user:kurser:ham_vt2023_l7
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user:kurser:ham_vt2023_l7 [2023/02/24 18:43] – user | user:kurser:ham_vt2023_l7 [2023/04/22 14:19] – Added portion about lobe patterns. 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. | ||
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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** |
- | Show some example | + | 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. |
- | * Ground plane/ | + | |
+ | 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. | ||
+ | |||
+ | A seletion of different antenna types, roughly in order of most common -> least common | ||
+ | * Dipole | ||
+ | * Monopole | ||
* Loop antennas | * Loop antennas | ||
- | * Yagi | + | * Yagi (Yagi-Uda) |
- | * log periodic | + | * Patch |
- | * Parabolic | + | * Quad |
- | * etc. | + | * PCB/PIFA |
+ | * Waveguide slot | ||
+ | * Bowtie | ||
+ | * Spiral/ | ||
+ | * Vivaldi | ||
+ | * J-pole | ||
+ | |||
+ | 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: | ||
+ | * Log-periodic | ||
+ | |||
+ | 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. | ||
+ | |||
+ | All antennas can be characterised using //at least// the following parameters: | ||
+ | * Input impedance, Z_in | ||
+ | * Standing wave ratio, SWR | ||
+ | * Operational frequency band | ||
+ | * Resonance frequency, f_0 | ||
+ | * Directivity, | ||
+ | * Antenna gain | ||
+ | * Far-field distance = d_f > 2*D^2/ | ||
+ | * Radiation efficiency, eta | ||
+ | * Radiation pattern, E & H patterns | ||
+ | * 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 a vector network analyser, or an antenna analyser. Let's do that at ETA. | ||
+ | |||
+ | ==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// (bomm). | ||
+ | |||
+ | Often, antennas are shielded from the evil world of rain, snow, pidgeons | ||
+ | |||
+ | ==Direction== | ||
+ | |||
+ | The most ideal antenna is a single charge floating in free space, radiating in 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. 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 high directivity, | ||
+ | |||
+ | *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, can have lobes that are shaped practically in any way you want it to be shaped. 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/ | ||
+ | |||
+ | 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. | ||
+ | |||
+ | ==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. | ||
+ | |||
+ | |||
+ | SE words: vertikal 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 orientation. | ||
+ | |||
+ | Common misconception: | ||
+ | |||
+ | |||
+ | |||
- | Talk about polarization. | ||
Common mode current on coax = bad! | Common mode current on coax = bad! | ||
+ | |||
== Propagation == | == Propagation == | ||
Line 80: | Line 171: | ||
Fading example with cellphone if possible | Fading example with cellphone if possible | ||
+ | |||
+ | Radio Antenna Fundamentals Part 1 1947 | ||
+ | |||
+ | https:// | ||
user/kurser/ham_vt2023_l7.txt · Last modified: 2024/02/13 18:08 by user