user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/04/22 19:08] – Added p.e.p. and e.i.r.p. example exam questions on directivity. user | user:kurser:ham_vt2023_l7 [2023/04/22 19:52] – user | ||
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* Quad | * Quad | ||
* Waveguide slot | * Waveguide slot | ||
+ | * Ground plane antenna (GP-antenna) | ||
+ | * Horn antenna | ||
* Spiral/ | * Spiral/ | ||
* Vivaldi | * Vivaldi | ||
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How small that illuminated segment of the sphere is as opposed to size the entire sphere, 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 resulting lobe pattern from the driven element, is then by-design usually deformed using other elements that are in fact not connected to the cable that fed the antenna. All elements on the antenna that are not driving the EM wave, are known as parasitic elements (SE: parasitelement). | The resulting lobe pattern from the driven element, is then by-design usually deformed using other elements that are in fact not connected to the cable that fed the antenna. All elements on the antenna that are not driving the EM wave, are known as parasitic elements (SE: parasitelement). | ||
- | These parasitic elements have different purposes. They could be used to direct the lobe pattern into some wanted direction; such elements are known as // | + | These parasitic elements have different purposes. They could be used to direct the lobe pattern into some wanted direction; such elements are known as // |
Example: the Yagi-Uda antenna, has one driven element on its boom. To get the signal pointing forwards, three dipole directors are mounted at the front of the antenna. And to get the lobe pattern pointing less backwards, a reflector is mounted at its back. | Example: the Yagi-Uda antenna, has one driven element on its boom. To get the signal pointing forwards, three dipole directors are mounted at the front of the antenna. And to get the lobe pattern pointing less backwards, a reflector is mounted at its back. | ||
In practice, reflectors are usually checkerboard-shaped meshes of wires. | In practice, reflectors are usually checkerboard-shaped meshes of wires. | ||
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==A stricter definition of directivity== | ==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). | 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). | ||
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+ | \\ | ||
+ | ==How wide is a lobe?== | ||
+ | * TODO: -3 dB point (SE: halvvärdesbredd)\\ | ||
+ | * TODO: The angle spanned by the circular segment, defines the // | ||
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Some antennas transmit mainly using the electric field, some transmit mainly using the magnetic field, and some use both. Let's imagine an antenna with an electric (E) field that is vertically polarised, and a magnetic (H) field that is horisontally polarised. Let's describe the E field like a cosine with some phase, and the H field like a sine with some phase. If we stand directly in front of where the antenna is pointing, we could see both the cosine and sine waves like composants in a complex vector. As time progresses, and the E cosine goes up/down while the H sine goes left/right, that complex vector would be spinning around in a circle as time progresses. This type of polarisation is known as a circular polarisation; | Some antennas transmit mainly using the electric field, some transmit mainly using the magnetic field, and some use both. Let's imagine an antenna with an electric (E) field that is vertically polarised, and a magnetic (H) field that is horisontally polarised. Let's describe the E field like a cosine with some phase, and the H field like a sine with some phase. If we stand directly in front of where the antenna is pointing, we could see both the cosine and sine waves like composants in a complex vector. As time progresses, and the E cosine goes up/down while the H sine goes left/right, that complex vector would be spinning around in a circle as time progresses. This type of polarisation is known as a circular polarisation; | ||
- | Circular polarisation is common in public FM radio broadcast (rundradio). The advantage is that the receiver can be rotated at almost any direction with respect to the ground, and still be somewhat optimal to the transmitter. Another example is in satellite-to-Earth | + | Circular polarisation is common in public FM radio broadcast (SE: rundradio). The advantage is that the receiver can be rotated at almost any direction with respect to the ground, and still be somewhat optimal to the transmitter. Another example is in satellite-to-Earth |
- | Common misconception: | + | Common misconception: |
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== Common mode current on coax = bad! == | == Common mode current on coax = bad! == | ||
TODO | TODO | ||
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+ | \\ | ||
+ | == Antenna impedance matching == | ||
+ | TODO | ||
+ | |||
+ | |||
+ | |||
+ | |||
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==Far-field== | ==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/ | + | 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, | Meaning, that for very high frequencies, |
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user