Differences

This shows you the differences between two versions of the page.

--- user:kurser:ham_vt2023_l7 [2023/04/24 19:07] – Added some exam questions. user
+++ user:kurser:ham_vt2023_l7 [2024/02/13 18:08] (current) – user
@@ Line 1: / Line 1: @@
 =====ETA313-07: Theory Lesson 3: Antennas, Transmission lines, Propagation =====
-[[user:kurser:ham_vt2023|Back to course information]]
+[[user:kurser:ham_vt2024|Back to course information]]
 **Recommended reading: KonCEPT page 191-229 (chapter 7 + 8) **
@@ Line 28: / Line 28: @@
 **Antennas**
+\\ de SA6KRZ
 In this segment, possible exam questions have been marked //**POSSIBLE EXAM QUESTIONS**//.
@@ Line 82: / Line 83: @@
   * 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?)
+Many of these parameters can be analysed using a vector network analyser or an antenna analyser.
 \\
@@ Line 139: / Line 140: @@
 \\
 ==How wide is a lobe?==
-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).\\
@@ Line 178: / Line 179: @@
 Size-wise, antenna //elements// are usually 0.25 · λ or 0.5 · λ in length/size. An antenna with several elements of different lengths, can usually operate fairly well across a span of frequencies.
-//Why have I never seen an antenna that is 1.0 · λ in length?//
-\\
-A full-wave antenna is by definition resonant. A positive voltage on the antenna is cancelled by a negative voltage on the antenna. No field leaves the antenna. It's gain is thus 0; it doesn't work.
@@ Line 198: / Line 194: @@
-==Velocity factor (SE: förkortningsfaktor)==
+==Realistic design factors (SE: förkortningsfaktor)==
-Light travels slower in various media. Fact is that antenna component dimensions have to be scaled with the velocity factor in the relevant media. Often, you may see that some factor is given as 0.95, 0.96 or 0.98 etc. For instance, aluminium tubing typically has a factor of 0.96.
+Antenna component dimensions have to be scaled down when making real antennas. There are various factors that all make up a 0-1 scaling factor, that effectively makes it so that your antenna is shorter than what it would have been if you only accounted for the wavelength of some frequency. Often, you may see that some factor is given as 0.95, 0.96 or 0.98 etc. For instance, aluminium tubing typically has a factor of 0.96.
 \\
 //**POSSIBLE EXAM QUESTION**//
 \\
-You are building a full-wavelength delta-loop antenna for 7.1 MHz. The wire has a velocity factor of 0.95. How much wire will you use in total?
+You are building a full-wavelength delta-loop antenna for 7.1 MHz. The wire has a scaling factor of 0.95. How much wire will you use in total?
   * 20.07 m
@@ Line 214: / Line 210: @@
 Answer: ( 300 / (1.0 · 7.1) ) · 0.95 = 40.14 m
 \\
-Whether full-wavelength antennas are a good idea or not is irrelevant, that was not part of the question.
 \\