GB/T 44119-2024Measurement method of antenna factor for 1 m method radiated disturbance (English PDF)

1 Scope

The document describes a method of measuring the antenna factor for a radiated disturbance measurement distance of 1 m.

It applies to the measurement of biconical antennas, resonant dipole antennas, log-periodic dipole array (LPDA) antennas, double-ridged waveguide horn antennas, standard gain horn antennas and similar antennas.

2 Normative references

GB/T 4365-2003, Electrotechnical terminology - Electromagnetic compatibility.

GB/T 6113.106-2024, Specification for radio disturbance and immunity measuring apparatus and methods - Part 1-6: Radio disturbance and immunity measuring apparatus - EMC antenna calibration.

Dated references apply in the edition cited; undated references apply in their latest edition, including any amendments.

3 Terms and definitions

The terms and definitions of GB/T 4365-2003 and GB/T 6113.106-2024 apply, together with the following.

3.1 antenna: a transducer that converts the guided electromagnetic energy of a feeder into a wave radiated into space, and the reverse. The note states that in this document the term antenna also covers the balun where a balun is a necessary part of normal operation. From GB/T 6113.106-2024, 3.1.1.1.

3.2 antenna factor: the ratio of the electric field strength of a plane wave incident in the direction of the mechanical boresight, that is the main axis of the antenna, measured in free space, to the voltage produced across a specified load connected to the antenna. The note refers to 4.2 for further information and states that antenna factor is also used as a general term, written AF. From GB/T 6113.106-2024, 3.1.2.1.

3.3 antenna gain: the ratio of the radiated power density in a given direction of the antenna to the mean radiated power density.

3.4 balun: a device used between transmission lines for conversion from balanced to unbalanced or from unbalanced to balanced. Note 1 gives as an example the coupling of balanced antenna elements to an unbalanced feeder such as a coaxial cable, and states that a balun may have an inherent impedance transformation other than 1 to 1. Note 2 states that in this document the term also refers to the handle of a biconical antenna, usually shaped as a metal tube or rod. From GB/T 6113.106-2024, 3.1.1.13.

3.5 biconical antenna: a symmetrical antenna formed by two conical radiating elements sharing a common axis and fed from the adjacent apexes of the two cones. Note 1 states that for use in the VHF band a biconical antenna is usually made of two conical wire cages, each cage usually having a cross bar connecting the centre conductor to one of the outer wires in order to remove a narrow band resonance, and that this shorting cross bar affects the characteristics of the antenna above 215 MHz. Note 2 states that in this document a biconical antenna whose end to end length is 1.3 m to 1.4 m, typically 1.37 m, is called a traditional biconical antenna, to distinguish it from small biconical antennas whose upper frequency limit is above 300 MHz. From GB/T 6113.106-2024, 3.1.1.2.

3.6 horn antenna: an antenna formed by a section of waveguide whose cross section increases progressively towards the open end, the open end being called the aperture. The note states that tapered horn antennas of rectangular waveguide are widely used at microwave frequencies above about 1 GHz, that double-ridged waveguide horn antennas, sometimes called DRG horn antennas because of their two ridges, cover a very wide frequency range, and that the main lobe of some of these antennas splits into several lobes at the higher frequencies. From GB/T 6113.106-2024, 3.1.1.5.

3.7 log-periodic dipole array antenna, LPDA antenna: an antenna made up of an array of linear dipoles whose length and spacing increase logarithmically from the tip to the rear of the antenna as the frequency falls. From GB/T 6113.106-2024, 3.1.1.7.

3.8 resonant dipole antenna, tuned dipole antenna: an antenna formed by two collinear straight conductors of the same length placed end to end and separated by a small gap that forms a balanced feed, the length of each conductor being approximately a quarter wavelength so that at a particular frequency, with the dipole in free space, the reactance of the input impedance measured across the gap is zero. The note states that in this document the term linear dipole means two collinear straight conductors, as distinct from the dipoles of a biconical dipole or of an LPDA array. From GB/T 6113.106-2024, 3.1.1.9.

