user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/04/22 12:12] – 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. | ||
Line 23: | Line 23: | ||
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== | + | \\ |
+ | \\ | ||
- | //Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.// | ||
- | 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. | ||
- | // | + | **Antennas** |
+ | |||
+ | In this segment, possible exam questions have been marked // | ||
+ | |||
+ | \\ | ||
+ | ==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== | ||
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. | ||
- | Different | + | A seletion of different |
* Dipole | * Dipole | ||
* Monopole | * Monopole | ||
- | * Small loop | + | * 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 | ||
+ | * Bowtie | ||
+ | |||
+ | Antennas may often be combined together for different effects. Such a group of antennas acting together as one, is known as an //antenna array// (SE: gruppantenn). Some antenna array designs are more common than others, for example: | ||
+ | * Log-periodic dipole array (LDPA) antenna | ||
- | Antennas may often be combined together for different effects. These group of antennas acting together as one, are denoted //antenna arrays// (gruppantenner). Some antenna arrays are more common than others, for example: | + | ... however, in practice there is nothing prohibiting an antenna |
- | * Log-periodic dipole | + | |
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: | ||
* Input impedance, Z_in | * Input impedance, Z_in | ||
* Standing wave ratio, SWR | * Standing wave ratio, SWR | ||
- | * Operational frequency | + | * Operational frequency |
* Resonance frequency, f_0 | * Resonance frequency, f_0 | ||
- | * Directivity, | + | * Directivity, |
- | * Antenna gain, dBi, dBd | + | * Antenna gain |
* Far-field distance = d_f > 2*D^2/ | * Far-field distance = d_f > 2*D^2/ | ||
* Radiation efficiency, eta | * Radiation efficiency, eta | ||
Line 61: | Line 80: | ||
* Polarisation and x-pol suppression | * Polarisation and x-pol suppression | ||
- | Of these parameters, only input impedance | + | Most of these parameters |
+ | |||
+ | \\ | ||
+ | ==Introductory important terminology== | ||
+ | |||
+ | 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// (SE: bom). | ||
+ | |||
+ | Often, antennas are shielded from the evil world of rain, snow, pidgeons etc. by placing them inside a plastic cover shell. This cover shell is known as a //radome// (SE: radom). | ||
+ | |||
+ | \\ | ||
+ | ==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, | ||
+ | |||
+ | How small that illuminated segment of the sphere is, is known as the //antenna directivity// | ||
+ | |||
+ | A higher directivity means that more of the emitted/ | ||
+ | |||
+ | \\ | ||
+ | ==dBd== | ||
+ | |||
+ | Practically, | ||
+ | |||
+ | A dipole antenna is -2.15 dB less " | ||
+ | \\ | ||
+ | //dBd = directivity in dB with respect to an ideal dipole antenna.// | ||
+ | |||
+ | \\ | ||
+ | == Lobes (SE: Lober) == | ||
+ | |||
+ | 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 // | ||
+ | |||
+ | 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 | ||
+ | |||
+ | |||
+ | \\ | ||
+ | ==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== | ||
+ | |||
+ | All antennas emit/ | ||
+ | |||
+ | Very often, it's fully possible to just look an an antenna, and figure out its polarisation. "Which way are the antenna elements pointing?" | ||
+ | |||
+ | Why polarisation is important: typically, the best transmission efficiency between two antennas, is achieved when their polarisations are matching. | ||
+ | |||
+ | \\ | ||
+ | ==Complicated: | ||
+ | |||
+ | Some antennas transmit mainly using the electric field, some transmit mainly using the magnetic field, and some use both. Let' | ||
+ | |||
+ | Circular polarisation is common in public FM radio broadcast (rundradio). The advantage is that the receiver can be rotated | ||
+ | |||
+ | Common misconception: | ||
+ | |||
+ | |||
+ | \\ | ||
+ | == Common mode current on coax = bad! == | ||
+ | TODO | ||
- | Talk about antenna polarisation. | ||
- | Common mode current on coax = bad! | ||
+ | \\ | ||
== Propagation == | == Propagation == | ||
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user