Differences

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--- user:kurser:ham_vt2023_l7 [2023/04/22 19:55] – Added clarification to the Yagi-Uda antenna being built from other antenna types. user
+++ user:kurser:ham_vt2023_l7 [2023/04/22 20:26] – user
@@ Line 82: / Line 82: @@
   * 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.
+Many of these parameters can be analysed using a vector network analyser or an antenna analyser. (Let's do that at ETA?)
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@@ Line 95: / Line 95: @@
 ==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: ??).
+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, 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.
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 == Antenna impedance matching ==
-TODO
+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.
+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.
+Overcoming impedance differences in antennas may for instance be done using matching networks.
+Matching an antenna gets harder as the operational band of the antenna grows larger.