4 Basic principle of the measurement method

4.1 General. Where there is no ground reflection, the antenna factor under far field conditions can be determined and calculated. Moving from the far field to a distance of 1 m, as described in this document, generally changes the antenna factor by 0 dB to 4 dB; for LPDA antennas that change is intended, since it corrects the field strength from the phase centre position at each frequency on the antenna to the tip of the antenna at 1 m from the emitting source. The main factors affecting the antenna factor are the spacing between the antennas, the height of the antenna above the ground plane, the orientation of the antenna with respect to the ground plane, and the size, flatness and conductivity of the ground plane; for a 1 m spacing, however, the measurement method of Clause 5 is designed to reduce the influence of the ground plane.

4.2 Antenna factor. For radiated disturbance measurement an antenna factor AF is to be specified, which converts the voltage U at the input of the measuring receiver, in volts, into the field strength E in volts per metre or into the same quantity as the unit of the radiated disturbance limit, decibels relative to one microvolt per metre; this is Formula (1). If U is in microvolts, the field strength E can be expressed in decibels relative to one microvolt per metre; this is Formula (2).

The 1 m antenna factor is an antenna factor based on a gain measurement, but that antenna factor is measured under the particular condition in which the antenna is actually used in a radiated disturbance compliance measurement, that is at 1 m from the emitting source. The antenna gain in that condition is an apparent gain including mismatch loss, called the 1 m gain, and it therefore differs from the true antenna gain. Formula (3) relates AF to the apparent gain in a 50 ohm system, where G is the numerical gain of the antenna and the wavelength is in metres; the derivation of AF follows Annex A.

4.3 Two identical antennas method. The 1 m gain means that the antenna factor derived from it is intended to determine the field strength at 1 m from the emitting source, which matters in particular for LPDA antennas, where the point at which the field is picked up on the antenna is more than 1 m from the source. Other influences that make the 1 m gain differ from the far field gain are the significant mutual coupling between a pair of identical biconical antennas or horn antennas, and the fact that 1 m is less than the distance required for the far field region in which the antenna establishes a plane wave.

The 1 m gain can be calculated by Formula (4), which uses two identical antennas with their axes aligned and their polarizations matched; for antennas that are not identical, the three antenna method of 4.4 is used. The key to Formula (4) gives the received power and the transmitted power in watts, the numerical gains of the transmitting and receiving antennas, the distance between the antennas in metres and the wavelength in metres. If the two gains are equal, Formula (5) follows. If both the receiving system and the transmitting system are matched, that is at 50 ohm, voltage measurements may replace power measurements, giving Formula (6), whose key gives the numerical gain of the antenna, determined from two voltage measurements and therefore a dimensionless ratio, with the calculation of near field gain covered in 4.6.2, and the port voltages of the receiving and transmitting antennas in volts.

Identical antennas are antennas of the same manufacturer, model and design. If identical antennas are not available, the three antenna method is to be used, and similar antennas do not meet the requirement for identical antennas. Most biconical antennas have wire cage dimensions similar enough, but it also matters that the two antennas have the same balun transformation, most commonly 50 ohm or 200 ohm. Most LPDA antennas are similar enough if they fit within a common template, an isosceles triangle with a base of 0.72 m and a height of 0.63 m; this excludes the less common high gain antennas, which are almost twice that size and would lead to a larger measurement uncertainty. LPDA antennas designed to cover 200 MHz to 1 GHz should meet the template. Antennas whose dimensions differ by not more than plus or minus 2 percent are generally considered identical.

4.4 Three antenna method. The three antenna method allows the antenna factor of a single antenna to be calculated and does not depend on having completely identical antennas. Its requirements are: a) no height scan is needed at a spacing of 1 m and the antennas stay at a fixed height; b) the measurement distance is fixed at 1 m; c) the requirements on the antenna measurement site do not apply, because both antennas are raised either to a height that reduces the ground effect or to 3 m, whichever is the lower.

For this arrangement the ground reflection is negligible or is not detected by the antenna under calibration (AUC). For the fixed distance of 1 m the quantity EmaxD is 16.9 dB, the more so for directional antennas above 1 GHz, and that value can be used in Formula (7) to calculate the antenna factor. The key to Formula (7) gives the frequency in megahertz; the three quantities A1, A2 and A3, each of them twenty times the logarithm of the ratio of transmitted to received port voltage, in decibels; the port voltage of each transmitting antenna and of each receiving antenna in volts; and the antenna factors of antennas 1, 2 and 3 in decibels per metre. Using the three antenna method reduces the influence of any one antenna on the measurement process.

