user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/04/22 15:30] – Added segment on driven and parasitic elements. user | user:kurser:ham_vt2023_l7 [2023/04/22 18:30] – Added segment on antenna gain and efficiency. user | ||
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* Directivity, | * Directivity, | ||
* Antenna gain | * Antenna gain | ||
- | * Far-field distance = d_f > 2*D^2/lambda, given d_f >> D, d_f >> | + | * Far-field distance = d_f > 2*D^2/λ, given d_f >> D, d_f >> |
- | * Radiation efficiency, | + | * Radiation efficiency, |
* Radiation pattern, E & H patterns | * Radiation pattern, E & H patterns | ||
* Polarisation and x-pol suppression | * Polarisation and x-pol suppression | ||
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==Direction== | ==Direction== | ||
- | The most ideal antenna is a single charge floating in free space, radiating as a sphere in all directions. Such a single charge is known academically as an //isotropic radiator//. Practically, | + | The most ideal antenna is a single charge floating in free space, radiating as a sphere in all directions. Such a single charge is known academically as an //isotropic radiator//. Practically, |
- | How small that illuminated segment of the sphere is, is known as the //antenna directivity// | + | How small that illuminated segment of the sphere is as opposed to size the entire sphere, is known as the //antenna directivity// |
A higher directivity means that more of the emitted/ | A higher directivity means that more of the emitted/ | ||
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The direction with the highest directivity of the antenna, is known as the //main lobe// (SE: huvudloben). Thus, the smaller lobes are known as // | The direction with the highest directivity of the antenna, is known as the //main lobe// (SE: huvudloben). Thus, the smaller lobes are known as // | ||
- | Simple antennas, like a monopole antenna, only have a single lobe. Very complex antennas, may have lobes that are shaped practically in any way imaginable. Example: an antenna in a satellite orbiting above a nation, might have an antenna with a lobe pattern that is shaped according to the borders of that nation. The antenna is thus good at transmitting/ | + | Simple antennas, like a monopole antenna, only have a single lobe. Very complex antennas, may have lobes that are shaped practically in any way imaginable. Example: an antenna in a satellite orbiting above a nation, might have an antenna with a lobe pattern |
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In practice, reflectors are usually checkerboard-shaped meshes of wires. | In practice, reflectors are usually checkerboard-shaped meshes of wires. | ||
+ | \\ | ||
+ | ==A stricter definition of directivity== | ||
+ | The directivity of an antenna, is the maximum transmitted power in the main lobe, divided by the average power transmitted across the entire sphere. From this ratio, we may derive that the directivity D = (4*pi) / (beam area). | ||
+ | |||
+ | \\ | ||
+ | ==Gain and antenna efficiency== | ||
+ | In a datasheet, you will typically find an entry for //gain// (SE: antennvinst). Different sources will have different opinions on what is the gain of the antenna. Here, we choose to define the antenna gain as the //power gain// of the antenna. The power gain G is related to the directivity of the antenna as\\ | ||
+ | G = η * D | ||
+ | |||
+ | ... where η is the so-called efficiency factor of the antenna. In practice, there will be unexpected ohmic losses in the antenna, leading to an η that is smaller than 1. | ||
+ | |||
+ | It is thus useful to discuss a real antenna in terms of its gain, rather than in terms of its theoretical directivity. | ||
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TODO | TODO | ||
+ | \\ | ||
+ | ==Far-field== | ||
+ | All commonly used formulas related to antennas, assume simplifications that happen once we are standing at a large distance away from the antenna. The far-field distance = d_f > 2*D^2/ | ||
+ | |||
+ | Meaning, that for very high frequencies, | ||
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user