
Figure 3 illustrates a general measurement device designed for transmitters powering separate antennas This is applicable for setups featuring multiple broadcasting transmitters operating in the kilometer, hectometer, and decameter bands, as well as for mixed television and FM broadcasting stations.
Les tensions de sortie des coupleurs directifs sont mesurées à l'aide d'un analyseur de spectre indiquant l'amplitude de tous les produits d'interférence sur une trace unique (voir figure 1).
Autrement, un récepteur d'essai sélectif peut être utilisé.
Ce dernier sera généralement suffisant, mais l'analyseur de spectre a l'avantage de représenter tous les signaux simultanément, ce qui rend le choix des fréquences intéressantes relativement aisé.
La puissance se calcule à partir des tensions mesurées, en utilisant les formules (A.4-1) et
(A.4-2) Les variations du coefficient de couplage en fonction de la fréquence doivent être prises en compte, ce qui nécessite des corrections supplémentaires.
Ce dispositif permet de mesurer l'ensemble des produits d'interférence intéressants à l'extérieur du canal.
Measurements in transmitter feeders
To measure the transmitter system, connect the test equipment as illustrated in figure 1 First, with the stop filters removed, measure the unmodulated carrier frequencies to establish a reference level for intermodulation products (IPs); use the lowest power transmission as the reference if the transmitter powers differ Next, activate all stop filters and adjust them for maximum attenuation at the designated carrier frequency, aiming for approximately 55 dB attenuation, as determined by the spectrum analyzer's dynamic range Finally, calculate the expected frequencies of IPs using the specified formulae and measure their levels relative to the reference carrier with the spectrum analyzer.
Precautions to be observed when making measurements
To prevent intermodulation products (IPs) from being generated within the spectrum analyzer, ensure that input levels are sufficiently low, which can be verified by attenuating the input by 3 dB and observing a corresponding 3 dB reduction in all IPs Additionally, confirm that the identified IPs originate from the transmitter system by sequentially turning off the transmitters Finally, adjust the measured IP values to account for performance variations at both the carrier and IP frequencies.
– stop filter insertion loss (this is particularly important for any IP close to a carrier frequency);
The measurement outlined in section 4.2 does not reflect the true radiated level from the antenna To determine the actual radiated level, it is essential to adjust the results as specified in section 4.3 c), considering the antenna's frequency response and/or Voltage Standing Wave Ratio (VSWR).
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To conduct off-air measurements, connect the test equipment to a calibrated wideband receiving antenna positioned at the measurement site Perform the tests outlined in sections 4.2 b), 4.2 c), and 4.2 d) During the measurements, adhere to the precautions specified in sections 4.3 a) and 4.3 b), and adjust the results as necessary according to section 4.3 c) Additionally, consider the gain difference of the receiving antenna between the carrier frequencies and the intermodulation product frequencies.
4.5 Précautions spéciales à prendre pour effectuer des mesures hors ondes
The quality of received signals is critically influenced by the location of the receiving antenna It is essential to select the measurement site at a sufficient distance and at an appropriate angle, ensuring that the receiving antenna is positioned vertically and horizontally within the main radiation lobes of the transmitting antenna.
When directive arrays are involved, the antenna radiation patterns can vary significantly if there is a substantial difference between the carrier frequencies and the intermodulation product frequencies.
In these cases, off-air measurements typically do not provide useful information unless there is a specific interest in the level of intermodulation products in a particular direction or location, such as when there is a risk of interference with aviation services.
Cet effet est particulièrement significatif pour les centres VHF utilisant des émetteurs couplés,
In a system with two completely independent chains per program, each transmitter in a coupled set feeds a separate half-antenna When intermodulation products arise due to insufficient isolation between the antennas, the interference signal's phase becomes erratic Consequently, the phase of the intermodulation products generated in the two half-antennas will not have any direct correlation with the carrier phase.
Dans ce cas, le diagramme de rayonnement de l'antenne sera probablement différent entre la fréquence porteuse et chaque produit d'intermodulation.
It is crucial to ensure that off-air measurements have not been distorted by local reflections or multipath propagation effects To achieve this, it is advisable to repeat measurements by relocating the antenna to several nearby positions to verify the consistency of the results However, locations with significant variations in the relative level of the carrier and intermodulation product components should be avoided.
5 Méthodes de mesure pour une configuration ou un service d'émetteurs particuliers
5.1 Mesures pour émetteurs de radiodiffusion en VHF
Pour effectuer des mesures sur les émetteurs de Radiodiffusion en ondes métriques conformément à l'UIT-R Recommandation 329-6, les équipement suivants (voir figures 1 et 2) sont nécessaires:
– un analyseur de spectre: bande passante de 3 kHz à 1 MHz, dynamique d'au moins
– un coupleur directif: directivité de 20 dB à 30 dB, par exemple;
– des filtres réjecteurs (un par émission): affaiblissement des fréquences indésirables d'environ 55 dB, perte d'insertion d'environ 2 dB, par exemple;
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Off-air measurements
To conduct accurate measurements, connect the test equipment to a calibrated broad-band receiving antenna at the designated measurement location Follow the testing procedures outlined in sections 4.2 b), 4.2 c), and 4.2 d) While performing measurements, adhere to the precautions specified in sections 4.3 a) and 4.3 b), and apply corrections as detailed in section 4.3 c) when necessary Additionally, adjust the results to account for variations in receiving antenna gain at both the carrier and IP frequencies.