4.5 1 m gain. 4.5.1 The 1 m gain is needed because the antenna is to be used for radiated disturbance measurement at a distance of 1 m. The gain measurement method of this document does not give the true far field free space antenna gain, because measuring that gain requires a far field separation between the antennas of one wavelength for wire antennas and, for horn antennas, of twice the square of the largest dimension of the radiating aperture divided by the wavelength. The procedures of Clause 4 and Clause 5 serve only to determine the 1 m gain.

4.5.2 Antenna spacing. With the antennas 1 m apart the 1 m gain may be measured by a suitable method. This is the same as the distance between the antenna and the equipment under test (EUT) in an electromagnetic compatibility (EMC) compliance measurement. Some antennas, for example horn antennas, have an electrical centre used for theoretical calculation and for fixing the antenna position. For LPDA antennas, which have no defined electrical centre or whose electrical centre is a function of frequency, the nearest point method is used. Figure 1 shows the reference planes for an antenna spacing of 1 m.

4.5.3 Number of measurements needed. Measurements are to be made at enough frequencies to describe the 1 m gain within the specified operating bandwidth of the antenna, at the frequencies specified in 5.3 g). Any anomaly in the 1 m gain characteristic can be identified by a swept frequency measurement of the insertion loss between a pair of antennas: where a sharp resonance is present, other measurement frequencies should be chosen so as to capture the 1 m gain or the 1 m antenna factor in the resonance region.

4.6 Determination of the 1 m antenna factor. 4.6.1 The AF for a measurement at 1 m is calculated by Formula (3), with the gain determined at the 1 m spacing of 4.3. 4.6.2 The AF is calculated from Formula (3) using the gain; to simplify the calculation a logarithmic form may be used. Where the 1 m gain is a numerical value, the AF in decibels per metre is obtained from Formula (8), whose key gives the wavelength in metres; where the 1 m gain is in decibels, the AF in decibels per metre is obtained from Formula (9). The worked example given at 4.6.2 reads: at 200 MHz the 1 m gain is 10 dB and twenty times the logarithm of the ratio 9.73 to the wavelength is 16 dB/m, so that AF is 16 dB/m minus 10 dB/m, that is 6 dB/m.

4.7 Use of the antenna factor. Adding the appropriate antenna factor and the cable loss to the voltage at the input of the measuring receiver, expressed in decibels relative to 1 microvolt, gives the field strength, expressed in decibels relative to 1 microvolt per metre.

5 Measurement of the 1 m gain

5.1.1 Measurement equipment for the two identical antennas method: a) a signal generator with an output impedance of 50 ohm, able to produce the test level over the frequency range specified for the antenna type; b) two attenuators of 6 dB and 50 ohm; c) a measuring receiver or spectrum analyser tuned over the frequency range specified for the antenna type, whose input impedance is to be 50 ohm with a voltage standing wave ratio (VSWR) not greater than 1.25, an isolating attenuator being allowed at the input of the measuring receiver to achieve a VSWR of 1.25; d) coaxial cables of 50 ohm characteristic impedance with suitable connectors for matched connection to the antennas, the 6 dB attenuators, the signal generator and the measuring receiver or spectrum analyser; e) an adapter for joining two coaxial cables; f) alternatively a network analyser may be used, with matching attenuators of 6 dB giving a VSWR not greater than 2 at the ports matched to the antennas.

5.1.2 Measurement arrangement. The arrangement is shown in Figure 2. The area of the measurement site is to be free of obstacles and reflections. The site is to have a ground plane so as to simulate an open area. The arrangement shown in Figure 2 may be changed, or an anechoic chamber may be used, if the correlation of the measurement results with the reference arrangement of the outdoor site of Figure 2 is less than 1 dB.

The initial antenna measurement height is defined as 3 m to the centre of the antenna. For dipole type antennas the antenna factor varies with polarization and both polarizations are to be measured. For aperture type (horn) antennas, if the measurement is made at a height of 3 m, the difference between the antenna factors of the two polarizations obtained by the method of this document is to be less than 1 dB. Considerations on measurement uncertainty are given in Annex B. The key to Figure 2 gives the distance between the antennas in metres and the height of the antennas above the ground plane in metres.

