Introduction to commonly used antennas for electromagnetic compatibility (EMC).

Antennas are widely used in EMC and RF testing and measurement. Common antennas are as follows:

1. Biconical antenna:

Commonly used for RSE alternative testing.

Common operating frequency band: 30MHz~300MHz

2. Logarithmic antenna:

Commonly used for NSA calibration in radiation sites.

Common operating frequency band: 30MHz~1GHz

3. Logarithmic periodic antenna:

Commonly used for radiated disturbance/stray low frequency testing.

Common operating frequency band: 30MHz~3GHz

4. Triple loop antenna:

Commonly used for magnetic field radiation testing of lighting products.

Common operating frequency band: 9KHz-30MHz

5. Horn antenna:

Commonly used for radiation disturbance/radiation spurious high frequency testing.

Common operating frequency band: 1GHz~18GHz

6. Dipole antenna:

It is commonly used in the measurement of field attenuation and antenna coefficient.

Common operating frequency band: 30MHz~4GHz

7. Loop antenna:

Commonly used for low-frequency magnetic field testing.

Common operating frequency band: 9KHz-30MHz

During EMC and RF testing, the following basic concepts need to be mastered:

1. Polarization direction of antenna

Customers often ask what is vertical and what is horizontal. The antenna radiates electromagnetic waves into the surrounding space. Electromagnetic waves consist of electric and magnetic fields. It is stipulated that the direction of the electric field is the polarization direction of the antenna. The commonly used antenna is unipolar. The following illustration shows two basic monopolarization scenarios

2. Lobe width

Beam width refers to the angle between two half power points in the direction of the antenna peak response. The beam width has two components, the E-plane and the H-plane, which are not necessarily exactly equal. If the gain of an antenna is designed to be positive, its beam width and gain are often exactly the opposite. The pattern usually has two or more lobes, with the lobe with the highest radiation intensity being called the main lobe and the remaining lobe being called the side lobe or the side lobe. On both sides of the maximum radiation direction of the main lobe, the angle between two points where the radiation intensity decreases by 3 dB (power density decreases by half) is defined as the lobe width (also known as beam width or main lobe width or half power angle). The narrower the lobe width, the better the directionality, the farther the action distance, and the stronger the anti-interference ability.

3. Antenna gain

Gain refers to the ratio of the power density of the signal generated by the actual antenna and the ideal radiation unit at the same point in space under the condition of equal input power. It quantitatively describes the degree to which an antenna concentrates its input power. The gain is obviously closely related to the antenna pattern. The narrower the main lobe of the pattern, the smaller the side lobe, and the higher the gain. The physical meaning of gain can be understood in this way - to generate a signal of a certain size at a certain point over a certain distance, if an ideal non directional point source is used as a transmitting antenna, it requires 100 W of input power, while if a directional antenna with a gain of G=13 dB=20 is used as a transmitting antenna, the input power only needs 100/20=5 W. In other words, the gain of an antenna, in terms of its radiation effect in the maximum radiation direction, is the multiple by which the input power is amplified compared to an ideal point source with no directionality.

4. Antenna coefficient (AF)

The antenna coefficient in free space is an inherent parameter of the antenna itself. The antenna coefficient represents the relationship between the radiation field of the antenna and the antenna input voltage. AF has the following relationship with gain:

AF=E/U

(E - Electric field intensity of uniform plane waves incident on the reference plane of the receiving antenna; U - Output voltage of the receiving antenna)

5. Bandwidth

Bandwidth refers to the frequency coverage of an antenna. If the bandwidth is expressed as a portion of the rated frequency range of the antenna, the bandwidth of a nonresonant antenna is greater than the bandwidth of a resonant antenna, while the bandwidth of a low gain antenna is greater than the bandwidth of a high gain antenna. For broadband, balanced unbalanced converters or matching network antennas, the bandwidth has a greater impact than the antenna coefficient.

6. Impedance

The impedance of an antenna is usually considered very little because all EMC test equipment is designed with a load impedance of 50 Ω, and the impedance of an EMC antenna is usually designed to be approximately 50 Ω within its frequency range. However, testers should also be aware of the possible problems caused by impedance mismatches, especially for low-frequency magnetic field loop antennas, where the impedance of the antenna often varies with frequency, However, many low-frequency loop antennas do not have matching networks to compensate for this change.

7. Standing Wave Ratio (VSWR)

The standing wave ratio is an indirect parameter that measures whether the impedances of two RF devices match. VSWR is very important to most users for several complex reasons. To put it simply, in general, the impedance displayed by a feeder is the sum of the rated impedance of the feeder and the load impedance. Therefore, impedance mismatches may occur at both ends of the feeder. In this way, most signals will be reflected at the load, and then reflected again at the source along the feeder. When accurate measurements are required, "VSWR can become a problem when the signal source is sensitive to impedance mismatches, or when the loss of the feeder is significant.".

8. Dimensions

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. The need to reduce unwanted coupling to the ground or to surrounding objects will also affect the size, but on the contrary, if you want the antenna to have a good low-frequency response, high gain, or wide bandwidth, you need to increase the size of the antenna.