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C&T RF Antennas Inc www.lcantennas.com www.ctrfantennas.com [email protected] Antenna Performance Parameters There are many critical parameters that affect antenna performance, which can usually be adjusted during the antenna design process, such as resonance frequency, impedance, gain, aperture or radiation pattern, polarization, efficiency, and bandwidth. In addition, the transmitting antenna has maximum rated power, and the receiving antenna has noise suppression parameters. 1. Resonance frequency The resonant frequency and electrical resonance are related to the electrical length of the antenna. The electrical length is usually the physical length of the wire divided by the ratio of the wave propagation speed in free space to the speed in the wire. The electrical length of the antenna is usually expressed by the wavelength. Antennas are generally tuned at a certain frequency and are effective in a frequency band centered on this resonant frequency. But other antenna parameters (especially the radiation pattern and impedance) vary with frequency, so the resonant frequency of the antenna may only be close to the center frequency of these more important parameters. The antenna can resonate at a frequency corresponding to the length of the target wavelength component number relationship. Some antennas are designed with multiple resonant frequencies, while others are relatively effective over a wide frequency band. The most common broadband antenna is a log-periodic antenna, but its gain is much smaller than that of a narrowband antenna. Please Contact us for more information, thank you. Coco Lu (+86)13412239096 C&T RF Antennas Inc www.lcantennas.com www.ctrfantennas.com [email protected] 2. Gain Gain refers to the logarithm of the ratio of the intensity of the antenna radiation pattern of the antenna's strongest radiation directly to the intensity of the reference antenna. If the reference antenna is an omnidirectional antenna, the unit of gain in dBi. For example, the gain of the dipole antenna is 2.14dBi. Dipole antennas are also often used as reference antennas (this is because a perfect omnidirectional reference antenna cannot be manufactured). In this case, the gain of the antenna is in dBd. Antenna gain is a passive phenomenon. The antenna does not increase excitation, but only redistributes it to radiate more energy in a certain direction than an omnidirectional antenna. If the antenna gain is positive in some directions, its gain in other directions will be negative due to the conservation of energy of the antenna. Therefore, the gain that the antenna can achieve must be balanced between the antenna's coverage and its gain. For example, the gain of the dish antenna on the spacecraft is very large, but the coverage area is very narrow, so it must point accurately to the earth; while the broadcast transmitting antenna needs to radiate in all directions, its gain is very small. The gain of a dish antenna is proportional to the aperture (reflection area), the surface accuracy of the antenna reflection surface, and the frequency of transmission/reception. Generally speaking, the larger the aperture, the greater the gain, and the higher the frequency, the greater the gain, but the error of the surface accuracy at higher frequencies will result in a great decrease in gain. Aperture and radiation patterns are closely related to gain. Aperture refers to the cross-sectional shape of the beam in the direction of the highest gain, which is two-dimensional (sometimes the aperture is expressed as the radius of a circle approximate to the cross-section or the angle of the beam cone). The radiation pattern is a three-dimensional graph representing gain, but usually, only the horizontal and vertical two-dimensional sections of the radiation pattern are considered. The radiation pattern of a high gain antenna is often accompanied by side lobes. Sidelobe refers to the beams other than the main lobe (the beam with the highest gain) in the gain. When systems such as radars need to determine the direction of the signal, the sidelobe will affect the quality of the antenna. The sidelobe will also reduce the main lobe gain due to power distribution. 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 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 like this: A signal of a certain size is generated at a certain point at a certain distance. If an ideal non-directional point source is used as the transmitting antenna, 100W input power is required, and a certain directional antenna with a gain of G = 13 dB = 20 is used as the transmitting Please Contact us for more information, thank you. Coco Lu (+86)13412239096 C&T RF Antennas Inc www.lcantennas.com www.ctrfantennas.com [email protected] antenna. When using an antenna, the input power is only 100/20 = 5W. In other words, the gain of a certain antenna, in terms of the radiation effect in its maximum radiation direction, is the multiple of the input power amplification compared with an ideal point source without directionality. The gain of the half-wave symmetrical oscillator is G=2.15dBi. Four half-wave symmetrical oscillators are arranged up and down along the vertical line to form a vertical four-element array, with a gain of approximately G=8.15dBi (dBi unit indicates that the comparison object is an ideal point source with uniform radiation in all directions). If a half-wave symmetric oscillator is used as the comparison object, the unit of gain is dBd. The gain of the half-wave symmetrical oscillator is G=0dBd (because it is compared with itself, the ratio is 1, and the logarithm is zero.) The gain of the vertical quaternion array is about G=8.15–2.15=6dBd. Gain characteristics ⑴ The antenna is a passive device and cannot generate energy. The antenna gain is only the ability to effectively concentrate energy in a specific direction to radiate or receive electromagnetic waves. ⑵The antenna gain is produced by the superposition of dipole elements. The higher the gain, the longer the antenna length. ⑶ The higher the antenna gain, the better the directivity, the more concentrated the energy, and the narrower the lobe. 3. Bandwidth The bandwidth of an antenna refers to the frequency range in which it works effectively, usually centered on its resonant frequency. The antenna bandwidth can be increased by the following techniques, such as the use of thicker metal wires, the use of metal meshes to approximate thicker metal wires, antenna elements with thinner tips (such as in a feed horn), and a single integrated multi-antenna Components, use characteristic impedance to select the correct antenna. Small antennas are usually easy to use but have unavoidable limitations in bandwidth, size, and efficiency. 4. Impedance Impedance is similar to the refractive index in optics. Radio waves traveling through different parts of the antenna system (radio, feeder, antenna, free space) will encounter impedance differences. At each interface, depending on the impedance matching, part of the energy of the electric wave will be reflected back to the source, forming a certain standing wave on the feeder. At this time, the ratio of the maximum energy to the minimum energy of the electric wave can be measured, which is called the standing wave ratio (SWR). A standing wave ratio of 1:1 is ideal. A standing wave ratio of 1.5:1 is regarded as a critical value in low-energy applications where energy consumption is more critical. The standing wave ratio of up to 6:1 can also appear in the corresponding equipment. Minimizing Please Contact us for more information, thank you. Coco Lu (+86)13412239096 C&T RF Antennas Inc www.lcantennas.com www.ctrfantennas.com [email protected] the impedance difference (impedance matching) of various interfaces will reduce the standing wave ratio and maximize the energy transmission between the various parts of the antenna system. The complex impedance of an antenna relates to the electrical length of the antenna during operation. By adjusting the impedance of the feeder, that is, the feeder is used as an impedance converter, the impedance of the antenna can be matched with the feeder and the radio. It is more common to use antenna tuners, baluns, impedance converters, matching networks including capacitors and inductors, or matching sections such as gamma matching. 5. Radiation pattern The half-wave dipole antenna (same as above) gain (dBi) radiation pattern is a graphical description of the relative field strength of the antenna emitted or received. Since the antenna radiates to a three-dimensional space, several figures are needed to describe it. If the antenna radiation is symmetrical with respect to a certain axis (such as dipole antennas, helical antennas, and some parabolic antennas), only one pattern is required. Different antenna supplier/users have different standards and drawing formats for the pattern. 6. Characteristic impedance The ratio of voltage to current everywhere on an infinite transmission line is defined as the characteristic impedance of the transmission line, which is represented by Z。. The calculation formula of the characteristic impedance of the coaxial cable is Z。=〔60/√εr〕×Log (D/d) [Ou]. In the formula, D is the inner diameter of the coaxial cable outer conductor copper mesh; d is the outer diameter of the coaxial cable core wire; εr is the relative permittivity of the insulating medium between conductors.
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