What are the antenna performance measurement standards
Antennas are key components of radio frequency and microwave equipment and are widely used in various applications such as radio and television broadcasting, radar, cellular transmission, and satellite communications. Antennas are used to send and receive radio waves, and the receiving and sending methods are determined by the receiver's design-omnidirectional antennas send and receive equally in all horizontal directions; directional antennas or high-gain antennas send and receive in designated directions. For example, receiving antennas in the form of wires, horns, openings, arrays, and dielectric rods are used to collect electromagnetic waves and extract electrical energy from them. Important characteristics related to antenna design include gain, radiation efficiency, aperture, directivity, bandwidth, polarization, radiation pattern, effective length, and resonance.
The received power of the antenna is related to the total area of the circular area used for the input signal (also known as the effective aperture). The calculation formula is: available antenna power (W) = power density (W/m2) × aperture (m2). The antenna gain is proportional to the aperture, and the gain can be increased by focusing the radio waves in a single direction while reducing the radio waves in other directions. Therefore, the larger the aperture, the higher the gain and the narrower the beam. In most cases, the larger the antenna size, the larger its maximum effective area.
Antenna directivity is used to measure the degree of radiant energy concentration in a specific direction, expressed as the ratio of the radiation intensity in a given direction to the average radiation intensity. In other words, antenna directivity refers to the ability of the antenna to concentrate energy in a specific direction when transmitting and receiving.
The antenna bandwidth refers to the working frequency range of the antenna, expressed as a percentage of the center frequency of the band. The bandwidth is a constant value relative to the frequency, and different types of antennas have different bandwidth limitations.
Polarization refers to the orientation of an electromagnetic wave's electric field, usually described as an ellipse. The electromagnetic waves emitted by the antenna can be polarized vertically or horizontally. The initial polarization of radio waves is determined by the antenna. For example, if the electromagnetic wave is polarized in the vertical direction, its electric field vector is a vertical vector, so a vertical antenna is required. Circular polarization is the result of the combination of horizontal and vertical waves. The electric field vector rotates around the circle in the direction of propagation, one rotation per radio frequency cycle.
Since the radiated power of the antenna is not equal in all directions, the antenna radiation pattern (also called the antenna polar diagram) has become an important tool for quickly assessing the overall antenna response. The radiation pattern of the transmitting antenna is a graph of the antenna radiated power field strength at various angles. The radiation pattern is usually depicted in the antenna axis plane (E plane) or the plane perpendicular to the antenna axis (H plane), and the unit is usually decibels (dB).
The effective length represents the electromagnetic wave transmitting and receiving efficiency of the antenna, and is used to determine the induced voltage generated when the electric wave acts on the open port of the antenna. For the receiving antenna, the effective length refers to the length and direction of the uniform current required to generate the same electric field as the transmitting antenna. The effective length provides a useful tool for determining the influence of the polarization mismatch between the radio wave emitted by the transmitting antenna and the receiving antenna.
Antennas are used to transmit radio waves to transmit information to a certain distance without the need for transmission lines. For radio receivers or transmitters, antennas are essential for signal transmission between radio and television broadcasting, cellular networks, Wi-Fi equipment, radar and GPS, and remote control equipment. The radio waves sent and received by the antenna can generally be polarized by adjusting the antenna axis. Due to the wide variety of transceiver equipment, in order to meet the corresponding transmission requirements, there are also various antenna types.
Antenna performance is usually characterized by basic radio frequency indicators such as antenna gain and antenna efficiency. The antenna needs to cover all target frequency ranges with good impedance matching performance and high radiation performance. For receiving and transmitting functions, many antenna characteristics are common characteristics between the two, which simplifies the test and measurement of each performance index described below. For example, calculating the gain of the transmitting antenna is more beneficial than calculating the area of the receiving antenna; similarly, for a large radio telescope, measuring the received power pattern is more useful than measuring the transmitting power pattern. It can be seen that this type of interoperability between the transmitting and receiving functions simplifies the calculation and measurement of the antenna.
The unit of gain is decibel (dBi), which is a performance index in terms of directivity and electrical efficiency. For antennas, gain is used to measure the degree of directionality of the radiation pattern—the radiation power of high-gain antennas is more concentrated in a certain direction; while the radiation angle of low-gain antennas is wider. High-gain antennas can propagate signals in a single direction to a greater extent, so longer transmission distances can be achieved without increasing signal strength, but they need to be accurately directed toward the receiver. On the contrary, although the transmission distance of a low-gain antenna is relatively short, it does not need to be directed towards the receiver. For example, the comparison between a high-gain satellite antenna and a low-gain cell phone built-in omni-directional antenna falls into this category.
An isotropic antenna is a hypothetical model with the same signal radiation power in all directions, which is used as a basis for comparison in the calculation of physical antenna gain. Although there are no physical antennas with isotropic radiation patterns in practice, many types of antennas have uniform radiation patterns on the horizontal plane. In this sense, antennas can be classified into directional antennas and omnidirectional antennas according to their applications.
The directional antenna is used to maximize the electromagnetic field coupling in the direction of the corresponding device. This type of antenna is particularly suitable for small-scale environments, because it can be tuned to achieve the best use as follows-after focusing, the smaller the percentage of the signal radiation angle to 360 degrees, the longer the maximum signal propagation distance.
Since the omnidirectional antenna receives and transmits on a 360-degree radius, its signal radiates uniformly in all directions. At the design level, an antenna with a size much smaller than the transmission wavelength cannot achieve high directivity. Therefore, the gain cannot measure the overall efficiency of the antenna, it can only determine the radiation output efficiency of the antenna in a single direction.
Radiation efficiency is used to determine the power required to achieve a specific level of performance. This parameter is an effective measure of antenna power efficiency and represents the ability of the antenna to utilize the power fed to the port. The definition of the radiation efficiency of an antenna refers to the ratio of the power delivered to the antenna to the power radiated by the antenna.
In an ideal situation, the antenna should convert all the power fed to its port into radiated electromagnetic energy for propagation to the surrounding space. However, in actual applications, part of the power fed to its antenna port will be lost. The causes of such losses include the mismatch between the antenna element and the feed network, and the natural loss caused by the resistance of the conductor as the antenna manufacturing material.
Since the radiation efficiency does not involve the radiation direction, it can be used as a performance index to measure the efficiency of cellular devices and other devices with omnidirectional radiation patterns in antenna design. On the contrary, if the antenna needs to radiate in a specific direction to achieve a design with directional characteristics in the radiation pattern, the gain is given priority as its performance index. Currently, more and more applications require omnidirectional signals. In these applications, radiation efficiency has gradually become the preferred test method. Through the radiation efficiency, the efficiency and performance of all areas around the antenna can be determined.