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--- user:kurser:ham_vt2023_l7 [2023/04/22 13:51] – Added segments to the Antenna section. user
+++ user:kurser:ham_vt2023_l7 [2023/04/22 14:19] – Added portion about lobe patterns. user
@@ Line 24: / Line 24: @@
 **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.
+Alternative: //Antenna = two port that converts energy from propagating in a transmission line to propagation in free-space.//
 //Antenna types.//
@@ Line 33: / Line 34: @@
 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 antenna types, roughly in order of most common -> least common
+A seletion of different antenna types, roughly in order of most common -> least common
   * Dipole
   * Monopole
-  * Small loop
+  * Loop antennas
   * Yagi (Yagi-Uda)
   * Patch
@@ Line 45: / Line 46: @@
   * Spiral/Helical
   * 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:
@@ Line 54: / Line 56: @@
   * Input impedance, Z_in
   * Standing wave ratio, SWR
-  * Operational frequency
+  * Operational frequency band
   * Resonance frequency, f_0
-  * Directivity, D
+  * Directivity, D, dBi, dBd
-  * Antenna gain, dBi, dBd
+  * Antenna gain
   * Far-field distance = d_f > 2*D^2/lambda, given d_f >> D, d_f >> lambda
   * Radiation efficiency, eta
@@ Line 81: / Line 83: @@
 A high directivity, 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 the other directions.
+*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 my 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 have one "very good" direction, and a set of "less good but OK" directions. How well an antenna is emitting to a particular direction, is known as a lobe.
+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.
+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, 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/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.
 ==Polarisation==