Special precautions to be observed when making off-air measurements
The effectiveness of signal reception is significantly influenced by the positioning of the receiving antenna It is essential to select a measuring site that is adequately distanced and oriented correctly, ensuring that the receiving antenna is situated within the primary horizontal and vertical radiation lobes of the transmitting antenna.
Where directional arrays are involved, the radiation patterns may differ considerably if the frequency spacing between carrier and the intermodulation product frequencies is significant.
Off-air measurements are typically not significant, except when intermodulation product levels are crucial in specific directions or locations, particularly where there is a potential for interference with aeronautical services.
The impact of intermodulation products is crucial in VHF installations that utilize parallel transmitters, where two independent transmission chains serve each service, with each transmitter connected to a separate half of the antenna In this setup, the imperfect isolation of the antennas leads to the generation of intermodulation products, resulting in a random phase for the interfering signal Consequently, the phases of the intermodulation products produced in the two halves of the antenna do not maintain a consistent relationship with the carrier phase.
As a result, the radiation pattern of the antenna is likely to be different for the carrier frequency and each intermodulation product.
To ensure accurate off-air measurements, it is crucial to verify that they are not affected by local reflections or multipath effects This can be achieved by repositioning the receiving antenna at several nearby locations and repeating the measurements to confirm the consistency of the results.
Locations where significant variations in the relative levels of the carrier and IP components are observed shall be avoided.
5 Methods of measurement for specific transmitter service or configuration
Measurements for VHF radio transmitters
In order to make measurements on VHF broadcasting transmitters operating in accordance with ITU-R Recommendation 329-6, the following equipment requirements (see figures 1 and 2) apply:
– spectrum analyzer: bandwidth 3 kHz to 1 MHz, dynamic range at least 70 dB, for example;
– directional coupler: 20 dB to 30 dB directivity, for example;
– stop filters (one for each transmission): attenuation at unwanted frequency approximately
55 dB, insertion loss approximately 2 dB, for example;
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– un filtre passe-bande accordable: bande passante de ±100 kHz à 3 dB (pour les mesures hors ondes), par exemple;
– un amplificateur large bande à faible bruit: gain de 20 dB (pour les mesures hors ondes), par exemple.
5.2 Mesures en cas d'antennes adjacentes
Figure 3 illustrates a general measurement device designed for transmitters powering separate antennas This is particularly applicable for setups involving multiple broadcasting transmitters operating in the kilometer, hectometer, and decameter bands, as well as for mixed television and FM broadcasting stations.
Les tensions de sortie des coupleurs directifs sont mesurées à l'aide d'un analyseur de spectre indiquant l'amplitude de tous les produits d'interférence sur une trace unique (voir figure 1).
Autrement, un récepteur d'essai sélectif peut être utilisé.
Ce dernier sera généralement suffisant, mais l'analyseur de spectre a l'avantage de représenter tous les signaux simultanément, ce qui rend le choix des fréquences intéressantes relativement aisé.
La puissance se calcule à partir des tensions mesurées, en utilisant les formules (A.4-1) et
(A.4-2) Les variations du coefficient de couplage en fonction de la fréquence doivent être prises en compte, ce qui nécessite des corrections supplémentaires.
Ce dispositif permet de mesurer l'ensemble des produits d'interférence intéressants à l'extérieur du canal.
5.3 Mesures en cas d'antenne commune
La figure 4 montre la disposition d'un centre d'émission dans lequel deux émetteurs ou plus sont couplés sur une antenne commune par l'intermédiaire d'un circuit de combinaison.
Dans ce cas, les produits d'intermodulation sont mesurés sur le coupleur directif commun
The measurement device shown in Figures 1 and 2 is used to calculate levels using formulas (A.4-1) and (A.4-2) However, this device does not allow for accurate measurement of second-order intermodulation components outside the channel, specifically those where \( p = q = 1 \), as the interference product frequencies from each transmitter coincide Additionally, varying phase relationships, often caused by modulation processes, can lead to significant fluctuations in the resulting amplitude.
On the other hand, the second-order intermodulation products outside the channel (p = q = 1, o = 2) will be significantly reduced in this configuration, as the two transmitters are decoupled through selective networks However, this is not always the case for installations with strongly coupled antennas (see Figure 3).
Dans le cas d'émetteurs partageant la même antenne, si les produits de rang 2 sont relativement élevés, une deuxième méthode, représentée à la figure 5, peut être utilisée.
Elle nécessite des coupleurs directifs à la sortie des émetteurs, c'est-à-dire en amont des circuits de combinaison, comme dans le cas de l'installation représentée à la figure 3.
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– tuneable bandpass filter: ±100 kHz at 3 dB points (for off-air measurements), for example;
– wide band, low noise amplifier: 20 dB gain (for off-air measurements), for example.
