user:kurser:ham_vt2023_l7
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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== | + | \\ |
+ | \\ | ||
- | Show some example | + | |
- | * Ground plane/ | + | **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. | ||
+ | |||
+ | 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 | + | * (Parabolic dish reflectors) |
- | * Parabolic | + | * PCB/PIFA |
- | * etc. | + | * Patch |
+ | * Quad | ||
+ | * Waveguide slot | ||
+ | * Ground plane antenna (GP-antenna) | ||
+ | * Horn antenna | ||
+ | * Spiral/ | ||
+ | * 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 | ||
+ | |||
+ | ... 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 usually built using 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: | ||
+ | * 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/λ, given d_f >> D, d_f >> λ | ||
+ | * Radiation efficiency, η | ||
+ | * Radiation pattern, E & H patterns | ||
+ | * Polarisation and x-pol suppression | ||
+ | |||
+ | Many of these parameters can be analysed using 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// (SE: bom). | ||
+ | |||
+ | 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 as a sphere equally strongly in all directions. Such a single charge is known academically as an //isotropic radiator//. Practically, | ||
+ | |||
+ | 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/ | ||
+ | |||
+ | \\ | ||
+ | ==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 (SE: strålningsdiagram) that is shaped according to the borders of that nation. The antenna is thus good at transmitting/ | ||
+ | |||
+ | |||
+ | \\ | ||
+ | ==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. | ||
+ | |||
+ | |||
+ | \\ | ||
+ | ==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). | ||
+ | |||
+ | \\ | ||
+ | ==How wide is a lobe?== | ||
+ | * TODO: -3 dB point (SE: halvvärdesbredd)\\ | ||
+ | * TODO: The angle spanned by the circular segment, defines the // | ||
+ | |||
+ | \\ | ||
+ | ==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. | ||
+ | |||
+ | \\ | ||
+ | ==Emitted power: p.e.p. and e.i.r.p== | ||
+ | The HAM radio band plan sets radiation emission limits in terms of power, either as **p.e.p.** or **e.i.r.p.** (peak emitted power, equivalent isotropic radiated power). | ||
+ | |||
+ | * The p.e.p. limit defines how much peak power may be fed into your antenna. | ||
+ | * The e.i.r.p. limit takes antenna gain into account as well, meaning that no lobe may peak above a certain power. | ||
+ | |||
+ | \\ | ||
+ | // | ||
+ | \\ | ||
+ | In the 5.3515-5.3665 MHz band, a HAM radio operator may at most transmit with 15 W e.i.r.p. Are you allowed to transmit at 5.36 MHz with 10 W using a +3 dB gain directional antenna? | ||
+ | \\ | ||
+ | \\ | ||
+ | Answer: No, this is not allowed. An antenna with +3 dB gain, would make a 10 W radiated emission seem as if we're transmitting with 20 W, which is above the e.i.r.p. limit. | ||
+ | |||
+ | \\ | ||
+ | // | ||
+ | \\ | ||
+ | In the 1850-1900 kHz band, a HAM radio operator may at most transmit with 10 W p.e.p. Are you allowed to transmit at 1875 kHz with 10 W using a +20 dB gain directional antenna? | ||
+ | \\ | ||
+ | \\ | ||
+ | Answer: Yes, this is allowed, since p.e.p. sets the limit of how much power is fed into the antenna, and does not account for directivity. | ||
+ | |||
+ | |||
+ | |||
+ | |||
+ | \\ | ||
+ | ==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'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 (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 transmissions. | ||
+ | |||
+ | Common misconception: | ||
+ | |||
+ | |||
+ | \\ | ||
+ | == Common mode current on coax = bad! == | ||
+ | TODO | ||
+ | |||
+ | \\ | ||
+ | == Antenna impedance matching == | ||
+ | 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, | ||
- | Talk about polarization. | ||
- | Common mode current on coax = bad! | ||
+ | \\ | ||
== Propagation == | == Propagation == | ||
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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