A Yagi-Uda Antenna (Yagi Antenna), named after the co-inventors Hidetsugu Yagi and Shintaro Uda, is a direction antenna with a single driven dipole section with a series of parasitic elements (directors) that re-radiate energy from the main dipole thus manipulating the antenna pattern. The parasitic elements of a Yagi antenna are within the near field of the main dipole, and hence significantly alter the antenna pattern as compared to a dipole without parasitic elements. The overall effect results in a much more directional antenna pattern, with the antenna pattern being dedicated by the spacing and length of the main dipole and parasitic elements.
Yagi Antennas are a type of fixed antenna and are often mounted on a mast or pole and can be used indoors or outdoors depending on their IP rating. Some examples of Yagi Antennas include the FMANLP1005 Yagi Antenna which operates in the 5 GHz band to provides highly focused directional connectivity. The FMANLP1010 Yagi Antenna offers 14 dBi of gain along with vertical polarization and operates in the 3 GHz band to address 5G, LTE, CMDA, LoRA, IoT, WIFI applications.
A Yagi antenna also typically has a reflector section positioned “behind” the driven dipole element to further aid in increasing the directivity of the antenna. The reflector is often a series of conductive shafts, or mesh, typically about 5% longer than the driven element, and can consist of one or more shafts spaced “behind” the driven element. The reflector may also wrap around the “sides” or “top” and “bottom” of the Yagi antenna to further concentrate the antenna radiation in the forward direction.
The impact of the reflector is to increase the gain in the forward direction by reducing the radiation in the reverse direction of the antenna and focusing that radiation instead in the forward direction of the antenna (adds around 4 or 5 dB of gain to the forward direction. Though careful design can largely reduce sidelobe development, sidelobes are still developed that reduce the overall gain of the Yagi antenna.
The directors also add gain in the forward direction, by capturing and re-radiating the radiation that would otherwise spread out in a dipole antenna pattern. In this way, the Yagi antenna gain is a function of the number of directors added to the front of the antenna. There is a point at which the benefit to gain of designing in additional direction begins to diminish, with a maximum Yagi antenna gain limited to around 20 dB for a single antenna. By adding more directors the beamwidth of the Yagi antenna narrows along with increasing gain, which results in a practical limit for certain applications.
Moreover, the spacing and size of the Yagi antenna directors and mechanical structure to support the antenna structure results in a practical limit for the gain and directivity of low frequency Yagi antennas. This is due to the larger spacing and director lengths needed for efficient operation at lower frequencies. Hence, high gain Yagi antennas with high directivity are also naturally long antennas. A common figure-of-merit for a Yagi-Uda antenna is the front-to-back ratio. This is simply a ratio of the antenna radiation in the forward direction of the antenna and the reverse direction of the antenna, often expressed in a log ratio (dB).
The bandwidth of a Yagi antenna is also a function of the size of the Yagi antenna, namely the length, diameter, and spacing of the elements. Yagi antennas typically have a maximum percent bandwidth that is a few percent of the designed center frequency. The number of elements also reduces the bandwidth of a Yagi antenna, so a higher gain Yagi antenna also has a narrower bandwidth along with higher directivity. Therefore, when designing a Yagi-Uda antenna, there are generally trade-offs with the key performance parameters of gain/directivity, frequency of operation, bandwidth, size, and front-to-back ratio.
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