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user:kurser:ham_vt2023_l7 [2025/03/07 18:32] – useruser:kurser:ham_vt2023_l7 [2026/03/02 17:18] (current) – loka
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 **Antennas** **Antennas**
 \\ de SA6KRZ \\ de SA6KRZ
 +
 +Updated 2026-03-02
  
 In this segment, possible exam questions have been marked //**POSSIBLE EXAM QUESTIONS**//. In this segment, possible exam questions have been marked //**POSSIBLE EXAM QUESTIONS**//.
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 \\ \\
-== Lobes (SE: Lober) ==+== Lobes (SE: Strålkäglor) ==
  
-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).+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: strålkägla).
  
-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.+The direction with the highest directivity of the antenna, is known as the //main lobe//. Thus, the smaller lobes are known as //sidelobes//. The main lobe defines which way the antenna is pointing, both by convention and often in practice.
  
 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. 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.
 +
 +A different example is the antenna running along the edge of your smartphone, disguised as a metal border rim. Having such a weird shape allows for a very high bandwidth, but the antenna must be paired with an always-on autotuner inside the phone for matching.
  
  
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 Stand in front of a transmitting antenna with a power detector. Assume that the strongest power in the main lobe is denoted P. As you move to the side, you see the power going down. The point at which the power has dropped 3 dB, is known as the //-3 dB point// (SE: halvvärdesbredd). By convention, the lobe is said to end at this point. Even though there is some power being transmitted beyond that point, as you move to the side further. Another (less common) datasheet specification, is also the -10 dB point. Stand in front of a transmitting antenna with a power detector. Assume that the strongest power in the main lobe is denoted P. As you move to the side, you see the power going down. The point at which the power has dropped 3 dB, is known as the //-3 dB point// (SE: halvvärdesbredd). By convention, the lobe is said to end at this point. Even though there is some power being transmitted beyond that point, as you move to the side further. Another (less common) datasheet specification, is also the -10 dB point.
 \\ \\
-Now, let's assume that you've found the -3 dB points to the left and to the right of the main lobe. If you draw a triangle between these two points and the antenna, you'll create some angle α at point of the triangle (at the antenna). This angle is known as the //beamwidth// of the lobe (SE: öppningsvinkel).\\+Now, let's assume that you've found the -3 dB points to the left and to the right of the main lobe. If you draw a triangle between these two points and the antenna, you'll create some angle α at apex point of the triangle (at the antenna). This angle is known as the //beamwidth// of the lobe (SE: öppningsvinkel).\\
  
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 ==Polarisation== ==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.+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, //horizontal// 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?"+Very often, it's fully possible to simply look an an antenna, and figure out its polarisation. "Where are the antenna elements pointing?"
  
 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.
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 \\ \\
 == Antenna impedance matching == == Antenna impedance matching ==
-Different antenna types have different ideal input impedances. And, the the input impedance is typically frequency dependent. For instance, a given antenna may look like a 50 ohm impedance at 5 MHz, but may look more like an open circuit at 10 MHz.+Different antenna types have different ideal input impedances. And, the the input impedance is typically frequency dependent. For instance, a given antenna may look like a 50 Ω impedance at 5 MHz, but may look more like an open circuit at 10 MHz.
  
-Example: the ideal dipole antenna has a 73 ohm input impedance. Feeding this antenna with a coaxial cable of 50 ohm characteristic impedance, leads to a 50 ohm -> 73 ohm impedance interconnect. This impedance difference will result in signal reflections, which are unwanted for several reasons.+Example: the ideal dipole antenna has a 73 Ω input impedance. Feeding this antenna with a coaxial cable of 50 Ω characteristic impedance, leads to a 50 Ω -> 73 Ω impedance interconnect. This impedance difference will result in signal reflections, which are unwanted for several reasons.
  
 Overcoming impedance differences in antennas may for instance be done using matching networks. Overcoming impedance differences in antennas may for instance be done using matching networks.
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user/kurser/ham_vt2023_l7.txt · Last modified: 2026/03/02 17:18 by loka