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Analysis of EMC Test Antenna Parameters

Release time:

2023-09-05 15:48

Source:

　　1 Overview

In electromagnetic compatibility (EMC) testing, the antenna plays a vital role. Antennas are receivers and transmitters of electromagnetic waves that capture or emit signals and transmit them to test equipment or test samples.

Receiving electromagnetic radiation: The antenna can receive electromagnetic radiation in the external environment, including signals in the radio spectrum. In the test, the antenna can be used to detect the electromagnetic emission level of the device.

Emission of electromagnetic radiation: In some EMC tests, it is necessary to simulate the emission of electromagnetic radiation from the device to the surrounding environment. The antenna can be used as a transmitter to transmit electromagnetic waves of a specific frequency and power to evaluate the interference effect of the device on other devices.

The selection and positioning of the antenna is critical to the accuracy and repeatability of the test results. Proper antenna selection can ensure accurate measurements of the device's radiation and sensitivity. This article mainly introduces the antennas commonly used in EMC testing and the parameter requirements that these antennas need to meet.

　　2 EMC test antenna classification

In the EMC test, the measuring antenna mainly includes the following types according to the frequency division:

A,9kHz ~ 30MHz frequency band

-monopole antenna (rod antenna)

-loop antenna

B,30MHz ~ 1000MHz band

-Tuned dipole antenna

-Biconical antenna

-Logarithmic antenna

-Composite antenna

C,1GHz ~ 18GHz and above

-Horn antenna

-Log periodic dipole whole column antenna

　　3 Parameter characteristics of the antenna

3.1 antenna coefficient

The antenna factor (Antenna factor) is a parameter used to measure the response and transmission efficiency of an antenna to electromagnetic waves. It describes the ability of an antenna to convert received electromagnetic waves into electrical signals that are introduced into a cable or other device at the receiving or transmitting end of the antenna. Antenna coefficients are usually expressed in units of decibels dB(m-1).

The antenna coefficient is a comprehensive result of factors such as antenna size, structure, material and design. It depends on the effective area, radiation pattern, gain and frequency response of the antenna. Different types of antennas have different antenna coefficients.

The calculation formula commonly used in radiation disturbance test is as follows:

E(dBμV m)=U(dBμV) Fa[(dB m-1)] L(dB)

　　

A dipole-like antenna for measuring the electric field should be used between 30MHz and 1000MHz, and a free space antenna coefficient needs to be used.

　　

A,9 kHz to 30 MHz loop antenna

The interference phenomenon in this frequency range is that the magnetic field component plays a major role, and the main measuring antenna is a loop antenna. Important parameters of the loop antenna include the magnetic field antenna coefficient and shielding performance.

The magnetic field antenna coefficient AF = Hincident / Vreceived, in dB(S/m) or dB(A/m/V).

The unit of magnetic field strength is dB(µA/m) or µA/m, and the magnetic field strength can be obtained by dividing the electric field strength by 377Ω, I .e. H(µA/m)= E(µV/m)/377Ω.

The electric field strength in dB(µV/m) minus 51.5dB yields the magnetic field strength in dB(µA/m), I .e., H[dB(µA/m)]= E[dB(µV/m)]-51.5dB(Ω).

The less-than-ideal shielding of the loop antenna produces an electric field response. The identification of the electric field response of the loop antenna should be evaluated by rotating the loop plane in a uniform field so that it is parallel to the electric field vector. The response measured when the loop plane is parallel to the flux should be at least 20dB lower than the response measured when the loop plane is perpendicular to the flux.

B,9 kHz to 30 MHz monopole antenna

When measuring the radiated electric field component in this frequency band, a symmetrical or asymmetrical antenna can be used. When an asymmetric antenna is used, the measurement only represents the induction of the electric field to the vertical rod antenna.

CISPR 16-1-6 gives information on the calculation of the operating characteristics of a 1m long monopole antenna (rod antenna) and the characteristics of its matching network.

C,9 kHz ~ 30 MHz band large loop antenna system

In the frequency range of 9kHz to 30MHz, the radiated magnetic field component of a single EUT can be determined by a large loop antenna system (LLAS). The radiated disturbance is measured in the form of the induced current of the magnetic field in each loop antenna of the LLAS. LLAS consists of three mutually perpendicular, 2m diameter large circular antenna, CISPR 16-1-4 gives the magnetic field induced current and magnetic field strength between the conversion coefficient and verification method.

