Everyone knows that our materials have an effective shielding effect on EMC. Since it has an effective effect on prevention, how do we know the EMC situation in the traditional medium at work? Next, let the editor tell you one of the EMC test tools, the application of the antenna.
EMC and RF testing are quite common in measurement. Common antennas are as follows:
1. Biconical antenna:
It is often used for RSE substitution test.
Common working frequency band: 30MHz~300MHz
2. Logarithmic antenna:
It is often used for NSA calibration of radiation sites.
Common working frequency: 30MHz~1GHz
3. Log periodic antenna:
It is often used for low frequency test of radiated disturbance/radiated spurious.
Common working frequency band: 30MHz~3GHz
4. Three loop antenna:
It is often used for magnetic field radiation test of lighting products.
Common working frequency band: 9KHz~30MHz
5. Horn antenna:
It is often used for high frequency test of radiated disturbance/radiated spurious.
Common working frequency band: 1GHz~18GHz
6. Dipole antenna:
It is often used in the measurement of site attenuation and antenna coefficient.
Common working frequency band: 30MHz~4GHz
7. Loop antenna:
Often used for low-frequency magnetic field testing.
Common working frequency band: 9KHz~30MHz
The polarization direction of the antenna
Customers often ask what is vertical and what is horizontal. The antenna radiates electromagnetic waves to the surrounding space. Electromagnetic waves are composed of electric and magnetic fields. It is stipulated that the direction of the electric field is the polarization direction of the antenna. The generally used antenna is single-polarized. The following figure shows two basic single polarization situations
Antenna's polarization direction lobe width
The beam width refers to the angle between the two half power points in the direction of the peak response of the antenna. The beam width has two components, the E plane and the H plane, and the two are not necessarily equal. If the gain of a certain antenna is designed as If it is positive, its beam width and gain are often just the opposite. The pattern usually has two or more lobes, among which the lobe with the highest radiation intensity is called the main lobe, and the remaining lobes are called side lobes or side lobes. On both sides of the maximum radiation direction of the main lobe, the angle between the two points where the radiation intensity is reduced by 3 dB (the power density is reduced by half) is defined as the lobe width (also known as the beam width or the main lobe width or the half power angle). The narrower the lobe width, the better the directivity, and the longer the operating distance, the stronger the anti-interference ability.
Gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiating unit at the same point in space under the condition of equal input power. It quantitatively describes the degree to which an antenna concentrates the input power and radiates it. The gain obviously has a close relationship with the antenna pattern. The narrower the main lobe of the pattern 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 distance and a certain point, if an ideal non-directional point source is used as the transmitting antenna, an input power of 100W is required, and the gain is G When a directional antenna with = 13 dB = 20 is used as a transmitting antenna, the input power only needs to be 100/20 = 5W. In other words, the gain of a certain antenna, in terms of the radiation effect in the direction of its maximum radiation, is the multiple by which the input power is amplified compared to an ideal point source without directivity.
Antenna factor (AF)
The antenna coefficient in free space is an inherent parameter of the antenna itself. The antenna coefficient represents the relationship between the antenna's radiation field and the antenna input voltage. AF has the following relationship with gain:
(E-The electric field intensity of a uniform plane wave incident on the reference plane of the receiving antenna; U-The output voltage of the receiving antenna)
Bandwidth refers to the frequency coverage of the antenna. If the bandwidth is expressed as a part of the rated frequency range of the antenna, the bandwidth of a non-resonant antenna is greater than that of a resonant antenna, and the bandwidth of a low-gain antenna is greater than that of a high-gain antenna. The bandwidth of the balun or matching network antenna has a greater impact than the antenna coefficient.
The impedance of the antenna is usually considered very little, because the load impedance of all EMC test equipment is designed to be 50Ω, and the impedance of the EMC antenna is usually designed to be close to 50Ω in its frequency range. However, the tester should also be aware that the impedance is not The possible problems caused by matching, especially the low-frequency magnetic field loop antenna, the impedance of the antenna often changes with frequency, but many low-frequency loop antennas do not have a matching network to compensate for this change.
Standing wave ratio
Standing wave ratio is an indirect parameter that measures whether the impedances of two RF devices match. VSWR is very important to most users. There are several complicated reasons for this. Simply put, under normal circumstances, the impedance shown by the feeder is the sum of the feeder impedance and the load impedance. Therefore, at both ends of the feeder Impedance mismatch may occur. In this way, most of the signal will be reflected at the load, and then reflect again at the source along the feeder. When accurate measurement is required, or when the signal source is very sensitive to impedance mismatch At times, or when the loss of the feeder is heavy, VSWR will become a problem.
Size is a very important antenna characteristic. The need to control and move the antenna limits the actual size of the antenna. The need to use the antenna in a shielded room also limits the maximum size of the antenna. It is necessary to reduce the undesirable ground or surrounding objects. Coupling will also affect the size, but on the contrary, if the antenna has a good low frequency response, high gain or wide bandwidth, the size of the antenna must be increased.