Measurements for adjacent antennas
Figure 3 shows a general arrangement for transmitters feeding separate antennas e.g for LF,
MF and HF multiple transmitter installations, or stations for television and FM sound broadcasting.
The directional coupler output voltages are measured by means of a spectrum analyzer, indicating the amplitudes of all interference products on a single trace (see figure 1).
Alternatively, a selective test receiver may be used.
The spectrum analyzer is advantageous as it displays all signals at once, facilitating the selection of specific frequencies of interest, making it a convenient tool in many situations.
The powers are calculated by means of the formulae (A.4-1) and (A.4-2), based on the voltages measured The frequency dependency of the coupling coefficients shall be allowed for, requiring additional corrections.
With this set-up, all external interference products of interest can be measured.
Measurements for a common antenna
Figure 4 shows a layout of a transmitter site in which two or more transmitters are coupled to a common antenna via a combining unit.
In this case, the intermodulation products are measured at the common directional coupler
In the setup illustrated in figures 1 and 2, levels are determined using the formulas (A.4-1) and (A.4-2) However, the external second-order intermodulation components, specifically when \( p = q = 1 \), cannot be accurately measured due to the overlapping frequencies of the second-order interference products from each transmitter Additionally, variations in phase relationships, often influenced by modulation processes, can lead to significant fluctuations in the resultant amplitudes.
In this configuration, the second-order external interference products are significantly minimized due to the decoupling of the two transmitters by selective networks However, this reduction may not apply to installations with closely coupled antennas.
If the second-order products are relatively high for transmitters sharing a common antenna, a second method shown in figure 5 may be used.
Directional couplers at the outputs of the transmitters, i.e in front of the combining devices, are needed as in the case of an installation conforming to figure 3.
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It is advisable to measure the attenuation in the combining circuit between the transmitter input and the antenna for all frequencies of intermodulation products This allows for the correction of the levels measured at the transmitter output.
This method can be employed when there is an insufficient number of reject filters available to perform the measurements outlined in the first method, between the combination circuit and the antenna.
If an antenna adaptation unit (AAU) is employed, a directional coupler will not be available at the antenna's output In this scenario, directional couplers at the output of each transmitter must be utilized, enabling the measurement of radiated interference signals across all ranges.
Dans le cas d'un centre d'émission ó plusieurs émetteurs fonctionnant à des fréquences différentes sont réunis par groupes, chaque groupe peut être traité comme un émetteur unique
Intermodulation products are influenced by the total number of transmitters present at a site Similar to single transmitters, a nearby group of coupled transmitters can absorb some of the high-frequency energy emitted by the tested group of transmitters.
De la même manière, on aura recours au coupleur directif commun (DC), pour déterminer l'énergie absorbée, en mesurant les tensions U′ IF et U′ IR indiquées entre parenthèses
(figure 4), et en utilisant les formules (A.4-1) et (A.4-2).
The total number of intermodulation products generated by the emission center can be calculated using the formula (A.3-1), where \( x \) represents the total number of active transmitters, regardless of the number of groups.
Due to the presence of second-order interferences within each group of transmitters, a combination of methods 2 and 3, as outlined in sections 5.3.2 and 5.3.3, can be employed to determine the necessary values for calculating the levels of all intermodulation products outside the channel.
Results should be expressed in decibels relative to the carrier frequency (f m) and the order of intermodulation products (o) at the specified frequencies, in dBm, or as absolute values in mV.
The results can be displayed in a table or graph based on the carrier frequency (\(f_m\)) and the order of intermodulation products (\(o\)), or alternatively, as a photograph or screenshot of the spectrum analyzer's display if a sweep measurement method is employed.
In this case, it is essential to consider the weakening factors that vary with frequency Additionally, both incident and reflected values must be measured to calculate the net radiated value For the incident value, the display indication is only accurate in specific situations where the antenna is tuned to the frequencies of the intermodulation products Therefore, photographs of the display alone are typically insufficient to represent accurate results.
Comme il sera toujours nécessaire d'effectuer des calculs supplémentaires et des corrections, la meilleure solution est de représenter les résultats sous forme de tableau (voir 4.3 c) et
4.4 c)) Il est recommandé d'utiliser pour cela le tableau figurant à l'annexe B.
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Before measuring the levels at the transmitter output, it is essential to assess the attenuation of the combining unit from the transmitter input to the antenna at each IP frequency This measurement allows for necessary corrections to be made to the output levels.
This approach is applicable when there are not enough stop filters to perform the measurements outlined in the initial method between the combining equipment and the antenna.
When utilizing an antenna matching unit (AMU), a directional coupler is not positioned in front of the antenna Instead, the directional couplers located at the outputs of each transmitter will be employed, allowing for the measurement of all orders of radiated interfering signals.
At a transmitter site with multiple transmitters operating at various frequencies, each group of transmitters can be considered as a single unit, although intermodulation products depend on the total number of transmitters present Additionally, a nearby combined transmitter group may absorb RF energy from the group being tested, similar to the behavior of an individual transmitter.