LAS loop antenna factor requirements

D, antennas in the frequency band above 30 MHz

CISPR 16-1-6 gives the calibration methods and requirements for antenna coefficients in the frequency band above 30MHz, which are summarized as follows:

The antenna coefficients obtained by the standard site method SSM are quasi-free space antenna coefficients. The measurement conditions are as follows:

  • Transmitting antenna height: 2m
  • Receiving antenna height: 1m ~ 4m
  • Polarization direction: horizontal polarization
  • Transceiver antenna horizontal projection distance: 10m

The biconical antenna coefficient Fa,SSM measured by the standard site method SSM, shall be corrected by the correction coefficient △Fa,SSM specified in the standard:

Fa = Fa,SSM - △Fa,SSM

The correction factors for biconical antenna in CISPR 16-1-6 are as follows:

If the test is carried out according to FCC standard, Fa needs to be corrected according to GSCF in C63.5 when logarithmic and biconical antennas are used for measurement in the darkroom field.

3.2 antenna gain

Gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiating element at the same point in space under the condition that the input power is equal. It quantitatively describes the degree to which an antenna concentrates the input power. Obviously, the gain is closely related to the antenna pattern. The narrower the main lobe and the smaller the side lobe, the higher the gain.

The physical meaning of gain can be understood in this way. In order to generate a signal of a certain size at a certain point at a certain distance, if an ideal non-directional point source is used as a transmitting antenna, an input power of 100W is required, while when a directional antenna with a gain of G = 13 dB (20 times) is used as a transmitting antenna, the input power only needs 100/20=5W. In other words, the gain of an antenna, in terms of the radiation effect in its maximum radiation direction, is a multiple of the input power amplification compared with the ideal point source without directivity.

The most commonly used antenna gain unit is dBi, because the power density of an antenna in different positions in space is different, we often say that an antenna 8dBi,12dBi... refers to the antenna radiation power density of the strongest position of the gain, that is, the maximum gain of the antenna. Refer to the following figure for the gain interface of a directional antenna:

In order to maximize the power of the main radiation direction, it is necessary to minimize the side lobes and back lobes and concentrate the energy on the main lobe.

The typical gain of the antenna manufacturer will be given in the factory report. The following figure shows the gain data of a horn antenna:

3.3 can withstand power

Antenna power refers to the maximum power that can be transmitted to a single antenna without damaging the antenna port due to overload, usually expressed in Watts or dBm. The maximum input power guides us to use the antenna correctly and prevents the safety of the device from being compromised. It is important to note that the excess power provided to the antenna will also be discharged in the form of heat, so overvoltage may cause a fire.

The following figure is a graph of the radiated field strength using recommend power provided by the antenna manufacturer:

　　

Symmetry of 3.4 antenna

In a radiation measurement, a common mode current is present on the cable connected to the receiving antenna. If the balun is not perfectly balanced, this common mode current will generate an electromagnetic field that can be received by the receiving antenna, thus affecting the radiated disturbance measurement.

In general, there is no obvious DM/CM in log-periodic dipole array (LPDA), and the symmetry of biconical antenna and composite antenna needs to be checked.

antenna symmetry requirements:

　　|20lg(V1/V2)|<1dB

The test method is shown in the figure below. V1 is the initial measurement data, and V2 is the reading value after the receiving antenna rotates 180 degrees without changing any configuration.

Cross-polarization performance of 3.5 antenna

When the antenna is placed in a polarized electromagnetic field, the terminal voltage when the antenna is cross-polarized with the field should be at least 20dB lower than the terminal voltage when co-polarized. This requirement applies to the entire frequency range from 30MHz to 18GHz, especially for LPDA antennas and composite antennas.

The transmit and receive antennas are set to be vertically polarized, and the signal strength over the entire frequency range is recorded. The transmit antenna is rotated 90 ° and the difference between the signal strength and the co-polarization reading is recorded.

When the interference signal is 20dB lower than the useful signal, the maximum error generated to the useful signal is ± 0.9dB. Otherwise, this error should be considered in the measurement uncertainty determination.

