user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/04/22 14:19] – Added portion about lobe patterns. user | user:kurser:ham_vt2023_l7 [2023/04/22 15:30] – Added segment on driven and parasitic elements. user | ||
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**Recommended reading: KonCEPT page 191-229 (chapter 7 + 8) ** | **Recommended reading: KonCEPT page 191-229 (chapter 7 + 8) ** | ||
- | Start with transmission line (TL), the most difficult component. | + | Start with transmission line (TL), the most difficult component. |
characterized by: Characteristic impedance, Z_0, a geometry and material parameter. Length and the speed of light in the transmission line. Two metal conductors that guide the fields. | characterized by: Characteristic impedance, Z_0, a geometry and material parameter. Length and the speed of light in the transmission line. Two metal conductors that guide the fields. | ||
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balanced TL folded out -> dipole antenna! Nice SWR achieved. L = lambda/2 = 300/(2*f) ~=(0, | balanced TL folded out -> dipole antenna! Nice SWR achieved. L = lambda/2 = 300/(2*f) ~=(0, | ||
+ | |||
+ | \\ | ||
+ | \\ | ||
+ | |||
**Antennas** | **Antennas** | ||
- | What is an antenna? An antenna is a device that converts EM waves between a bound medium and a free medium, for instance between a cable and free space. | + | In this segment, possible exam questions have been marked // |
- | Alternative: | + | \\ |
+ | ==What is an antenna?== | ||
+ | An antenna is a device that converts EM waves between a bound medium and a free medium, for instance between a cable and free space. | ||
+ | |||
+ | Alternative: | ||
- | //Antenna types.// | + | \\ |
+ | ==Antenna types== | ||
There are many different types of antennas, since different antenna designs are optimised for solving different problems. There is no antenna that is perfect for every single operational situation. However, the opposite is true; there are indeed antennas that are quite bad at everything. | There are many different types of antennas, since different antenna designs are optimised for solving different problems. There is no antenna that is perfect for every single operational situation. However, the opposite is true; there are indeed antennas that are quite bad at everything. | ||
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* Loop antennas | * Loop antennas | ||
* Yagi (Yagi-Uda) | * Yagi (Yagi-Uda) | ||
+ | * (Parabolic dish reflectors) | ||
+ | * PCB/PIFA | ||
* Patch | * Patch | ||
* Quad | * Quad | ||
- | * PCB/PIFA | ||
* Waveguide slot | * Waveguide slot | ||
- | * Bowtie | ||
* Spiral/ | * Spiral/ | ||
* Vivaldi | * Vivaldi | ||
* J-pole | * J-pole | ||
+ | * Bowtie | ||
- | Antennas may often be combined together for different effects. | + | Antennas may often be combined together for different effects. |
- | * Log-periodic dipole array (LDPA) | + | * Log-periodic dipole array (LDPA) |
+ | |||
+ | ... however, in practice there is nothing prohibiting an antenna array from being built using any antenna type. | ||
Very often, specific antennas are are combination of other antenna types. For example, the very common Yagi-Uda antenna, is a combination of three (or more) dipole antennas, and one magnetic loop antenna. | Very often, specific antennas are are combination of other antenna types. For example, the very common Yagi-Uda antenna, is a combination of three (or more) dipole antennas, and one magnetic loop antenna. | ||
+ | |||
+ | \\ | ||
+ | ==Antenna parameters and characteristics== | ||
All antennas can be characterised using //at least// the following parameters: | All antennas can be characterised using //at least// the following parameters: | ||
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* Polarisation and x-pol suppression | * Polarisation and x-pol suppression | ||
- | Of these parameters, only input impedance and operational frequency are arguably easy to analyse. This analysis is done with either | + | Most of these parameters |
+ | \\ | ||
==Introductory important terminology== | ==Introductory important terminology== | ||
- | The actual metal sticks that point at different directions on an antenna, are commonly known as // | + | The actual metal sticks that point at different directions on an antenna, are commonly known as // |
- | These elements are often mounted to a frame of sorts, like a large metal bar that supports all elements. This bar is known as the //boom// (bomm). | + | These elements are often mounted to a frame of sorts, like a large metal bar that supports all elements. This bar is known as the //boom// (SE: bom). |
- | Often, antennas are shielded from the evil world of rain, snow, pidgeons etc. by placing them in a plastic cover, box etc. This plastic | + | Often, antennas are shielded from the evil world of rain, snow, pidgeons etc. by placing them inside |
+ | \\ | ||
==Direction== | ==Direction== | ||
- | The most ideal antenna is a single charge floating in free space, radiating | + | The most ideal antenna is a single charge floating in free space, radiating |
- | How small that illuminated segment of the sphere is, is known as the antenna directivity. The directivity is often given in dB with respect to that ideal //isotropic radiator//. The unit is thus *dBi*. A very large part of an antenna' | + | How small that illuminated segment of the sphere is, is known as the //antenna directivity//. The directivity is often given in dB with respect to that ideal //isotropic radiator//. The unit is thus **dBi**. A very large part of an antenna' |
- | A higher directivity means that more of the emitted/ | + | A higher directivity means that more of the emitted/ |
- | A high directivity, | + | \\ |
+ | ==dBd== | ||
- | *dBd* | + | Practically, |
- | Practically, | + | A dipole antenna is -2.15 dB less " |
- | + | \\ | |
- | A dipole antenna is -2.15 dB less " | + | |
//dBd = directivity in dB with respect to an ideal dipole antenna.// | //dBd = directivity in dB with respect to an ideal dipole antenna.// | ||
+ | \\ | ||
== Lobes (SE: Lober) == | == Lobes (SE: Lober) == | ||
- | Important: almost always, antennas do not emit/ | + | Important: almost always, antennas do not emit/ |
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, | + | Simple antennas, like a monopole antenna, only have a single lobe. Very complex antennas, |
- | In practice, it is very hard to design an antenna with such a complicated lobe pattern. Companies approach this problem for instance using evolutional AI algorithms that brute-force designs until the lobe pattern is achieved. Another very common approach, is to make a very large array of antennas, and control the phase of the signal reaching each antenna, in order to achieve a more complicated lobe pattern. | ||
+ | \\ | ||
+ | ==Shaping the lobes + Terminology 2== | ||
+ | The EM wave that we wish to transmit/ | ||
+ | |||
+ | 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 // | ||
+ | |||
+ | 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. | ||
+ | |||
+ | |||
+ | |||
+ | \\ | ||
==Polarisation== | ==Polarisation== | ||
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Why polarisation is important: typically, the best transmission efficiency between two antennas, is achieved when their polarisations are matching. | Why polarisation is important: typically, the best transmission efficiency between two antennas, is achieved when their polarisations are matching. | ||
- | + | \\ | |
- | SE words: vertikal | + | ==Complicated: circular |
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; | ||
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+ | \\ | ||
+ | == Common mode current on coax = bad! == | ||
+ | TODO | ||
- | Common mode current on coax = bad! | + | \\ |
- | + | ||
== Propagation == | == Propagation == | ||
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user