5.2 Three antenna method. The measurement procedure of the three antenna method needs the same measurement equipment and the same measurement arrangement as the two identical antennas method. When the three antenna method is used, the three antennas are taken two at a time: antenna 1 with antenna 2, antenna 1 with antenna 3, and antenna 2 with antenna 3. The antenna factor is calculated according to 4.4.

5.3 Measurement procedure. The measurement below uses a signal generator and a measuring receiver or spectrum analyser; as an alternative a network analyser may be used. With a measuring receiver or spectrum analyser, the following operations are to be carried out at each measurement frequency: a) adjust the output of the signal generator to obtain a reading on the measuring receiver at least 10 dB above the noise floor, making sure the receiver is tuned to the maximum response to the signal; b) adjust the alignment of the antennas to obtain the maximum reading on the measuring receiver and record the signal generator setting as the transmitted voltage; c) disconnect the measuring receiver cable and the signal generator cable from their antennas and interconnect the signal generator and the measuring receiver using the same cables and a 50 ohm adapter; d) reduce the output of the signal generator to obtain the same reading on the measuring receiver as in step b) and record the signal generator setting as the received voltage; e) calculate the gain at an antenna spacing of 1 m using Formula (6), the received and transmitted voltages being the signal generator readings recorded in steps d) and b) respectively; f) calculate the antenna factor using Formula (3) or Formula (7); g) carry out the measurement at the following frequency steps: 5 MHz from 20 MHz to 200 MHz; 50 MHz from 200 MHz to 1.0 GHz; 100 MHz from 1.0 GHz to 40 GHz.

Annex A Annex A (normative) Derivation of the antenna factor

The annex derives the antenna factor associated with any antenna of a given gain, starting from Formula (A.1). The key to Formula (A.1) gives the input voltage of the 50 ohm measuring receiver in volts, the effective height of the antenna in metres and the field strength in volts per metre. The factor of one half in the antenna factor comes from the assumption that, with a 50 ohm measuring receiver connected at the antenna port, the voltage at that port is divided by two.

Formula (A.2) and Formula (A.3) follow, with a key giving the maximum effective area, that is the area delivering the maximum power to a matched load, in square metres; a resistance of 50 ohm; the wave impedance of free space, taken as 377 ohm; and the directivity of the antenna. Substituting Formula (A.3) into Formula (A.2) gives Formula (A.4).

If a lossless 50 ohm transmission line is assumed between the antenna and the measuring receiver, and zero mismatch is assumed, then the directivity equals the gain, which gives Formula (A.5), whose key gives the numerical gain and the wavelength in metres. If the voltage is to be converted into the field strength, Formula (A.6) follows from Formula (A.5). The antenna factor that converts the reading of the measuring receiver in volts into the field strength in volts per metre is given by Formula (A.7).

The annex closes by stating that if the field strength is derived from the voltage induced in the antenna or from the open circuit voltage at the antenna terminals, the antenna factor is 4.87 divided by the wavelength, multiplied by the square root of the gain. The printed formulas themselves are damaged in the source text and are not reproduced here.

Annex B Annex B (informative) Considerations on measurement uncertainty

B.1 General considerations. The measurement uncertainties that apply fall into two classes. The first is the measurement uncertainty of the antenna factor itself: depending on the expertise of the calibration laboratory and the capability of its equipment, some laboratories can offer a lower antenna factor uncertainty than others. The second is the measurement uncertainty associated with the antenna calibration method when the antenna is used for radiated disturbance measurement.

Within the second class the largest contribution is the mutual coupling between biconical antennas. The cage elements of biconical antennas are only 0.5 m apart during calibration, so the presence of the opposite biconical antenna is built into the antenna factor of the AUC. In a radiated disturbance measurement the opposite antenna is replaced by the EUT, the receiving antenna no longer behaves as it did during calibration and the antenna factor is no longer valid. The change caused by the difference between the coupling during EUT measurement and the coupling during antenna calibration is assessed by measurement uncertainty. The worst case EUT is a large metal plane, giving a distance of 2 m between the antenna and its image; compared with the 1 m calibration distance the coupling is smaller and the error is smaller. The antenna factor is estimated to be 1.3 dB high, so that the measured field strength is also 1.3 dB high. An alternative solution is to use the free space antenna factor, in which case the measurement uncertainty introduced by coupling with the EUT can be reduced to about 0.5 dB.

