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--- user:kurser:ham_vt2023_l7 [2023/02/24 18:43] – user
+++ user:kurser:ham_vt2023_l7 [2023/04/22 18:30] – Added segment on antenna gain and efficiency. user
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 [[user:kurser:ham_vt2023|Back to course information]]
-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.
 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) ~=(0,96*lambda/2)
-==Antennas==
+\\
+\\
-Show some example antennas.
-  * Ground plane/verticals
+**Antennas**
+In this segment, possible exam questions have been marked //**POSSIBLE EXAM QUESTIONS**//.
+\\
+==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 = two port that converts energy from propagating in a transmission line to propagation in free-space.//
+\\
+==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
-  * Yagi
+  * Yagi (Yagi-Uda)
-  * log periodic
+  * (Parabolic dish reflectors)
-  * Parabolic
+  * PCB/PIFA
-  * etc.
+  * Patch
+  * Quad
+  * Waveguide slot
+  * Spiral/Helical
+  * 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
+... 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.
+\\
+==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, D, dBi, dBd
+  * 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
+Most 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 //elements// (SE: antennelement, spröt).
+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, antennas are real objects and thus not single charges, meaning that the "radiation sphere" (made-up word) is very much not a sphere. Most antennas only //illuminate// (actual term) a smaller part of that hypothetic sphere, meaning that most of the EM field is sent/recieved from/to the antenna at that specific direction in space. The illuminated area is known as the //beam area// (SE: ??).
+How small that illuminated segment of the sphere is as opposed to size the entire sphere, 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's design specification, is its directivity. Different types of antennas have very different directivities.
+A higher directivity means that more of the emitted/recieved field is sent through a smaller portion of that sphere around the antenna. The antenna is in a way "pointing more" towards one direction. A high directivity thus means that the antenna is very good as emitting/receiving to that specific direction. And as a drawback, the antenna become worse at emitting/receiving in every other direction.
+\\
+==dBd==
+Practically, it is not uncommon to describe the directivity of some antenna with respect to something more real. Such as, a dipole antenna's directivity. How much more "directive" (made-up word) is our antenna, in comparison to a dipole antenna?
+A dipole antenna is -2.15 dB less "directive" than an isotropic radiator. Thus, the unit **dBd** = dBi - 2.15.
+\\
+//dBd = directivity in dB with respect to an ideal dipole antenna.//
+\\
+== Lobes (SE: Lober) ==
+Important: almost always, antennas do not emit/receive in a single direction. Most antennas emit/receive in one "very good" direction, and a small set of "less good but OK" directions. How well an antenna is emitting to a particular direction, is known as a //lobe// (SE: lob).
+The direction with the highest directivity of the antenna, is known as the //main lobe// (SE: huvudloben). Thus, the smaller lobes are known as //sidelobes// (SE: sidolober). The main lobe in practice defines which way the antenna is pointing.
+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/receiving to/from that nation, and worse at transmitting/receiving to/from locations outside of that nation's borders. 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/recieve to the outside world, will travel along a cable into the antenna. Where that cable meets the antenna, is almost always at an antenna element. In a way, you could say that this antenna element is what is in fact transmitting inside of the antenna. This element is known as the //driven// element (SE: drivelement). An antenna may have several driven elements.
+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 //directors// (SE: direktorer). Or, those elements could be to block off the lobe pattern from emitting/reciving into some direction; such elements are known as //reflectors// (SE: reflektorer).
+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).
+\\
+==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.
+\\
+==Polarisation==
+All antennas emit/receive EM waves. Meaning, that the antenna emits electric and magnetic fields. By convention, the orientation of the //electric// field is said to define the //polarisation// of the antenna. Example: let's say that the antenna is //vertically// polarised, then that would mean that the E-field is sent from the antenna like a sine wave moving up (and down) along the Z-axis, i.e. vertically with respect to the ground. Vice versa, //horisontal// polarisation means that the E-field is propagating like a sine wave that is laying down flat with respect to the ground.
+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: 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; the emitted/received EM field is in a way "spinning around" in a circle.
+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: even though many circularly-polarised antennas are shaped like round objects, it is not necessary to have a round antenna to create circular polarisation. The circular polarisation itself stems from the E and H field composants spinning a complex vector around in a circle, with respect to the direction of the propagating EM wave.
+\\
+== Common mode current on coax = bad! ==
+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/lambda, given d_f >> D, d_f >> lambda.
+Meaning, that for very high frequencies, our simplifications and thus our formulas, are valid already fairly close to the antenna. The opposite of the far-field is known as the near-field, where most of our formulas stop being valid, and in practice the behaviour of the antenna has to be numerically simulated.
-Talk about polarization.
-Common mode current on coax = bad!
+\\
 == Propagation ==
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 Fading example with cellphone if possible
+Radio Antenna Fundamentals Part 1 1947
+https://www.youtube.com/watch?v=JHSPRcRgmOw&ab_channel=GerryTrenwith