What is the symmetrical antenna meaning?

Balanced feed

Due to the symmetry of the symmetrical antenna, when balanced feeding is used, the currents on the corresponding line segments of the two halves should also be symmetrically distributed, and the pattern in the symmetry plane also has symmetry. The symmetrical antenna actually used, due to the influence of stray capacitance, such as the influence of the distributed capacitance to the ground, may also make the two halves unbalanced, thereby causing the asymmetry of the pattern. When a symmetrical antenna is connected to an unbalanced feeder (such as an axis), a balanced-unbalanced converter must be used to ensure the balance of the two halves of the antenna. Common converters include quarter-wave choke sleeves, U-tubes and transmission line impedance transformation. device, etc.

Symmetrical antenna also has a narrow understanding, that is, the center-fed dipole antenna is called a symmetrical antenna, also known as a dipole antenna). The length of each arm is l, and when the wire diameter is very small, the current on it can be approximately considered as a sinusoidal distribution. is the direction of the E surface when the arm lengths are different, and the direction diagram of the H surface is a circle. When l/λ is greater than 0.5, side lobes appear; when λ≥0.75, the maximum radiation direction deviates from the direction perpendicular to the antenna axis, usually limited to use in the range of λ≤0.625, where λ is the working wavelength. Antennas equal to λ/4 and λ/2 are called half-wave antennas and full-wave antennas, respectively, and half-wave symmetrical antennas are the most commonly used.

Dipole antenna

The dipole antenna can be regarded as formed by the open-circuit parallel bi-wires, and its input impedance Zin. Resonance occurs when it is slightly smaller than an integer multiple of λ/4. The smaller the characteristic impedance of the antenna, the smoother the change of input impedance with frequency. Therefore, the method of thickening the antenna wire diameter is often used to reduce its characteristic impedance to broaden the working frequency band. In order to reduce weight and save materials, several wires can be used to form cage-shaped or grid-shaped arms in the medium and short wave bands to increase the equivalent diameter of the antenna. Around λ=0.25, the input resistance is about 70 ohms. The half-power lobe widths of the half-wave and full-wave dipole antennas are 78° and 47°, respectively, and the directivity coefficients are 1.64 and 2.41, respectively. If λ is small, the pattern is similar to that of the fundamental oscillator, and the half-power lobe width is close to 90°. If l/λ increases, stronger directivity can be obtained, but if it exceeds 0.625, the directivity coefficient will decrease. The Hertzian antenna used by H.R. Hertz to demonstrate the existence of radio waves through experiments is a symmetrical antenna. Dipole antennas are the most widely used antennas in the shortwave and ultrashortwave bands. In the short-wave band, a dipole antenna erected horizontally on the ground is also called a π-shaped antenna, and the angle (elevation angle) between its maximum radiation direction and the ground is controlled by the erection height. In the ultrashort wave band, the dipole antenna can be used independently or as an array element of a strong directional antenna array. In the microwave band, it is generally used as a feed.

Optimized design

The length of the antenna affects the operating frequency. In the next simulation design, the antenna length will be optimized and the most suitable result will be selected. The size of the ground plate on the back of the antenna has a great influence on the radiation performance of the antenna. Considering the omnidirectional radiation of the antenna and the influence of the floor on the bandwidth and center frequency of the antenna, the length of the floor is less than the length of the feeder. Secondly, in order to obtain an intermediate frequency suitable for the WiMAX frequency band, a slot is designed on the floor, so that the short-circuit section of the slot forms a virtual feed point relative to the microstrip feeder, and the desired frequency band is excited to form a notch and adjust the length of the slot. , width and distance from the feeder to adjust the radiation performance of the antenna, and find the appropriate slot parameters to achieve the design results. Finally, considering each structural parameter of the antenna design comprehensively, select the appropriate value to achieve the design requirements. Due to the parasitic impedance generated during bending, connection and transmission, parasitic radiation will be generated, which will lead to an increase in the side lobe level of the antenna and a decrease in the gain. In order to To reduce the influence of parasitic impedance, the connection of the antenna pole is cut with a cut angle instead of a right angle. There are two symmetrical slits on the ground plate on the back of the antenna. The length of the ground plate is between 11 and 13 mm. The longer the length of the ground plate, the higher the return loss of the antenna in the high-frequency resonant frequency band is less than -10 dB bandwidth. The larger the value, and the lower frequency resonant frequency moves to the left, the -10 dB bandwidth of the low frequency resonant frequency band decreases accordingly, and it can be clearly seen that the return loss value of the low frequency band is getting smaller and smaller, and the low frequency radiation performance of the antenna is getting better and better. it is good. However, when the length of the ground plate continues to increase to 14 mm, the return loss of the antenna in the high-frequency resonant frequency band of the antenna is smaller than the -10 dB bandwidth, and the return loss value of the low-frequency resonant frequency band increases significantly, resulting in the low frequency of the antenna. Radiation performance deteriorates significantly. When the length of the ground plate is 11 mm, the minimum bandwidth of the high-frequency resonance frequency band cannot cover the WLAN 5.8 GHz. In addition, the return loss value of the low-frequency band is large, and the radiation performance is not good.

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