The second largest contribution in the second class applies to LPDA antennas used at 200 MHz. The phase centre of such an antenna varies with frequency, and at 200 MHz the field strength measured by the antenna is at its greatest distance from the emitting source, usually 1.65 m. The calibration method is designed to correct for the variation of the phase centre with frequency, but because the opposite antenna is another LPDA antenna, its own phase centre variation is not compensated. Compensation can be achieved by using the three antenna method with a pair of small biconical antennas designed for the frequency range of the LPDA antenna, the small biconical antenna simulating the surface of the EUT emitting source. The error introduced is about 0.7 dB at 200 MHz and falls linearly with frequency to 0.1 dB at 1 GHz. Using small biconical antennas increases the reflection from the antenna mast, and if the measurement site is indoors a larger room or more absorbing material may be needed.

The preferred solution is to use two identical LPDA antennas with their tips 1 m apart and to apply a correction that reduces the residual phase centre error: for LPDA antennas the correction added to the antenna factor is 0.85 minus 0.6 times the frequency in megahertz divided by 800, in decibels.

It is to be noted that for an LPDA antenna 0.65 m long, using a free space antenna factor that applies at the phase centre can produce an error of up to 4.4 dB. That is the worst case error, which arises when the field strength is expected to be measured at 1 m from the EUT and the tip of the LPDA antenna is 1 m from the surface of the EUT, but the field is picked up by the dipole of the LPDA antenna that resonates at 200 MHz. To extrapolate the field strength to the tip of the antenna, the correction to the antenna factor is calculated as twenty times the logarithm of the ratio between, on the one hand, the sum of the distance from the antenna tip to the EUT, that is 1 m, and the distance from the tip of the LPDA antenna to the position of the active element, that is the phase centre, and on the other hand the distance from the antenna tip to the EUT.

A further example in the second class is the mutual coupling between a pair of horn antennas. The aperture of the EUT may differ from that of the horn antenna used to calibrate the receiving horn antenna, so that for a general EUT the measurement uncertainty of about 1 dB already included in the antenna factor may be larger than the uncertainty produced by reflection from the EUT. The free space antenna factor can be used for all the antennas listed in Clause 1, with a phase centre correction for LPDA antennas.

Radiated disturbance tests come in several arrangements, and which of the 1 m antenna factor and the free space antenna factor has the greater technical merit is still disputed. For a conventional double-ridged waveguide horn antenna working over 1 GHz to 18 GHz the 1 m antenna factor is about 1 dB higher than the free space antenna factor in the Fresnel region. For biconical antennas the free space antenna factor may be more correct than the 1 m antenna factor obtained by the method of this document. A small improvement in the accuracy of the antenna factor will have little effect on the total measurement uncertainty of a radiated disturbance measurement carried out in a shielded room that is not an anechoic chamber or that is only partly lined with absorbing material, which is typically 8 dB. At frequencies as low as 20 MHz, where the wavelength is 15 m, measuring with an antenna spacing as small as 1 m magnifies the influence of the opposite antenna of the pair, which does not adequately represent an EUT. Extrapolating the field strength from the rear of an LPDA antenna to its tip does not reliably give the actual field strength at the tip, because standing waves in a non-ideal free space environment affect the field at the rear of the antenna. For a shielded room partly lined with absorbing material a measurement uncertainty of 6 dB may be allowed above 80 MHz, but where the room is too small for waves to propagate at the lower frequencies a measurement uncertainty of 10 dB may be more appropriate.

B.2 Antenna gain in the near field region. Over the frequency range from 20 MHz to close to 45 MHz an antenna spacing of 1 m means that the antennas are in each other's inductive near field. In that case the antenna factor calculated from Formula (6), which applies in the far field, is in error, the antenna factor being 7 dB low at 20 MHz and 4 dB low at 30 MHz. The spacing in Formula (6) is replaced by an equivalent near field term, given as Formula (B.1), whose key gives the effective distance value applied in Formula (6), the actual spacing of 1 m, the phase constant taken as two pi divided by the wavelength, and the wavelength in metres. At distances greater than one wavelength the effective distance converges to the actual spacing, so a single formula can cover the whole frequency range.