3.6 antenna pattern (lobe)

Patterns usually have two or more lobes, of which the lobe with the highest radiation intensity is called the main lobe, and the remaining lobes are called side lobes or side lobes. Please see the figure below. On both sides of the maximum radiation direction of the main lobe, the angle between two points where the radiation intensity is reduced by 3 dB (power density is reduced by half) is defined as the lobe width (also known as the beam width or main lobe width or half power angle). The narrower the lobe width, the better the direction, the longer the distance, the stronger the anti-interference ability.

The antenna manufacturer shall provide the type test results to prove the radiation pattern satisfied by the receiving antenna. Usually the boresight direction is the maximum radiation direction of the antenna. The following table shows the antenna lobe widths included in the antenna factory report.

The gray shaded area in the polar plot in the figure below is defined by the maximum height h, the maximum width w, and the measurement distance d of the EUT. In order to make the half-power lobe width of the receiving antenna fully cover the EUT, the half-power lobe width shall not be located in the shaded area of the E-plane and H-plane directional diagrams shown in the figure below.

Half Power Lobe Width Measureable Maximum EUT Width Half Power Lobe Width Measureable Maximum EUT Height

3.7 Return Loss or Standing Wave (VSWR)

Regarding the standing wave, we first understand the impedance of the antenna. The ratio of the signal voltage to the signal current at the antenna input is called the input impedance of the antenna. The input impedance has a resistive component Rin and a reactive component Xin, I .e., Zin = Rin j Xin. The presence of the reactance component reduces the antenna's extraction of signal power from the feed line, so the reactance component must be as zero as possible, that is, the input impedance of the antenna should be as pure as possible. In fact, even a well-designed and well-tuned antenna always contains a small reactance component in its input impedance. For any antenna, people can always through the antenna impedance debugging, in the required operating frequency range, so that the input impedance of the imaginary part is very small and the real part is quite close to 50 ohms, so that the input impedance of the antenna is Zin = Rin = 50 ohms, which is necessary for the antenna to be in good impedance matching with the feeder.

The VSWR of the antenna is called Swr, which is calculated using the formula, and the equivalent return loss is calculated as follows:

Return loss = -20lg | S11 | and Swr = 1 | S11 | / 1 | S11 |

Use the network analyzer to calibrate the reflection of the antenna end port, record S11, and use the above formula to convert to return loss or VSWR.

CISPR 16-1-4(GB 6113.104) requires that the return loss of the antenna connected to the antenna feeder should not be less than 10dB. In order to meet this requirement, it may be necessary to connect a matching attenuator on the cable line of the feeder. At the same time, C63.5 also mentioned the antenna feeder end attenuator to meet the standing wave ratio is not greater than 2:1 requirements.

The following figure shows the standing wave ratio of the composite antenna VULB 9162. When there is no terminating attenuator, the standing wave in the low frequency band is very high. After terminating the 3dB or 6dB attenuator, the standing wave is obviously reduced. Therefore, when we use this antenna, the antenna feeder port needs to be terminated with a 6dB attenuator.

3.8 phase center and reference point

After the electromagnetic wave radiated by the antenna leaves the antenna at a certain distance, its equiphase surface is approximately a spherical surface, and the spherical center of the spherical surface is the equivalent phase center of the antenna. The phase center should be a theoretical point. That is, it is theoretically considered that the signal radiated by the antenna is radiated outward with this point as the center of the circle. This point is the so-called phase center. But in the actual antenna basically does not exist, because your antenna can not be so perfect, so the actual antenna phase center is a region.

Reference point (marking point), a well-defined point on the antenna. Biconical antenna and loop antenna are usually the intersection of the center and axis of the antenna. The manufacturer will give the antenna when it leaves the factory, and the midpoint of the central axis of the antenna is generally selected as the reference point. Horn antenna usually selects the front edge of the horn mouth as the reference point.

In the antenna calibration, for the arrangement of the antenna geometry, the test distance d is defined as the distance between the projections of the two antenna reference points on the ground plane. In order to reduce the uncertainty in determining Fa, the following d-phase can be used instead for accurate measurement of the transceiver antenna spacing.

In EMC disturbance measurement, it is required to measure the electric field strength at a given distance, which is measured from the front surface of the EUT. In order to obtain higher accuracy, a correction can be given according to the following formula for a given frequency.

　　4 Conclusion

Antennas play an important role in EMC testing. They are a key part of the test system. Knowing the key parameters and significance of antennas can help laboratories improve the accuracy of testing and ensure the reliability of test results.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

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